TransformStream cleanup using "Transformer.cancel"

This commit adds a "cancel" hook to "Transformer". This allows users to
perform resource cleanup when the readable side of the TransformStream
is cancelled, or the writable side is aborted.

To preserve existing behavior, when the readable side is cancelled with
a reason, the writable side is always immediately aborted with that same
reason. The same is true in the reverse case. This means that the
status of both sides is always either "closed", "erroring", or
"erroring" when the "cancel" hook is called.

"flush" and "cancel" are never both called. As per existing behaviour,
when the writable side is closed the "flush" hook is called. If the
readable side is cancelled while a promise returned from "flush" is
still pending, "cancel" is not called. In this scenario the readable
side ends up in the "errored" state, while the writable side ends up in
the "closed" state.

index.bs

@@ -5489,13 +5489,15 @@ dictionary Transformer {
   TransformerStartCallback start;
   TransformerTransformCallback transform;
   TransformerFlushCallback flush;
+  TransformerCancelCallback cancel;
   any readableType;
   any writableType;
 };
 
 callback TransformerStartCallback = any (TransformStreamDefaultController controller);
 callback TransformerFlushCallback = Promise<undefined> (TransformStreamDefaultController controller);
 callback TransformerTransformCallback = Promise<undefined> (any chunk, TransformStreamDefaultController controller);
+callback TransformerCancelCallback = Promise<undefined> (any reason, TransformStreamDefaultController controller);
 </xmp>
 
 <dl>
@@ -5558,6 +5560,25 @@ callback TransformerTransformCallback = Promise<undefined> (any chunk, Transform
    {{Transformer/flush|flush()}}; the stream is already in the process of successfully closing down,
    and terminating it would be counterproductive.)
 
+  <dt><dfn dict-member for="Transformer" lt="cancel">cancel(<var ignore>reason</var>, <var ignore>controller</var>)</dfn></dt>
+  <dd>
+   <p>A function called when the [=writable side=] is aborted, or when the [=readable side=] is
+   cancelled.
+
+   <p>Typically this is used to clean up underlying transformer resources when the  stream is
+   aborted or cancelled.
+
+   <p>If the cancellation process is asynchronous, the function can return a promise to signal
+   success or failure; the result will be communicated to the caller of
+   {{WritableStream/abort()|stream.writable.abort()}} or
+   {{ReadableStream/cancel()|stream.readable.cancel()}}. Throwing an exception is treated the same
+   as returning a rejected promise.
+
+   <p>(Note that there is no need to call
+   {{TransformStreamDefaultController/terminate()|controller.terminate()}} inside
+   {{Transformer/cancel|cancel()}}; the stream is already in the process of cancelling/aborting, and
+   terminating it would be counterproductive.)
+
  <dt><dfn dict-member for="Transformer">readableType</dfn></dt>
  <dd>
   <p>This property is reserved for future use, so any attempts to supply a value will throw an
@@ -5570,9 +5591,9 @@ callback TransformerTransformCallback = Promise<undefined> (any chunk, Transform
 </dl>
 
 The <code>controller</code> object passed to {{Transformer/start|start()}},
-{{Transformer/transform|transform()}}, and {{Transformer/flush|flush()}} is an instance of
-{{TransformStreamDefaultController}}, and has the ability to enqueue [=chunks=] to the [=readable
-side=], or to terminate or error the stream.
+{{Transformer/transform|transform()}}, {{Transformer/flush|flush()}}, and
+{{Transformer/cancel|cancel()}} is an instance of {{TransformStreamDefaultController}}, and has the
+ability to enqueue [=chunks=] to the [=readable side=], or to terminate or error the stream.
 
