Don't use Dispatcher for suspendReadableAndRead and TLS initialization on Node.js

io/js/src/main/scala/fs2/io/internal/facade/events.scala

@@ -34,6 +34,8 @@ import scala.scalajs.js
 @nowarn212("cat=unused")
 private[io] trait EventEmitter extends js.Object {
 
+  protected[io] def on(eventName: String, listener: js.Function0[Unit]): this.type = js.native
+
   protected[io] def on[E](eventName: String, listener: js.Function1[E, Unit]): this.type = js.native
 
   protected[io] def on[E, F](eventName: String, listener: js.Function2[E, F, Unit]): this.type =

io/js/src/main/scala/fs2/io/ioplatform.scala

@@ -31,7 +31,6 @@ import cats.effect.std.Queue
 import cats.effect.syntax.all._
 import cats.syntax.all._
 import fs2.concurrent.Channel
-import fs2.io.internal.MicrotaskExecutor
 import fs2.io.internal.facade
 
 import java.nio.charset.Charset
@@ -58,53 +57,107 @@ private[fs2] trait ioplatform {
   def suspendReadableAndRead[F[_], R <: Readable](
       destroyIfNotEnded: Boolean = true,
       destroyIfCanceled: Boolean = true
-  )(thunk: => R)(implicit F: Async[F]): Resource[F, (R, Stream[F, Byte])] =
-    (for {
-      dispatcher <- Dispatcher.sequential[F]
-      channel <- Channel.unbounded[F, Unit].toResource
-      error <- F.deferred[Throwable].toResource
-      readableResource = for {
-        readable <- Resource.makeCase(F.delay(thunk)) {
-          case (readable, Resource.ExitCase.Succeeded) =>
+  )(thunk: => R)(implicit F: Async[F]): Resource[F, (R, Stream[F, Byte])] = {
+
+    final class Listener {
+      private[this] var readable = false
+      private[this] var error: Either[Throwable, Boolean] = null
+      private[this] var ended = false
+      private[this] var callback: Either[Throwable, Boolean] => Unit = null
+
+      def handleReadable(): Unit =
+        if (callback eq null) {
+          readable = true
+        } else {
+          val cb = callback
+          callback = null
+          cb(Right(true))
+        }
+
+      def handleEnd(): Unit = {
+        ended = true
+        if (!readable && (callback ne null)) {
+          callback(Right(false))
+        }
+      }
+
+      def handleError(e: js.Error): Unit = {
+        error = Left(js.JavaScriptException(e))
+        if (callback ne null) {
+          callback(error)
+        }
+      }
+
+      private[this] def next: F[Boolean] = F.async { cb =>
+        F.delay {
+          if (error ne null) {
+            cb(error)
+            None
+          } else if (readable) {
+            readable = false
+            cb(Right(true))
+            None
+          } else if (ended) {
+            cb(Right(false))
+            None
+          } else {
+            callback = cb
+            Some(F.delay { callback = null })
+          }
+        }
+      }
+
+      def readableEvents: Stream[F, Unit] = {
+        def go: Pull[F, Unit, Unit] =
+          Pull.eval(next).flatMap { continue =>
+            if (continue)
+              Pull.outUnit >> go
+            else
+              Pull.done
+          }
+
+        go.streamNoScope
+      }
+
+    }
+
+    Resource
+      .eval(F.delay(new Listener))
+      .flatMap { listener =>
+        Resource
+          .makeCase {
             F.delay {
-              if (!readable.readableEnded & destroyIfNotEnded)
-                readable.destroy()
+              val readable = thunk
+              readable.on("readable", () => listener.handleReadable())
+              readable.once("error", listener.handleError(_))
+              readable.once("end", () => listener.handleEnd())
+              readable
             }
-          case (readable, Resource.ExitCase.Errored(_)) =>
-            // tempting, but don't propagate the error!
-            // that would trigger a unhandled Node.js error that circumvents FS2/CE error channels
-            F.delay(readable.destroy())
-          case (readable, Resource.ExitCase.Canceled) =>
-            if (destroyIfCanceled)
+          } {
+            case (readable, Resource.ExitCase.Succeeded) =>
+              F.delay {
+                if (!readable.readableEnded & destroyIfNotEnded)
+                  readable.destroy()
+              }
+            case (readable, Resource.ExitCase.Errored(_)) =>
+              // tempting, but don't propagate the error!
+              // that would trigger a unhandled Node.js error that circumvents FS2/CE error channels
               F.delay(readable.destroy())
-            else
-              F.unit
-        }
-        _ <- readable.registerListener[F, Any]("readable", dispatcher)(_ => channel.send(()).void)
-        _ <- readable.registerListener[F, Any]("end", dispatcher)(_ => channel.close.void)
-        _ <- readable.registerListener[F, Any]("close", dispatcher)(_ => channel.close.void)
-        _ <- readable.registerListener[F, js.Error]("error", dispatcher) { e =>
-          error.complete(js.JavaScriptException(e)).void
-        }
-      } yield readable
-      // Implementation note: why run on the MicrotaskExecutor?
-      // In many cases creating a `Readable` starts async side-effects (e.g. negotiating TLS handshake or opening a file handle).
-      // Furthermore, these side-effects will invoke the listeners we register to the `Readable`.
-      // Therefore, it is critical that the listeners are registered to the `Readable` _before_ these async side-effects occur:
-      // in other words, before we next yield (cede) to the event loop. Because an arbitrary effect `F` (particularly `IO`) may cede at any time,
-      // our only recourse is to run the entire creation/listener registration process on the microtask executor.
-      readable <- readableResource.evalOn(MicrotaskExecutor)
-      stream =
-        (channel.stream
-          .concurrently(Stream.eval(error.get.flatMap(F.raiseError[Unit]))) >>
-          Stream
-            .evalUnChunk(
-              F.delay(
-                Option(readable.read())
-                  .fold(Chunk.empty[Byte])(Chunk.uint8Array)
+            case (readable, Resource.ExitCase.Canceled) =>
+              if (destroyIfCanceled)
+                F.delay(readable.destroy())
+              else
+                F.unit
+          }
+          .fproduct { readable =>
+            listener.readableEvents.adaptError { case IOException(ex) => ex } >>
+              Stream.evalUnChunk(
+                F.delay(Option(readable.read()).fold(Chunk.empty[Byte])(Chunk.uint8Array(_)))
               )
-            )).adaptError { case IOException(ex) => ex }
-    } yield (readable, stream)).adaptError { case IOException(ex) => ex }
+          }
+      }
+      .adaptError { case IOException(ex) => ex }
+  }
 
