defmodule DBConnection.Stream do
  defstruct [:conn, :query, :params, :opts]

  @type t :: %__MODULE__{conn: DBConnection.conn(), query: any, params: any, opts: Keyword.t()}
end

defimpl Enumerable, for: DBConnection.Stream do
  def count(_), do: {:error, __MODULE__}

  def member?(_, _), do: {:error, __MODULE__}

  def slice(_), do: {:error, __MODULE__}

  def reduce(stream, acc, fun), do: DBConnection.reduce(stream, acc, fun)
end

defmodule DBConnection.PrepareStream do
  defstruct [:conn, :query, :params, :opts]

  @type t :: %__MODULE__{conn: DBConnection.conn(), query: any, params: any, opts: Keyword.t()}
end

defimpl Enumerable, for: DBConnection.PrepareStream do
  def count(_), do: {:error, __MODULE__}

  def member?(_, _), do: {:error, __MODULE__}

  def slice(_), do: {:error, __MODULE__}

  def reduce(stream, acc, fun), do: DBConnection.reduce(stream, acc, fun)
end

defmodule DBConnection do
  @moduledoc """
  A behaviour module for implementing efficient database connection
  client processes, pools and transactions.

  `DBConnection` handles callbacks differently to most behaviours. Some
  callbacks will be called in the calling process, with the state
  copied to and from the calling process. This is useful when the data
  for a request is large and means that a calling process can interact
  with a socket directly.

  A side effect of this is that query handling can be written in a
  simple blocking fashion, while the connection process itself will
  remain responsive to OTP messages and can enqueue and cancel queued
  requests.

  If a request or series of requests takes too long to handle in the
  client process a timeout will trigger and the socket can be cleanly
  disconnected by the connection process.

  If a calling process waits too long to start its request it will
  timeout and its request will be cancelled. This prevents requests
  building up when the database can not keep up.

  If no requests are received for an idle interval, the pool will
  ping all stale connections which can then ping the database to keep
  the connection alive.

  Should the connection be lost, attempts will be made to reconnect with
  (configurable) exponential random backoff to reconnect. All state is
  lost when a connection disconnects but the process is reused.

  The `DBConnection.Query` protocol provide utility functions so that
  queries can be encoded and decoded without blocking the connection or pool.

  ## Connection pools

  DBConnection connections support using different pools via the `:pool` option
  passed to `start_link/2`. The default pool is `DBConnection.ConnectionPool`.
  Another supported pool that is commonly used for tests is `DBConnection.Ownership`.

  For now, using *custom* pools is not supported since the API for pools is not
  public.

  ## Errors

  Most functions in this module raise a `DBConnection.ConnectionError` exception
  when failing to check out a connection from the pool.
  """
  require Logger

  alias DBConnection.Holder

  require Holder

  defstruct [:pool_ref, :conn_ref, :conn_mode]

  defmodule EncodeError do
    defexception [:message]
  end

  defmodule TransactionError do
    defexception [:status, :message]

    def exception(:idle),
      do: %__MODULE__{status: :idle, message: "transaction is not started"}

    def exception(:transaction),
      do: %__MODULE__{status: :transaction, message: "transaction is already started"}

    def exception(:error),
      do: %__MODULE__{status: :error, message: "transaction is aborted"}
  end

  @typedoc """
  Run or transaction connection reference.
  """
  @type t :: %__MODULE__{pool_ref: any, conn_ref: reference}
  @type conn :: GenServer.server() | t
  @type query :: DBConnection.Query.t()
  @type params :: any
  @type result :: any
  @type cursor :: any
  @type status :: :idle | :transaction | :error

  @type start_option ::
          {:after_connect, (t -> any) | {module, atom, [any]} | nil}
          | {:after_connect_timeout, timeout}
          | {:connection_listeners, [Process.dest()] | nil | {[Process.dest()], any}}
          | {:backoff_max, non_neg_integer}
          | {:backoff_min, non_neg_integer}
          | {:backoff_type, :stop | :exp | :rand | :rand_exp}
          | {:configure, (keyword -> keyword) | {module, atom, [any]} | nil}
          | {:idle_interval, non_neg_integer}
          | {:idle_limit, non_neg_integer}
          | {:max_restarts, non_neg_integer}
          | {:max_seconds, pos_integer}
          | {:name, GenServer.name()}
          | {:pool, module}
          | {:pool_size, pos_integer}
          | {:queue_interval, non_neg_integer}
          | {:queue_target, non_neg_integer}
          | {:show_sensitive_data_on_connection_error, boolean}

  @typedoc """
  An option you can pass to DBConnection functions (*deprecated*).

  > #### Deprecated {: .warning}
  >
  > This option is deprecated since v2.6.0. Use `t:connection_option/0` instead.

  """
  @type option :: connection_option

  @typedoc """
  An option you can pass to DBConnection functions.
  """
  @typedoc since: "2.6.0"
  @type connection_option ::
          {:log, (DBConnection.LogEntry.t() -> any) | {module, atom, [any]} | nil}
          | {:queue, boolean}
          | {:timeout, timeout}
          | {:deadline, integer | nil}

  @doc """
  Connect to the database. Return `{:ok, state}` on success or
  `{:error, exception}` on failure.

  If an error is returned it will be logged and another
  connection attempt will be made after a backoff interval.

  This callback is called in the connection process.
  """
  @callback connect(opts :: Keyword.t()) ::
              {:ok, state :: any} | {:error, Exception.t()}

  @doc """
  Checkouts the state from the connection process. Return `{:ok, state}`
  to allow the checkout or `{:disconnect, exception, state}` to disconnect.

  This callback is called immediately after the connection is established
  and the state is never effetively checked in again. That's because
  DBConnection keeps the connection state in an ETS table that is moved
  between the different clients checking out connections. There is no
  `checkin` callback. The state is only handed back to the connection
  process during pings and (re)connects.

  This callback is called in the connection process.
  """
  @callback checkout(state :: any) ::
              {:ok, new_state :: any} | {:disconnect, Exception.t(), new_state :: any}

  @doc """
  Called when the connection has been idle for a period of time. Return
  `{:ok, state}` to continue or `{:disconnect, exception, state}` to
  disconnect.

  This callback is called if no callbacks have been called after the
  idle timeout and a client process is not using the state. The idle
  timeout can be configured by the `:idle_interval` and `:idle_limit`
  options. This function can be called whether the connection is checked
  in or checked out.

  This callback is called in the connection process.
  """
  @callback ping(state :: any) ::
              {:ok, new_state :: any} | {:disconnect, Exception.t(), new_state :: any}

  @doc """
  Handle the beginning of a transaction.

  Return `{:ok, result, state}`/`{:ok, query, result, state}` to continue,
  `{status, state}` to notify caller that the transaction can not begin due
  to the transaction status `status`, or `{:disconnect, exception, state}`
  to error and disconnect. If `{:ok, query, result, state}` is returned,
  the query will be used to log the begin command. Otherwise, it will be
  logged as `begin`.

  A callback implementation should only return `status` if it
  can determine the database's transaction status without side effect.

  This callback is called in the client process.
  """
  @callback handle_begin(opts :: Keyword.t(), state :: any) ::
              {:ok, result, new_state :: any}
              | {:ok, query, result, new_state :: any}
              | {status, new_state :: any}
              | {:disconnect, Exception.t(), new_state :: any}

  @doc """
  Handle committing a transaction. Return `{:ok, result, state}` on successfully
  committing transaction, `{status, state}` to notify caller that the
  transaction can not commit due to the transaction status `status`,
  or `{:disconnect, exception, state}` to error and disconnect.

