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Hotswap.scala
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Hotswap.scala
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/*
* Copyright (c) 2013 Functional Streams for Scala
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
* FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
package fs2
import cats.ApplicativeError
import cats.syntax.all._
import cats.effect.kernel.{Concurrent, Ref, Resource}
import cats.effect.kernel.implicits._
/** Supports treating a linear sequence of resources as a single resource.
*
* A `Hotswap[F, R]` instance is created as a `Resource` and hence, has
* a lifetime that is scoped by the `Resource`. After creation, a `Resource[F, R]`
* can be swapped in to the `Hotswap` by calling `swap`. The acquired resource
* is returned and is finalized when the `Hotswap` is finalized or upon the next
* call to `swap`, whichever occurs first.
*
* For example, the sequence of three resources `r1, r2, r3` are shown in the
* following diagram:
*
* {{{
* >----- swap(r1) ---- swap(r2) ---- swap(r3) ----X
* | | | | |
* Creation | | | |
* r1 acquired | | |
* r2 acquired | |
* r1 released r3 acquired |
* r2 released |
* r3 released
* }}}
*
* This class is particularly useful when working with pulls that cycle through
* resources -- e.g., writing bytes to files, rotating files every N bytes or M seconds.
* Without `Hotswap`, such pulls leak resources -- on each file rotation, a file handle
* or at least an internal resource reference accumulates. With `Hotswap`, the `Hotswap`
* instance is the only registered resource and each file is swapped in to the `Hotswap`.
*
* Usage typically looks something like:
*
* {{{
* Stream.resource(Hotswap(mkResource)).flatMap { case (hotswap, r) =>
* // Use r, call hotswap.swap(mkResource) as necessary
* }
* }}}
*
* See `fs2.io.file.writeRotate` for an example of usage.
*/
sealed trait Hotswap[F[_], R] {
/** Allocates a new resource, closes the last one if present, and
* returns the newly allocated `R`.
*
* If there are no further calls to `swap`, the resource created by
* the last call will be finalized when the lifetime of
* this `Hotswap` (which is itself tracked by `Resource`) is over.
*
* Since `swap` closes the old resource immediately, you need to
* ensure that no code is using the old `R` when `swap` is called.
* Failing to do so is likely to result in an error on the
* _consumer_ side. In any case, no resources will be leaked by
* `swap`.
*
* If you try to call swap after the lifetime of this `Hotswap` is
* over, `swap` will fail, but it will ensure all resources are
* closed, and never leak any.
*/
def swap(next: Resource[F, R]): F[R]
/** Runs the finalizer of the current resource, if any, and restores
* this `Hotswap` to its initial state.
*
* Like `swap`, you need to ensure that no code is using the old `R` when
* `clear is called`. Similarly, calling `clear` after the lifetime of this
* `Hotswap` results in an error.
*/
def clear: F[Unit]
}
object Hotswap {
/** Creates a new `Hotswap` initialized with the specified resource.
* The `Hotswap` instance and the initial resource are returned.
*/
def apply[F[_]: Concurrent, R](
initial: Resource[F, R]
): Resource[F, (Hotswap[F, R], R)] =
create[F, R].evalMap(p => p.swap(initial).map(r => (p, r)))
/** Creates a new `Hotswap`, which represents a `Resource`
* that can be swapped during the lifetime of this `Hotswap`.
*/
def create[F[_]: Concurrent, R]: Resource[F, Hotswap[F, R]] = {
def raise[A](msg: String): F[A] =
ApplicativeError[F, Throwable].raiseError(new RuntimeException(msg))
def initialize = Concurrent[F].ref(().pure[F].some)
def finalize(state: Ref[F, Option[F[Unit]]]): F[Unit] =
state
.getAndSet(None)
.flatMap {
case None => raise[Unit]("Finalizer already run")
case Some(finalizer) => finalizer
}
Resource.make(initialize)(finalize).map { state =>
new Hotswap[F, R] {
override def swap(next: Resource[F, R]): F[R] =
Concurrent[F].uncancelable { _ =>
Concurrent[F].flatMap(next.allocated) { case (newValue, finalizer) =>
swapFinalizer(finalizer).as(newValue)
}
}
override def clear: F[Unit] =
swapFinalizer(().pure[F]).uncancelable
private def swapFinalizer(newFinalizer: F[Unit]): F[Unit] =
state.modify {
case Some(oldFinalizer) =>
newFinalizer.some -> oldFinalizer
case None =>
None -> (newFinalizer *> raise[Unit]("Cannot swap after proxy has been finalized"))
}.flatten
}
}
}
}