   /** `Pipe` that converts a stream of bytes to a stream that will emit a single `Readable`,
     * that ends whenever the resulting stream terminates.

io/js/src/main/scala/fs2/io/net/tls/TLSContextPlatform.scala

@@ -62,10 +62,36 @@ private[tls] trait TLSContextCompanionPlatform { self: TLSContext.type =>
               clientMode: Boolean,
               params: TLSParameters,
               logger: TLSLogger[F]
-          ): Resource[F, TLSSocket[F]] = (Dispatcher.sequential[F], Dispatcher.parallel[F])
-            .flatMapN { (seqDispatcher, parDispatcher) =>
-              if (clientMode) {
-                Resource.eval(F.deferred[Either[Throwable, Unit]]).flatMap { handshake =>
+          ): Resource[F, TLSSocket[F]] = {
+
+            final class Listener {
+              private[this] var value: Either[Throwable, Unit] = null
+              private[this] var callback: Either[Throwable, Unit] => Unit = null
+
+              def complete(value: Either[Throwable, Unit]): Unit =
+                if (callback ne null) {
+                  callback(value)
+                  callback = null
+                } else {
+                  this.value = value
+                }
+
+              def get: F[Unit] = F.async { cb =>
+                F.delay {
+                  if (value ne null) {
+                    cb(value)
+                    None
+                  } else {
+                    callback = cb
+                    Some(F.delay { callback = null })
+                  }
+                }
+              }
+            }
+
+            (Dispatcher.parallel[F], Resource.eval(F.delay(new Listener)))
+              .flatMapN { (parDispatcher, listener) =>
+                if (clientMode) {
                   TLSSocket
                     .forAsync(
                       socket,
@@ -79,22 +105,17 @@ private[tls] trait TLSContextCompanionPlatform { self: TLSContext.type =>
                         val tlsSock = facade.tls.connect(options)
                         tlsSock.once(
                           "secureConnect",
-                          () => seqDispatcher.unsafeRunAndForget(handshake.complete(Either.unit))
+                          () => listener.complete(Either.unit)
                         )
                         tlsSock.once[js.Error](
                           "error",
-                          e =>
-                            seqDispatcher.unsafeRunAndForget(
-                              handshake.complete(Left(new js.JavaScriptException(e)))
-                            )
+                          e => listener.complete(Left(new js.JavaScriptException(e)))
                         )
                         tlsSock
                       }
                     )
-                    .evalTap(_ => handshake.get.rethrow)
-                }
-              } else {
-                Resource.eval(F.deferred[Either[Throwable, Unit]]).flatMap { verifyError =>
+                    .evalTap(_ => listener.get)
+                } else {
                   TLSSocket
                     .forAsync(
                       socket,
@@ -117,24 +138,21 @@ private[tls] trait TLSContextCompanionPlatform { self: TLSContext.type =>
                                   .map(e => new JavaScriptSSLException(js.JavaScriptException(e)))
                                   .toLeft(())
                               else Either.unit
-                            seqDispatcher.unsafeRunAndForget(verifyError.complete(result))
+                            listener.complete(result)
                           }
                         )
                         tlsSock.once[js.Error](
                           "error",
-                          e =>
-                            seqDispatcher.unsafeRunAndForget(
-                              verifyError.complete(Left(new js.JavaScriptException(e)))
-                            )
+                          e => listener.complete(Left(new js.JavaScriptException(e)))
                         )
                         tlsSock
                       }
                     )
-                    .evalTap(_ => verifyError.get.rethrow)
+                    .evalTap(_ => listener.get)
                 }
               }
-            }
-            .adaptError { case IOException(ex) => ex }
+              .adaptError { case IOException(ex) => ex }
+          }
         }
 
       def fromSecureContext(context: SecureContext): TLSContext[F] =