  A callback implementation should only return `status` if it
  can determine the database's transaction status without side effect.

  This callback is called in the client process.
  """
  @callback handle_commit(opts :: Keyword.t(), state :: any) ::
              {:ok, result, new_state :: any}
              | {status, new_state :: any}
              | {:disconnect, Exception.t(), new_state :: any}

  @doc """
  Handle rolling back a transaction. Return `{:ok, result, state}` on successfully
  rolling back transaction, `{status, state}` to notify caller that the
  transaction can not rollback due to the transaction status `status` or
  `{:disconnect, exception, state}` to error and disconnect.

  A callback implementation should only return `status` if it
  can determine the database' transaction status without side effect.

  This callback is called in the client and connection process.
  """
  @callback handle_rollback(opts :: Keyword.t(), state :: any) ::
              {:ok, result, new_state :: any}
              | {status, new_state :: any}
              | {:disconnect, Exception.t(), new_state :: any}

  @doc """
  Handle getting the transaction status. Return `{:idle, state}` if outside a
  transaction, `{:transaction, state}` if inside a transaction,
  `{:error, state}` if inside an aborted transaction, or
  `{:disconnect, exception, state}` to error and disconnect.

  If the callback returns a `:disconnect` tuples then `status/2` will return
  `:error`.
  """
  @callback handle_status(opts :: Keyword.t(), state :: any) ::
              {status, new_state :: any}
              | {:disconnect, Exception.t(), new_state :: any}

  @doc """
  Prepare a query with the database. Return `{:ok, query, state}` where
  `query` is a query to pass to `execute/4` or `close/3`,
  `{:error, exception, state}` to return an error and continue or
  `{:disconnect, exception, state}` to return an error and disconnect.

  This callback is intended for cases where the state of a connection is
  needed to prepare a query and/or the query can be saved in the
  database to call later.

  This callback is called in the client process.
  """
  @callback handle_prepare(query, opts :: Keyword.t(), state :: any) ::
              {:ok, query, new_state :: any}
              | {:error | :disconnect, Exception.t(), new_state :: any}

  @doc """
  Execute a query prepared by `c:handle_prepare/3`. Return
  `{:ok, query, result, state}` to return altered query `query` and result
  `result` and continue, `{:error, exception, state}` to return an error and
  continue or `{:disconnect, exception, state}` to return an error and
  disconnect.

  This callback is called in the client process.
  """
  @callback handle_execute(query, params, opts :: Keyword.t(), state :: any) ::
              {:ok, query, result, new_state :: any}
              | {:error | :disconnect, Exception.t(), new_state :: any}

  @doc """
  Close a query prepared by `c:handle_prepare/3` with the database. Return
  `{:ok, result, state}` on success and to continue,
  `{:error, exception, state}` to return an error and continue, or
  `{:disconnect, exception, state}` to return an error and disconnect.

  This callback is called in the client process.
  """
  @callback handle_close(query, opts :: Keyword.t(), state :: any) ::
              {:ok, result, new_state :: any}
              | {:error | :disconnect, Exception.t(), new_state :: any}

  @doc """
  Declare a cursor using a query prepared by `c:handle_prepare/3`. Return
  `{:ok, query, cursor, state}` to return altered query `query` and cursor
  `cursor` for a stream and continue, `{:error, exception, state}` to return an
  error and continue or `{:disconnect, exception, state}` to return an error
  and disconnect.

  This callback is called in the client process.
  """
  @callback handle_declare(query, params, opts :: Keyword.t(), state :: any) ::
              {:ok, query, cursor, new_state :: any}
              | {:error | :disconnect, Exception.t(), new_state :: any}

  @doc """
  Fetch the next result from a cursor declared by `c:handle_declare/4`. Return
  `{:cont, result, state}` to return the result `result` and continue using
  cursor, `{:halt, result, state}` to return the result `result` and close the
  cursor, `{:error, exception, state}` to return an error and close the
  cursor, `{:disconnect, exception, state}` to return an error and disconnect.

  This callback is called in the client process.
  """
  @callback handle_fetch(query, cursor, opts :: Keyword.t(), state :: any) ::
              {:cont | :halt, result, new_state :: any}
              | {:error | :disconnect, Exception.t(), new_state :: any}

  @doc """
  Deallocate a cursor declared by `c:handle_declare/4` with the database. Return
  `{:ok, result, state}` on success and to continue,
  `{:error, exception, state}` to return an error and continue, or
  `{:disconnect, exception, state}` to return an error and disconnect.

  This callback is called in the client process.
  """
  @callback handle_deallocate(query, cursor, opts :: Keyword.t(), state :: any) ::
              {:ok, result, new_state :: any}
              | {:error | :disconnect, Exception.t(), new_state :: any}

  @doc """
  Disconnect from the database. Return `:ok`.

  This callback is called from the connection process. The first argument is
  either the exception from a `:disconnect` 3-tuple returned by a previous
  callback or an exception generated by the connection process.

  If the state is controlled by a client and it exits or times out while
  processing a request, the last known state will be sent and the exception
  will be a `DBConnection.ConnectionError`.

  When the connection is stopped, this callback will be invoked from `terminate`.
  The last known state will be sent and the exception will be a `DBConnection.ConnectionError`
  containing the reason for the exit. To have the same happen on unexpected
  shutdowns, you may trap exits from the `connect` callback.

  """
  @callback disconnect(err :: Exception.t(), state :: any) :: :ok

  @connection_module_key :connection_module

  @doc """
  Use `DBConnection` to set the behaviour.
  """
  defmacro __using__(_) do
    quote location: :keep do
      @behaviour DBConnection
    end
  end

  @doc """
  Starts and links to a database connection process.

  By default the `DBConnection` starts a pool with a single connection.
  The size of the pool can be increased with `:pool_size`. A separate
  pool can be given with the `:pool` option.

  ### Options

    * `:backoff_min` - The minimum backoff interval (default: `1_000`)
    * `:backoff_max` - The maximum backoff interval (default: `30_000`)
    * `:backoff_type` - The backoff strategy, `:stop` for no backoff and
    to stop, `:exp` for exponential, `:rand` for random and `:rand_exp` for
    random exponential (default: `:rand_exp`)
    * `:configure` - A function to run before every connect attempt to
    dynamically configure the options, either a 1-arity fun,
    `{module, function, args}` with options prepended to `args` or `nil` where
    only returned options are passed to connect callback (default: `nil`). This
    function is called *in the connection process*.
    * `:after_connect` - A function to run on connect using `run/3`, either
    a 1-arity fun, `{module, function, args}` with `t:DBConnection.t/0` prepended
    to `args` or `nil` (default: `nil`)
    * `:after_connect_timeout` - The maximum time allowed to perform
    function specified by `:after_connect` option (default: `15_000`)
    * `:connection_listeners` - A list of process destinations to send
      notification messages whenever a connection is connected or disconnected.
      See "Connection listeners" below
    * `:name` - A name to register the started process (see the `:name` option
      in `GenServer.start_link/3`)
    * `:pool` - Chooses the pool to be started (default: `DBConnection.ConnectionPool`). See
      ["Connection pools"](#module-connection-pools).
    * `:pool_size` - Chooses the size of the pool (default: `1`)
    * `:idle_interval` - Controls the frequency we check for idle connections
      in the pool. We then notify each idle connection to ping the database.
      In practice, the ping happens within `idle_interval <= ping < 2 * idle_interval`.
      Defaults to 1000ms.
    * `:idle_limit` - The number of connections to ping on each `:idle_interval`.
      Defaults to the pool size (all connections).
    * `:queue_target` and `:queue_interval` - See "Queue config" below
    * `:max_restarts` and `:max_seconds` - Configures the `:max_restarts` and
      `:max_seconds` for the connection pool supervisor (see the `Supervisor` docs).
      Typically speaking the connection process doesn't terminate, except due to
      faults in DBConnection. However, if backoff has been disabled, then they
      also terminate whenever a connection is disconnected (for instance, due to
      client or server errors)
    * `:show_sensitive_data_on_connection_error` - By default, `DBConnection`
      hides all information during connection errors to avoid leaking credentials
      or other sensitive information. You can set this option if you wish to
      see complete errors and stacktraces during connection errors