 <h4 id="ts-prototype">Constructor and properties</h4>
 
@@ -5726,6 +5747,10 @@ the following table:
    <th>Internal Slot</th>
    <th>Description (<em>non-normative</em>)</th>
  <tbody>
+  <tr>
+   <td><dfn>\[[cancelAlgorithm]]</dfn>
+   <td class="non-normative">A promise-returning algorithm, taking one argument (the [=reason=] for
+   cancellation), which communicates a requested cancellation to the [=transformer=]
   <tr>
    <td><dfn>\[[flushAlgorithm]]</dfn>
    <td class="non-normative">A promise-returning algorithm which communicates a requested close to
@@ -5819,8 +5844,7 @@ The following abstract operations operate on {{TransformStream}} instances at a
  1. Let |pullAlgorithm| be the following steps:
   1. Return ! [$TransformStreamDefaultSourcePullAlgorithm$](|stream|).
  1. Let |cancelAlgorithm| be the following steps, taking a |reason| argument:
-  1. Perform ! [$TransformStreamErrorWritableAndUnblockWrite$](|stream|, |reason|).
-  1. Return [=a promise resolved with=] undefined.
+  1. Return ! [$TransformStreamDefaultSourceCancelAlgorithm$](|stream|, |reason|).
  1. Set |stream|.[=TransformStream/[[readable]]=] to ! [$CreateReadableStream$](|startAlgorithm|,
     |pullAlgorithm|, |cancelAlgorithm|, |readableHighWaterMark|, |readableSizeAlgorithm|).
  1. Set |stream|.[=TransformStream/[[backpressure]]=] and
@@ -5854,6 +5878,14 @@ The following abstract operations operate on {{TransformStream}} instances at a
  1. Perform ! [$TransformStreamDefaultControllerClearAlgorithms$](|stream|.[=TransformStream/[[controller]]=]).
  1. Perform !
     [$WritableStreamDefaultControllerErrorIfNeeded$](|stream|.[=TransformStream/[[writable]]=].[=WritableStream/[[controller]]=], |e|).
+ 1. Perform ! [$TransformStreamUnblockWrite$](|stream|).
+</div>
+
+<div algorithm>
+ <dfn abstract-op lt="TransformStreamUnblockWrite"
+ id="transform-stream-unblock-write">TransformStreamUnblockWrite(|stream|)</dfn> performs the
+ following steps:
+
  1. If |stream|.[=TransformStream/[[backpressure]]=] is true, perform ! [$TransformStreamSetBackpressure$](|stream|,
     false).
 
@@ -5882,7 +5914,8 @@ The following abstract operations support the implementaiton of the
 <div algorithm>
  <dfn abstract-op lt="SetUpTransformStreamDefaultController"
  id="set-up-transform-stream-default-controller">SetUpTransformStreamDefaultController(|stream|,
- |controller|, |transformAlgorithm|, |flushAlgorithm|)</dfn> performs the following steps:
+ |controller|, |transformAlgorithm|, |flushAlgorithm|, |cancelAlgorithm|)</dfn> performs the
+ following steps:
 
  1. Assert: |stream| [=implements=] {{TransformStream}}.
  1. Assert: |stream|.[=TransformStream/[[controller]]=] is undefined.
@@ -5891,6 +5924,7 @@ The following abstract operations support the implementaiton of the
  1. Set |controller|.[=TransformStreamDefaultController/[[transformAlgorithm]]=] to
     |transformAlgorithm|.
  1. Set |controller|.[=TransformStreamDefaultController/[[flushAlgorithm]]=] to |flushAlgorithm|.
+ 1. Set |controller|.[=TransformStreamDefaultController/[[cancelAlgorithm]]=] to |cancelAlgorithm|.
 </div>
 
 <div algorithm>
@@ -5904,15 +5938,20 @@ The following abstract operations support the implementaiton of the
   1. If |result| is an abrupt completion, return [=a promise rejected with=] |result|.\[[Value]].
   1. Otherwise, return [=a promise resolved with=] undefined.
  1. Let |flushAlgorithm| be an algorithm which returns [=a promise resolved with=] undefined.
+ 1. Let |cancelAlgorithm| be an algorithm which returns [=a promise resolved with=] undefined.
  1. If |transformerDict|["{{Transformer/transform}}"] [=map/exists=], set |transformAlgorithm| to an
     algorithm which takes an argument |chunk| and returns the result of [=invoking=]
     |transformerDict|["{{Transformer/transform}}"] with argument list «&nbsp;|chunk|,
     |controller|&nbsp;» and [=callback this value=] |transformer|.
  1. If |transformerDict|["{{Transformer/flush}}"] [=map/exists=], set |flushAlgorithm| to an
     algorithm which returns the result of [=invoking=] |transformerDict|["{{Transformer/flush}}"]
     with argument list «&nbsp;|controller|&nbsp;» and [=callback this value=] |transformer|.
+ 1. If |transformerDict|["{{Transformer/cancel}}"] [=map/exists=], set |cancelAlgorithm| to an
+    algorithm which takes an argument |reason| and returns the result of [=invoking=]
+    |transformerDict|["{{Transformer/cancel}}"] with argument list «&nbsp;|reason|,
+    |controller|&nbsp;» and [=callback this value=] |transformer|.
  1. Perform ! [$SetUpTransformStreamDefaultController$](|stream|, |controller|,
-    |transformAlgorithm|, |flushAlgorithm|).
+    |transformAlgorithm|, |flushAlgorithm|, |cancelAlgorithm|).
 </div>
 