  ### Example

      {:ok, conn} = DBConnection.start_link(mod, [idle_interval: 5_000])

  ## Queue config

  Handling requests is done through a queue. When DBConnection is
  started, there are two relevant options to control the queue:

    * `:queue_target` in milliseconds, defaults to 50ms
    * `:queue_interval` in milliseconds, defaults to 1000ms

  Our goal is to wait at most `:queue_target` for a connection.
  If all connections checked out during a `:queue_interval` takes
  more than `:queue_target`, then we double the `:queue_target`.
  If checking out connections take longer than the new target,
  then we start dropping messages.

  For example, by default our target is 50ms. If all connections
  checkouts take longer than 50ms for a whole second, we double
  the target to 100ms and we start dropping messages if the
  time to checkout goes above the new limit.

  This allows us to better plan for overloads as we can refuse
  requests before they are sent to the database, which would
  otherwise increase the burden on the database, making the
  overload worse.

  ## Connection listeners

  The `:connection_listeners` option allows one or more processes to be notified
  whenever a connection is connected or disconnected. A listener may be a remote
  or local PID, a locally registered name, or a tuple in the form of
  `{registered_name, node}` for a registered name at another node.

  Each listener process may receive the following messages where `pid`
  identifies the connection process:

    * `{:connected, pid}`
    * `{:disconnected, pid}`

  If the value of `:connection_listeners` is a tuple like `{listeners, term}`, then
  the messages are these instead:

    * `{:connected, pid, term}`
    * `{:disconnected, pid, term}`

  Note the disconnected messages are not guaranteed to be delivered if the
  `pid` for connection crashes. So it is recommended to monitor the connected
  `pid` if you want to track all disconnections.

  Here is an example of a `:connection_listener` implementation:

      defmodule DBConnectionListener do
        use GenServer

        def start_link(opts) do
          GenServer.start_link(__MODULE__, [], opts)
        end

        def get_notifications(pid) do
          GenServer.call(pid, :read_state)
        end

        @impl true
        def init(stack) when is_list(stack) do
          {:ok, stack}
        end

        @impl true
        def handle_call(:read_state, _from, state) do
          {:reply, state, state}
        end

        @impl true
        def handle_info({:connected, _pid} = msg, state) do
          {:noreply, [msg | state]}
        end

        @impl true
        def handle_info({_other_states, _pid} = msg, state) do
          {:noreply, [msg | state]}
        end
      end

  You can then start it, pass the PID in the `connection_listeners`
  option on `DBConnection.start_link/2` and, when needed, can query the notifications:

      {:ok, pid} = DBConnectionListener.start_link([])
      {:ok, _conn} = DBConnection.start_link(SomeModule, [connection_listeners: [pid]])
      notifications = DBConnectionListener.get_notifications(pid)

  ### Tagging messages

  If you pass `{listeners, tag}` as an option, you can specify an arbitrary `tag` term that will
  be sent alongside all `:connected`/`:disconnected` messages. This is useful if you
  want to track information about the pool a connection belongs to or any other information.

  This feature is available since v2.6.0. Before this version `:connection_listeners` only
  accepted a list of listener processes.

  ## Telemetry

  A `[:db_connection, :connection_error]` event is published whenever a
  connection checkout receives a `%DBConnection.ConnectionError{}`.

  Measurements:

    * `:count` - A fixed-value measurement which always measures 1.

  Metadata

    * `:error` - The `DBConnection.ConnectionError` struct which triggered the event.

    * `:opts` - All options given to the pool operation

  """
  @spec start_link(module, [start_option()] | Keyword.t()) :: GenServer.on_start()
  def start_link(conn_mod, opts) do
    case child_spec(conn_mod, opts) do
      {_, {m, f, args}, _, _, _, _} -> apply(m, f, args)
      %{start: {m, f, args}} -> apply(m, f, args)
    end
  end

  @doc """
  Creates a supervisor child specification for a pool of connections.

  See `start_link/2` for options.
  """
  @spec child_spec(module, [start_option()] | Keyword.t()) :: :supervisor.child_spec()
  def child_spec(conn_mod, opts) do
    pool = Keyword.get(opts, :pool, DBConnection.ConnectionPool)
    pool.child_spec({conn_mod, opts})
  end

  @doc """
  Returns the names of all possible options that you can pass to `start_link/2`.

  This is mostly useful for library authors that base their library on top of
  `DBConnection`, since they can use the return value of this function to perform
  validation on options only passing down these options to DBConnection.

  See also `t:start_option/0`.
  """
  @doc since: "2.6.0"
  @spec available_start_options() :: [atom, ...]
  def available_start_options do
    [
      :after_connect,
      :after_connect_timeout,
      :connection_listeners,
      :backoff_max,
      :backoff_min,
      :backoff_type,
      :configure,
      :idle_interval,
      :idle_limit,
      :max_restarts,
      :max_seconds,
      :name,
      :pool,
      :pool_size,
      :queue_interval,
      :queue_target,
      :show_sensitive_data_on_connection_error
    ]
  end

  @doc """
  Returns the names of all possible options that you can pass to most functions
  in this module.

  This is mostly useful for library authors that base their library on top of
  `DBConnection`, since they can use the return value of this function to perform
  validation on options only passing down these options to DBConnection.

  See also `t:connection_option/0`.
  """
  @doc since: "2.6.0"
  @spec available_connection_options() :: [atom, ...]
  def available_connection_options do
    [:log, :queue, :timeout, :deadline]
  end

  @doc """
  Forces all connections in the pool to disconnect within the given interval
  in milliseconds.

  Once this function is called, the pool will disconnect all of its connections
  as they are checked in or as they are pinged. Checked in connections will be
  randomly disconnected within the given time interval. Pinged connections are
  immediately disconnected - as they are idle (according to `:idle_interval`).

  If the connection has a backoff configured (which is the case by default),
  disconnecting means an attempt at a new connection will be done immediately
  after, without starting a new process for each connection. However, if backoff
  has been disabled, the connection process will terminate. In such cases,
  disconnecting all connections may cause the pool supervisor to restart
  depending on the max_restarts/max_seconds configuration of the pool,
  so you will want to set those carefully.
  """
  @spec disconnect_all(conn, non_neg_integer, [connection_option()] | Keyword.t()) :: :ok
  def disconnect_all(conn, interval, opts \\ []) when interval >= 0 do
    pool = Keyword.get(opts, :pool, DBConnection.ConnectionPool)
    pool.disconnect_all(conn, interval, opts)
  end

  @doc """
  Prepare a query with a database connection for later execution.

  It returns `{:ok, query}` on success or `{:error, exception}` if there was
  an error.

  The returned `query` can then be passed to `execute/4` and/or `close/3`

  ### Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)
    * `:log` - A function to log information about a call, either
    a 1-arity fun, `{module, function, args}` with `t:DBConnection.LogEntry.t/0`
    prepended to `args` or `nil`. See `DBConnection.LogEntry` (default: `nil`)

  The pool and connection module may support other options. All options
  are passed to `c:handle_prepare/3`.

  ### Example

      DBConnection.transaction(pool, fn conn ->
        query = %Query{statement: "SELECT * FROM table"}
        query = DBConnection.prepare!(conn, query)
        try do
          DBConnection.execute!(conn, query, [])
        after
          DBConnection.close(conn, query)
        end
      end)

  """
  @spec prepare(conn, query, [connection_option()] | Keyword.t()) ::
          {:ok, query} | {:error, Exception.t()}
  def prepare(conn, query, opts \\ []) do
    meter = meter(opts)

    result =
      with {:ok, query, meter} <- parse(query, meter, opts) do
        run(conn, &run_prepare/4, query, meter, opts)
      end

    log(result, :prepare, query, nil)
  end

  @doc """
  Prepare a query with a database connection and return the prepared
  query. An exception is raised on error.

  See `prepare/3`.
  """
  @spec prepare!(conn, query, [connection_option()] | Keyword.t()) :: query
  def prepare!(conn, query, opts \\ []) do
    case prepare(conn, query, opts) do
      {:ok, result} -> result
      {:error, err} -> raise err
    end
  end

  @doc """
  Prepare a query and execute it with a database connection and return both the
  prepared query and the result, `{:ok, query, result}` on success or
  `{:error, exception}` if there was an error.

  The returned `query` can be passed to `execute/4` and `close/3`.

  ### Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)
    * `:log` - A function to log information about a call, either
    a 1-arity fun, `{module, function, args}` with `t:DBConnection.LogEntry.t/0`
    prepended to `args` or `nil`. See `DBConnection.LogEntry` (default: `nil`)

  ### Example

      query                = %Query{statement: "SELECT id FROM table WHERE id=$1"}
      {:ok, query, result} = DBConnection.prepare_execute(conn, query, [1])
      {:ok, result2}       = DBConnection.execute(conn, query, [2])
      :ok                  = DBConnection.close(conn, query)
  """
  @spec prepare_execute(conn, query, params, [connection_option()] | Keyword.t()) ::
          {:ok, query, result}
          | {:error, Exception.t()}
  def prepare_execute(conn, query, params, opts \\ []) do
    result =
      with {:ok, query, meter} <- parse(query, meter(opts), opts) do
        parsed_prepare_execute(conn, query, params, meter, opts)
      end

    log(result, :prepare_execute, query, params)
  end

  defp parsed_prepare_execute(conn, query, params, meter, opts) do
    with {:ok, query, result, meter} <-
           run(conn, &run_prepare_execute/5, query, params, meter, opts),
         {:ok, result, meter} <- decode(query, result, meter, opts) do
      {:ok, query, result, meter}
    end
  end

  @doc """
  Prepare a query and execute it with a database connection and return both the
  prepared query and result. An exception is raised on error.

  See `prepare_execute/4`.
  """
  @spec prepare_execute!(conn, query, [connection_option()] | Keyword.t()) :: {query, result}
  def prepare_execute!(conn, query, params, opts \\ []) do
    case prepare_execute(conn, query, params, opts) do
      {:ok, query, result} -> {query, result}
      {:error, err} -> raise err
    end
  end

  @doc """
  Execute a prepared query with a database connection and return
  `{:ok, query, result}` on success or `{:error, exception}` if there was an error.

  If the query is not prepared on the connection an attempt may be made to
  prepare it and then execute again.

  ### Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)
    * `:log` - A function to log information about a call, either
    a 1-arity fun, `{module, function, args}` with `t:DBConnection.LogEntry.t/0`
    prepended to `args` or `nil`. See `DBConnection.LogEntry` (default: `nil`)

  The pool and connection module may support other options. All options
  are passed to `handle_execute/4`.

  See `prepare/3`.
  """
  @spec execute(conn, query, params, [connection_option()] | Keyword.t()) ::
          {:ok, query, result} | {:error, Exception.t()}
  def execute(conn, query, params, opts \\ []) do
    result =
      case maybe_encode(query, params, meter(opts), opts) do
        {:prepare, meter} ->
          parsed_prepare_execute(conn, query, params, meter, opts)

        {:ok, params, meter} ->
          with {:ok, query, result, meter} <-
                 run(conn, &run_execute/5, query, params, meter, opts),
               {:ok, result, meter} <- decode(query, result, meter, opts) do
            {:ok, query, result, meter}
          end

        {_, _, _, _} = error ->
          error
      end

    log(result, :execute, query, params)
  end

  @doc """
  Execute a prepared query with a database connection and return the
  result. Raises an exception on error.

  See `execute/4`
  """
  @spec execute!(conn, query, params, [connection_option()] | Keyword.t()) :: result
  def execute!(conn, query, params, opts \\ []) do
    case execute(conn, query, params, opts) do
      {:ok, _query, result} -> result
      {:error, err} -> raise err
    end
  end

  @doc """
  Close a prepared query on a database connection and return `{:ok, result}` on
  success or `{:error, exception}` on error.

  This function should be used to free resources held by the connection
  process and/or the database server.

  ## Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)
    * `:log` - A function to log information about a call, either
    a 1-arity fun, `{module, function, args}` with `t:DBConnection.LogEntry.t/0`
    prepended to `args` or `nil`. See `DBConnection.LogEntry` (default: `nil`)

  The pool and connection module may support other options. All options
  are passed to `c:handle_close/3`.

  See `prepare/3`.
  """
  @spec close(conn, query, [connection_option()] | Keyword.t()) ::
          {:ok, result} | {:error, Exception.t()}
  def close(conn, query, opts \\ []) do
    conn
    |> run_cleanup(&run_close/4, [query], meter(opts), opts)
    |> log(:close, query, nil)
  end

  @doc """
  Close a prepared query on a database connection and return the result. Raises
  an exception on error.

  See `close/3`.
  """
  @spec close!(conn, query, [connection_option()] | Keyword.t()) :: result
  def close!(conn, query, opts \\ []) do
    case close(conn, query, opts) do
      {:ok, result} -> result
      {:error, err} -> raise err
    end
  end

  @doc """
  Acquire a lock on a connection and run a series of requests on it.

  The return value of this function is the return value of `fun`.

  To use the locked connection call the request with the connection
  reference passed as the single argument to the `fun`. If the
  connection disconnects all future calls using that connection
  reference will fail.

  `run/3` and `transaction/3` can be nested multiple times but a
  `transaction/3` call inside another `transaction/3` will be treated
  the same as `run/3`.

  > #### Checkout failures {: .warning}
  >
  > If we cannot check out a connection from the pool, this function raises a
  > `DBConnection.ConnectionError` exception.
  > If you want to handle these cases, you should rescue
  > `DBConnection.ConnectionError` exceptions when using `run/3`.

  ## Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)

  The pool may support other options.

  ## Example

      {:ok, res} = DBConnection.run(conn, fn conn ->
        DBConnection.execute!(conn, query, [])
      end)

  """
  @spec run(conn, (t -> result), [connection_option()] | Keyword.t()) :: result when result: var
  def run(conn, fun, opts \\ [])

  def run(%DBConnection{} = conn, fun, _) do
    fun.(conn)
  end

  def run(pool, fun, opts) do
    case checkout(pool, nil, opts) do
      {:ok, conn, _} ->
        old_status = status(conn, opts)

        try do
          result = fun.(conn)
          {result, run(conn, &run_status/3, nil, opts)}
        catch
          kind, error ->
            checkin(conn)
            :erlang.raise(kind, error, __STACKTRACE__)
        else
          {result, {:error, _, _}} ->
            checkin(conn)
            result

          {result, {^old_status, _meter}} ->
            checkin(conn)
            result

          {_result, {new_status, _meter}} ->
            err =
              DBConnection.ConnectionError.exception(
                "connection was checked out with status #{inspect(old_status)} " <>
                  "but it was checked in with status #{inspect(new_status)}"
              )

            disconnect(conn, err)
            raise err

          {_result, {kind, reason, stack, _meter}} ->
            :erlang.raise(kind, reason, stack)
        end

      {:error, err, _} ->
        raise err

      {kind, reason, stack, _} ->
        :erlang.raise(kind, reason, stack)
    end
  end

  @doc """
  Acquire a lock on a connection and run a series of requests inside a
  transaction. The result of the transaction fun is return inside an `:ok`
  tuple: `{:ok, result}`.

  To use the locked connection call the request with the connection
  reference passed as the single argument to the `fun`. If the
  connection disconnects all future calls using that connection
  reference will fail.

  `run/3` and `transaction/3` can be nested multiple times. If a transaction is
  rolled back or a nested transaction `fun` raises the transaction is marked as
  failed. All calls except `run/3`, `transaction/3`, `rollback/2`, `close/3` and
  `close!/3` will raise an exception inside a failed transaction until the outer
  transaction call returns. All `transaction/3` calls will return
  `{:error, :rollback}` if the transaction failed or connection closed and
  `rollback/2` is not called for that `transaction/3`.

  ### Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)
    * `:log` - A function to log information about begin, commit and rollback
    calls made as part of the transaction, either a 1-arity fun,
    `{module, function, args}` with `t:DBConnection.LogEntry.t/0` prepended to
    `args` or `nil`. See `DBConnection.LogEntry` (default: `nil`)

  The pool and connection module may support other options. All options
  are passed to `c:handle_begin/2`, `c:handle_commit/2` and
  `c:handle_rollback/2`.

  ### Example

      {:ok, res} = DBConnection.transaction(conn, fn conn ->
        DBConnection.execute!(conn, query, [])
      end)
  """
  @spec transaction(conn, (t -> result), [connection_option()] | Keyword.t()) ::
          {:ok, result} | {:error, reason :: any}
        when result: var
  def transaction(conn, fun, opts \\ [])

  def transaction(%DBConnection{conn_mode: :transaction} = conn, fun, _opts) do
    %DBConnection{conn_ref: conn_ref} = conn

    try do
      result = fun.(conn)
      conclude(conn, result)
    catch
      :throw, {__MODULE__, ^conn_ref, reason} ->
        fail(conn)
        {:error, reason}

      kind, reason ->
        stack = __STACKTRACE__
        fail(conn)
        :erlang.raise(kind, reason, stack)
    else
      result ->
        {:ok, result}
    end
  end

  def transaction(%DBConnection{} = conn, fun, opts) do
    case begin(conn, &run/4, opts) do
      {:ok, _} ->
        run_transaction(conn, fun, &run/4, opts)

      {:error, %DBConnection.TransactionError{}} ->
        {:error, :rollback}

      {:error, err} ->
        raise err
    end
  end

  def transaction(pool, fun, opts) do
    case begin(pool, &checkout/4, opts) do
      {:ok, conn, _} ->
        run_transaction(conn, fun, &checkin/4, opts)

      {:error, %DBConnection.TransactionError{}} ->
        {:error, :rollback}

      {:error, err} ->
        raise err
    end
  end

  @doc """
  Rollback a database transaction and release lock on connection.

  When inside of a `transaction/3` call does a non-local return, using a
  `throw/1` to cause the transaction to enter a failed state and the
  `transaction/3` call returns `{:error, reason}`. If `transaction/3` calls are
  nested the connection is marked as failed until the outermost transaction call
  does the database rollback.

  ### Example

      {:error, :oops} = DBConnection.transaction(pool, fun(conn) ->
        DBConnection.rollback(conn, :oops)
      end)
  """
  @spec rollback(t, reason :: any) :: no_return
  def rollback(conn, reason)

  def rollback(%DBConnection{conn_mode: :transaction} = conn, reason) do
    %DBConnection{conn_ref: conn_ref} = conn
    throw({__MODULE__, conn_ref, reason})
  end

  def rollback(%DBConnection{} = _conn, _reason) do
    raise "not inside transaction"
  end

  @doc """
  Return the transaction status of a connection.

  The callback implementation should return the transaction status according to
  the database, and not make assumptions based on client-side state.

  This function will raise a `DBConnection.ConnectionError` when called inside a
  deprecated `transaction/3`.

  ### Options

  See module documentation. The pool and connection module may support other
  options. All options are passed to `c:handle_status/2`.

  ### Example

      # outside of the transaction, the status is `:idle`
      DBConnection.status(conn) #=> :idle

      DBConnection.transaction(conn, fn conn ->
        DBConnection.status(conn) #=> :transaction

        # run a query that will cause the transaction to rollback, e.g.
        # uniqueness constraint violation
        DBConnection.execute(conn, bad_query, [])

        DBConnection.status(conn) #=> :error
      end)

      DBConnection.status(conn) #=> :idle
  """
  @spec status(conn, opts :: Keyword.t()) :: status
  def status(conn, opts \\ []) do
    case run(conn, &run_status/3, nil, opts) do
      {status, _meter} ->
        status

      {:error, _err, _meter} ->
        :error

      {kind, reason, stack, _meter} ->
        :erlang.raise(kind, reason, stack)
    end
  end

  @doc """
  Create a stream that will prepare a query, execute it and stream results
  using a cursor.

  ### Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)
    * `:log` - A function to log information about a call, either
    a 1-arity fun, `{module, function, args}` with `t:DBConnection.LogEntry.t/0`
    prepended to `args` or `nil`. See `DBConnection.LogEntry` (default: `nil`)

  The pool and connection module may support other options. All options
  are passed to `c:handle_prepare/3`, `c:handle_close/3`, `c:handle_declare/4`,
  and `c:handle_deallocate/4`.

  ### Example

      {:ok, results} = DBConnection.transaction(conn, fn conn ->
        query = %Query{statement: "SELECT id FROM table"}
        stream = DBConnection.prepare_stream(conn, query, [])
        Enum.to_list(stream)
      end)
  """
  @spec prepare_stream(t, query, params, [connection_option()] | Keyword.t()) ::
          DBConnection.PrepareStream.t()
  def prepare_stream(%DBConnection{} = conn, query, params, opts \\ []) do
    %DBConnection.PrepareStream{conn: conn, query: query, params: params, opts: opts}
  end

  @doc """
  Create a stream that will execute a prepared query and stream results using a
  cursor.

  ### Options

    * `:queue` - Whether to block waiting in an internal queue for the
    connection's state (boolean, default: `true`). See "Queue config" in
    `start_link/2` docs
    * `:timeout` - The maximum time that the caller is allowed to perform
    this operation (default: `15_000`)
    * `:deadline` - If set, overrides `:timeout` option and specifies absolute
    monotonic time in milliseconds by which caller must perform operation.
    See `System` module documentation for more information on monotonic time
    (default: `nil`)
    * `:log` - A function to log information about a call, either
    a 1-arity fun, `{module, function, args}` with `t:DBConnection.LogEntry.t/0`
    prepended to `args` or `nil`. See `DBConnection.LogEntry` (default: `nil`)

  The pool and connection module may support other options. All options
  are passed to `c:handle_declare/4` and `c:handle_deallocate/4`.

  ### Example

      DBConnection.transaction(pool, fn conn ->
        query = %Query{statement: "SELECT id FROM table"}
        query = DBConnection.prepare!(conn, query)
        try do
          stream = DBConnection.stream(conn, query, [])
          Enum.to_list(stream)
        after
          # Make sure query is closed!
          DBConnection.close(conn, query)
        end
      end)
  """
  @spec stream(t, query, params, [connection_option()] | Keyword.t()) :: DBConnection.Stream.t()
  def stream(%DBConnection{} = conn, query, params, opts \\ []) do
    %DBConnection.Stream{conn: conn, query: query, params: params, opts: opts}
  end

  @doc """
  Reduces a previously built stream or prepared stream.
  """
  def reduce(%DBConnection.PrepareStream{} = stream, acc, fun) do
    %DBConnection.PrepareStream{conn: conn, query: query, params: params, opts: opts} = stream

    declare = fn conn, opts ->
      {query, cursor} = prepare_declare!(conn, query, params, opts)
      {:cont, query, cursor}
    end

    enum = resource(conn, declare, &stream_fetch/3, &stream_deallocate/3, opts)
    enum.(acc, fun)
  end

  def reduce(%DBConnection.Stream{} = stream, acc, fun) do
    %DBConnection.Stream{conn: conn, query: query, params: params, opts: opts} = stream

    declare = fn conn, opts ->
      case declare(conn, query, params, opts) do
        {:ok, query, cursor} ->
          {:cont, query, cursor}

        {:ok, cursor} ->
          {:cont, query, cursor}

        {:error, err} ->
          raise err
      end
    end

    enum = resource(conn, declare, &stream_fetch/3, &stream_deallocate/3, opts)
    enum.(acc, fun)
  end

  @doc false
  def register_as_pool(conn_module) do
    Process.put(@connection_module_key, conn_module)
  end

  @doc """
  Returns connection module used by the given connection pool.

  When given a process that is not a connection pool, returns an `:error`.
  """
  @spec connection_module(conn) :: {:ok, module} | :error
  def connection_module(conn) do
    with pid when pid != nil <- pool_pid(conn),
         {:dictionary, dictionary} <- Process.info(pid, :dictionary),
         {@connection_module_key, module} <- List.keyfind(dictionary, @connection_module_key, 0),
         do: {:ok, module},
         else: (_ -> :error)
  end

  defp pool_pid(%DBConnection{pool_ref: Holder.pool_ref(pool: pid)}), do: pid
  defp pool_pid(conn), do: GenServer.whereis(conn)

  ## Helpers

  defp checkout(pool, meter, opts) do
    checkout = System.monotonic_time()
    pool_mod = Keyword.get(opts, :pool, DBConnection.ConnectionPool)

    caller = Keyword.get(opts, :caller, self())
    callers = [caller | Process.get(:"$callers") || []]

    try do
      pool_mod.checkout(pool, callers, opts)
    catch
      kind, reason ->
        stack = __STACKTRACE__
        {kind, reason, stack, past_event(meter, :checkout, checkout)}
    else
      {:ok, pool_ref, _conn_mod, checkin, _conn_state} ->
        conn = %DBConnection{pool_ref: pool_ref, conn_ref: make_ref()}
        meter = meter |> past_event(:checkin, checkin) |> past_event(:checkout, checkout)
        {:ok, conn, meter}

      {:error, err} ->
        {:error, err, past_event(meter, :checkout, checkout)}
    end
  end

  defp checkout(%DBConnection{} = conn, fun, meter, opts) do
    with {:ok, result, meter} <- fun.(conn, meter, opts) do
      {:ok, conn, result, meter}
    end
  end

  defp checkout(pool, fun, meter, opts) do
    with {:ok, conn, meter} <- checkout(pool, meter, opts) do
      case fun.(conn, meter, opts) do
        {:ok, result, meter} ->
          {:ok, conn, result, meter}

        error ->
          checkin(conn)
          error
      end
    end
  end

  defp checkin(%DBConnection{pool_ref: pool_ref}) do
    Holder.checkin(pool_ref)
  end

  defp checkin(%DBConnection{} = conn, fun, meter, opts) do
    return = fun.(conn, meter, opts)
    checkin(conn)
    return
  end

  defp checkin(pool, fun, meter, opts) do
    run(pool, fun, meter, opts)
  end

  defp disconnect(%DBConnection{pool_ref: pool_ref}, err) do
    _ = Holder.disconnect(pool_ref, err)
    :ok
  end

  defp stop(%DBConnection{pool_ref: pool_ref}, kind, reason, stack) do
    msg = "client #{inspect(self())} stopped: " <> Exception.format(kind, reason, stack)
    exception = DBConnection.ConnectionError.exception(msg)
    _ = Holder.stop(pool_ref, exception)
    :ok
  end

  defp handle_common_result(return, conn, meter) do
    case return do
      {:ok, result, _conn_state} ->
        {:ok, result, meter}

      {:error, err, _conn_state} ->
        {:error, err, meter}

      {:disconnect, err, _conn_state} ->
        disconnect(conn, err)
        {:error, err, meter}

      {:catch, kind, reason, stack} ->
        stop(conn, kind, reason, stack)
        {kind, reason, stack, meter}

      other ->
        bad_return!(other, conn, meter)
    end
  end

  @compile {:inline, bad_return!: 3}

  defp bad_return!(other, conn, meter) do
    try do
      raise DBConnection.ConnectionError, "bad return value: #{inspect(other)}"
    catch
      :error, reason ->
        stack = __STACKTRACE__
        stop(conn, :error, reason, stack)
        {:error, reason, stack, meter}
    end
  end

  defp parse(query, meter, opts) do
    try do
      DBConnection.Query.parse(query, opts)
    catch
      kind, reason ->
        stack = __STACKTRACE__
        {kind, reason, stack, meter}
    else
      query ->
        {:ok, query, meter}
    end
  end

  defp describe(conn, query, meter, opts) do
    try do
      DBConnection.Query.describe(query, opts)
    catch
      kind, reason ->
        stack = __STACKTRACE__
        raised_close(conn, query, meter, opts, kind, reason, stack)
    else
      query ->
        {:ok, query, meter}
    end
  end

  defp encode(conn, query, params, meter, opts) do
    try do
      DBConnection.Query.encode(query, params, opts)
    catch
      kind, reason ->
        stack = __STACKTRACE__
        raised_close(conn, query, meter, opts, kind, reason, stack)
    else
      params ->
        {:ok, params, meter}
    end
  end

  defp maybe_encode(query, params, meter, opts) do
    try do
      DBConnection.Query.encode(query, params, opts)
    rescue
      DBConnection.EncodeError -> {:prepare, meter}
    catch
      kind, reason ->
        stack = __STACKTRACE__
        {kind, reason, stack, meter}
    else
      params ->
        {:ok, params, meter}
    end
  end

  defp decode(query, result, meter, opts) do
    meter = event(meter, :decode)

    try do
      DBConnection.Query.decode(query, result, opts)
    catch
      kind, reason ->
        stack = __STACKTRACE__
        {kind, reason, stack, meter}
    else
      result ->
        {:ok, result, meter}
    end
  end

  defp prepare_declare(conn, query, params, opts) do
    result =
      with {:ok, query, meter} <- parse(query, meter(opts), opts) do
        parsed_prepare_declare(conn, query, params, meter, opts)
      end

    log(result, :prepare_declare, query, params)
  end

  defp parsed_prepare_declare(conn, query, params, meter, opts) do
    run(conn, &run_prepare_declare/5, query, params, meter, opts)
  end

  defp prepare_declare!(conn, query, params, opts) do
    case prepare_declare(conn, query, params, opts) do
      {:ok, query, cursor} ->
        {query, cursor}

      {:error, err} ->
        raise err
    end
  end

  defp declare(conn, query, params, opts) do
    result =
      case maybe_encode(query, params, meter(opts), opts) do
        {:prepare, meter} ->
          parsed_prepare_declare(conn, query, params, meter, opts)

        {:ok, params, meter} ->
          run(conn, &run_declare/5, query, params, meter, opts)

        {_, _, _, _} = error ->
          error
      end

    log(result, :declare, query, params)
  end

  defp deallocate(conn, query, cursor, opts) do
    conn
    |> run_cleanup(&run_deallocate/4, [query, cursor], meter(opts), opts)
    |> log(:deallocate, query, cursor)
  end

  defp run_prepare(conn, query, meter, opts) do
    with {:ok, query, meter} <- prepare(conn, query, meter, opts) do
      describe(conn, query, meter, opts)
    end
  end

  defp prepare(%DBConnection{pool_ref: pool_ref} = conn, query, meter, opts) do
    pool_ref
    |> Holder.handle(:handle_prepare, [query], opts)
    |> handle_common_result(conn, event(meter, :prepare))
  end

  defp run_prepare_execute(conn, query, params, meter, opts) do
    with {:ok, query, meter} <- run_prepare(conn, query, meter, opts),
         {:ok, params, meter} <- encode(conn, query, params, meter, opts) do
      run_execute(conn, query, params, meter, opts)
    end
  end

  defp run_execute(conn, query, params, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn
    meter = event(meter, :execute)

    case Holder.handle(pool_ref, :handle_execute, [query, params], opts) do
      {:ok, query, result, _conn_state} ->
        {:ok, query, result, meter}

      {:ok, _, _} = other ->
        bad_return!(other, conn, meter)

      other ->
        handle_common_result(other, conn, meter)
    end
  end

  defp raised_close(conn, query, meter, opts, kind, reason, stack) do
    with {:ok, _, meter} <- run_close(conn, [query], meter, opts) do
      {kind, reason, stack, meter}
    end
  end

  defp run_close(conn, args, meter, opts) do
    meter = event(meter, :close)
    cleanup(conn, :handle_close, args, meter, opts)
  end

  defp run_cleanup(%DBConnection{} = conn, fun, args, meter, opts) do
    fun.(conn, args, meter, opts)
  end

  defp run_cleanup(pool, fun, args, meter, opts) do
    with {:ok, conn, meter} <- checkout(pool, meter, opts) do
      try do
        fun.(conn, args, meter, opts)
      after
        checkin(conn)
      end
    end
  end

  defp cleanup(conn, fun, args, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn

    case Holder.cleanup(pool_ref, fun, args, opts) do
      {:ok, result, _conn_state} ->
        {:ok, result, meter}

      {:error, err, _conn_state} ->
        {:error, err, meter}

      {:disconnect, err, _conn_state} ->
        disconnect(conn, err)
        {:error, err, meter}

      {:catch, kind, reason, stack} ->
        stop(conn, kind, reason, stack)
        {kind, reason, stack, meter}

      other ->
        bad_return!(other, conn, meter)
    end
  end

  defp run(%DBConnection{} = conn, fun, meter, opts) do
    fun.(conn, meter, opts)
  end

  defp run(pool, fun, meter, opts) do
    with {:ok, conn, meter} <- checkout(pool, meter, opts) do
      try do
        fun.(conn, meter, opts)
      after
        checkin(conn)
      end
    end
  end

  defp run(%DBConnection{} = conn, fun, arg, meter, opts) do
    fun.(conn, arg, meter, opts)
  end

  defp run(pool, fun, arg, meter, opts) do
    with {:ok, conn, meter} <- checkout(pool, meter, opts) do
      try do
        fun.(conn, arg, meter, opts)
      after
        checkin(conn)
      end
    end
  end

  defp run(%DBConnection{} = conn, fun, arg1, arg2, meter, opts) do
    fun.(conn, arg1, arg2, meter, opts)
  end

  defp run(pool, fun, arg1, arg2, meter, opts) do
    with {:ok, conn, meter} <- checkout(pool, meter, opts) do
      try do
        fun.(conn, arg1, arg2, meter, opts)
      after
        checkin(conn)
      end
    end
  end

  defp meter(opts) do
    case Keyword.get(opts, :log) do
      nil -> nil
      log -> {log, []}
    end
  end

  defp event(nil, _),
    do: nil

  defp event({log, events}, event),
    do: {log, [{event, System.monotonic_time()} | events]}

  defp past_event(nil, _, _),
    do: nil

  defp past_event(log_events, _, nil),
    do: log_events

  defp past_event({log, events}, event, time),
    do: {log, [{event, time} | events]}

  defp log({:ok, res, meter}, call, query, params),
    do: log(meter, call, query, params, {:ok, res})

  defp log({:ok, res1, res2, meter}, call, query, params),
    do: log(meter, call, query, params, {:ok, res1, res2})

  defp log({ok, res, meter}, call, query, cursor) when ok in [:cont, :halt],
    do: log(meter, call, query, cursor, {ok, res})

  defp log({:error, err, meter}, call, query, params),
    do: log(meter, call, query, params, {:error, err})

  defp log({kind, reason, stack, meter}, call, query, params),
    do: log(meter, call, query, params, {kind, reason, stack})

  defp log(nil, _, _, _, result),
    do: log_result(result)

  defp log({log, times}, call, query, params, result) do
    entry = DBConnection.LogEntry.new(call, query, params, times, entry_result(result))

    try do
      log(log, entry)
    catch
      kind, reason ->
        stack = __STACKTRACE__
        log_raised(entry, kind, reason, stack)
    end

    log_result(result)
  end

  defp entry_result({kind, reason, stack})
       when kind in [:error, :exit, :throw] do
    msg = "an exception was raised: " <> Exception.format(kind, reason, stack)
    {:error, %DBConnection.ConnectionError{message: msg}}
  end

  defp entry_result({ok, res}) when ok in [:cont, :halt],
    do: {:ok, res}

  defp entry_result(other), do: other

  defp log({mod, fun, args}, entry), do: apply(mod, fun, [entry | args])
  defp log(fun, entry), do: fun.(entry)

  defp log_result({kind, reason, stack}) when kind in [:error, :exit, :throw] do
    :erlang.raise(kind, reason, stack)
  end

  defp log_result(other), do: other

  defp log_raised(entry, kind, reason, stack) do
    reason = Exception.normalize(kind, reason, stack)

    Logger.error(
      fn ->
        "an exception was raised logging #{inspect(entry)}: " <>
          Exception.format(kind, reason, stack)
      end,
      crash_reason: {crash_reason(kind, reason), stack}
    )
  catch
    _, _ ->
      :ok
  end

  defp crash_reason(:throw, value), do: {:nocatch, value}
  defp crash_reason(_, value), do: value

  defp run_transaction(conn, fun, run, opts) do
    %DBConnection{conn_ref: conn_ref} = conn

    try do
      result = fun.(%{conn | conn_mode: :transaction})
      conclude(conn, result)
    catch
      :throw, {__MODULE__, ^conn_ref, reason} ->
        reset(conn)

        case rollback(conn, run, opts) do
          {:ok, _} ->
            {:error, reason}

          {:error, %DBConnection.TransactionError{}} ->
            {:error, reason}

          {:error, %DBConnection.ConnectionError{}} ->
            {:error, reason}

          {:error, err} ->
            raise err
        end

      kind, reason ->
        stack = __STACKTRACE__
        reset(conn)
        _ = rollback(conn, run, opts)
        :erlang.raise(kind, reason, stack)
    else
      result ->
        case commit(conn, run, opts) do
          {:ok, _} ->
            {:ok, result}

          {:error, %DBConnection.TransactionError{}} ->
            {:error, :rollback}

          {:error, err} ->
            raise err
        end
    after
      reset(conn)
    end
  end

  defp fail(%DBConnection{pool_ref: pool_ref}) do
    case Holder.status?(pool_ref, :ok) do
      true -> Holder.put_status(pool_ref, :aborted)
      false -> :ok
    end
  end

  defp conclude(%DBConnection{pool_ref: pool_ref, conn_ref: conn_ref}, result) do
    case Holder.status?(pool_ref, :ok) do
      true -> result
      false -> throw({__MODULE__, conn_ref, :rollback})
    end
  end

  defp reset(%DBConnection{pool_ref: pool_ref}) do
    case Holder.status?(pool_ref, :aborted) do
      true -> Holder.put_status(pool_ref, :ok)
      false -> :ok
    end
  end

  defp begin(conn, run, opts) do
    case run.(conn, &run_begin/3, meter(opts), opts) do
      {:ok, conn, {query, result}, meter} ->
        query = String.Chars.to_string(query)
        log({:ok, conn, result, meter}, :begin, query, nil)

      {:ok, {query, result}, meter} ->
        query = String.Chars.to_string(query)
        log({:ok, result, meter}, :begin, query, nil)

      other ->
        log(other, :begin, :begin, nil)
    end
  end

  defp run_begin(conn, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn
    meter = event(meter, :begin)

    case Holder.handle(pool_ref, :handle_begin, [], opts) do
      {status, _conn_state} when status in [:idle, :transaction, :error] ->
        status_disconnect(conn, status, meter)

      {:ok, query, result, _conn_status} ->
        {:ok, {query, result}, meter}

      other ->
        handle_common_result(other, conn, meter)
    end
  end

  defp rollback(conn, run, opts) do
    conn
    |> run.(&run_rollback/3, meter(opts), opts)
    |> log(:rollback, :rollback, nil)
  end

  defp run_rollback(conn, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn
    meter = event(meter, :rollback)

    case Holder.handle(pool_ref, :handle_rollback, [], opts) do
      {status, _conn_state} when status in [:idle, :transaction, :error] ->
        status_disconnect(conn, status, meter)

      other ->
        handle_common_result(other, conn, meter)
    end
  end

  defp commit(conn, run, opts) do
    case run.(conn, &run_commit/3, meter(opts), opts) do
      {:rollback, {:ok, result, meter}} ->
        log(meter, :commit, :rollback, nil, {:ok, result})
        err = DBConnection.TransactionError.exception(:error)
        {:error, err}

      {query, other} ->
        log(other, :commit, query, nil)

      {:error, err, meter} ->
        log(meter, :commit, :commit, nil, {:error, err})

      {kind, reason, stack, meter} ->
        log(meter, :commit, :commit, nil, {kind, reason, stack})
    end
  end

  defp run_commit(conn, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn
    meter = event(meter, :commit)

    case Holder.handle(pool_ref, :handle_commit, [], opts) do
      {:error, _conn_state} ->
        {:rollback, run_rollback(conn, meter, opts)}

      {status, _conn_state} when status in [:idle, :transaction] ->
        {:commit, status_disconnect(conn, status, meter)}

      other ->
        {:commit, handle_common_result(other, conn, meter)}
    end
  end

  defp status_disconnect(conn, status, meter) do
    err = DBConnection.TransactionError.exception(status)
    disconnect(conn, err)
    {:error, err, meter}
  end

  defp run_status(conn, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn

    case Holder.handle(pool_ref, :handle_status, [], opts) do
      {status, _conn_state} when status in [:idle, :transaction, :error] ->
        {status, meter}

      {:disconnect, err, _conn_state} ->
        disconnect(conn, err)
        {:error, err, meter}

      {:catch, kind, reason, stack} ->
        stop(conn, kind, reason, stack)
        {kind, reason, stack, meter}

      other ->
        bad_return!(other, conn, meter)
    end
  end

  defp run_prepare_declare(conn, query, params, meter, opts) do
    with {:ok, query, meter} <- prepare(conn, query, meter, opts),
         {:ok, query, meter} <- describe(conn, query, meter, opts),
         {:ok, params, meter} <- encode(conn, query, params, meter, opts),
         {:ok, query, cursor, meter} <- run_declare(conn, query, params, meter, opts) do
      {:ok, query, cursor, meter}
    end
  end

  defp run_declare(conn, query, params, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn
    meter = event(meter, :declare)

    case Holder.handle(pool_ref, :handle_declare, [query, params], opts) do
      {:ok, query, result, _conn_state} ->
        {:ok, query, result, meter}

      {:ok, _, _} = other ->
        bad_return!(other, conn, meter)

      other ->
        handle_common_result(other, conn, meter)
    end
  end

  defp stream_fetch(conn, {:cont, query, cursor}, opts) do
    conn
    |> run(&run_stream_fetch/4, [query, cursor], meter(opts), opts)
    |> log(:fetch, query, cursor)
    |> case do
      {ok, result} when ok in [:cont, :halt] ->
        {[result], {ok, query, cursor}}

      {:error, err} ->
        raise err
    end
  end

  defp stream_fetch(_, {:halt, _, _} = state, _) do
    {:halt, state}
  end

  defp run_stream_fetch(conn, args, meter, opts) do
    [query, _] = args

    with {ok, result, meter} when ok in [:cont, :halt] <- run_fetch(conn, args, meter, opts),
         {:ok, result, meter} <- decode(query, result, meter, opts) do
      {ok, result, meter}
    end
  end

  defp run_fetch(conn, args, meter, opts) do
    %DBConnection{pool_ref: pool_ref} = conn
    meter = event(meter, :fetch)

    case Holder.handle(pool_ref, :handle_fetch, args, opts) do
      {:cont, result, _conn_state} ->
        {:cont, result, meter}

      {:halt, result, _conn_state} ->
        {:halt, result, meter}

      other ->
        handle_common_result(other, conn, meter)
    end
  end

  defp stream_deallocate(conn, {_status, query, cursor}, opts),
    do: deallocate(conn, query, cursor, opts)

  defp run_deallocate(conn, args, meter, opts) do
    meter = event(meter, :deallocate)
    cleanup(conn, :handle_deallocate, args, meter, opts)
  end

  defp resource(%DBConnection{} = conn, start, next, stop, opts) do
    start = fn -> start.(conn, opts) end
    next = fn state -> next.(conn, state, opts) end
    stop = fn state -> stop.(conn, state, opts) end
    Stream.resource(start, next, stop)
  end
end