 <div algorithm>
@@ -5931,6 +5970,7 @@ The following abstract operations support the implementaiton of the
 
  1. Set |controller|.[=TransformStreamDefaultController/[[transformAlgorithm]]=] to undefined.
  1. Set |controller|.[=TransformStreamDefaultController/[[flushAlgorithm]]=] to undefined.
+ 1. Set |controller|.[=TransformStreamDefaultController/[[cancelAlgorithm]]=] to undefined.
 </div>
 
 <div algorithm>
@@ -6021,8 +6061,13 @@ side=] of [=transform streams=].
  id="transform-stream-default-sink-abort-algorithm">TransformStreamDefaultSinkAbortAlgorithm(|stream|,
  |reason|)</dfn> performs the following steps:
 
- 1. Perform ! [$TransformStreamError$](|stream|, |reason|).
- 1. Return [=a promise resolved with=] undefined.
+ 1. Let |readable| be |stream|.[=TransformStream/[[readable]]=].
+ 1. [$ReadableStreamDefaultControllerError$](|readable|.[=ReadableStream/[[controller]]=], |reason|).
+ 1. Let |controller| be |stream|.[=TransformStream/[[controller]]=].
+ 1. Let |cancelPromise| be the result of performing
+    |controller|.[=TransformStreamDefaultController/[[cancelAlgorithm]]=], passing |reason|.
+ 1. Perform ! [$TransformStreamDefaultControllerClearAlgorithms$](|controller|).
+ 1. Return |cancelPromise|.
 </div>
 
 <div algorithm>
@@ -6062,6 +6107,30 @@ side=] of [=transform streams=].
  1. Return |stream|.[=TransformStream/[[backpressureChangePromise]]=].
 </div>
 
+
+<div algorithm>
+ <dfn abstract-op lt="TransformStreamDefaultSourceCancelAlgorithm"
+ id="transform-stream-default-source-cancel">TransformStreamDefaultSourceCancelAlgorithm(|reason|,
+ |stream|)</dfn> performs the following steps:
+
+ 1. Let |writable| be |stream|.[=TransformStream/[[writable]]=].
+ 1. If |writable|.[=WritableStream/[[state]]=] is not "`writable`", return
+    [=a promise resolved with=] undefined.
+ 1. Perform !
+    [$WritableStreamDefaultControllerErrorIfNeeded$](|writable|.[=WritableStream/[[controller]]=], |reason|).
+ 1. Perform ! [$TransformStreamUnblockWrite$](|stream|).
+ 1. Let |controller| be |stream|.[=TransformStream/[[controller]]=].
+ 1. Let |cancelPromise| be the result of performing
+    |controller|.[=TransformStreamDefaultController/[[cancelAlgorithm]]=], passing |reason|.
+ 1. Perform ! [$TransformStreamDefaultControllerClearAlgorithms$](|controller|).
+ 1. Return |cancelPromise|.
+
+ <p class="note">The early return prevents the cancellation algorithm from being called if the
+ writable side is already (in the process of being) closed. This is important, because the
+ cancellation algorithm <span class=allow-2119>must</span> not run if the flush algorithm has
+ already been run.
+</div>
+
 <h2 id="qs">Queuing strategies</h2>
 
 <h3 id="qs-api">The queuing strategy API</h3>
@@ -7106,9 +7175,11 @@ reason.
 <div algorithm="create a TransformStream">
  To <dfn export for="TransformStream" lt="set up|setting up">set up</dfn> a
  newly-[=new|created-via-Web IDL=] {{TransformStream}} |stream| given an algorithm <dfn export
- for="TransformStream/set up"><var>transformAlgorithm</var></dfn> and an optional algorithm <dfn
- export for="TransformStream/set up"><var>flushAlgorithm</var></dfn>, perform the following steps.
- |transformAlgorithm| and, if given, |flushAlgorithm|, may return a promise.
+ for="TransformStream/set up"><var>transformAlgorithm</var></dfn>, an optional algorithm <dfn
+ export for="TransformStream/set up"><var>flushAlgorithm</var></dfn>, and an optional algorithm <dfn
+ export for="TransformStream/set up"><var>cancelAlgorithm</var></dfn>, perform the following steps.
+ |transformAlgorithm|, if given, |flushAlgorithm|, and, if given, |cancelAlgorithm|, may return a
+ promise.
 
  1. Let |writableHighWaterMark| be 1.
  1. Let |writableSizeAlgorithm| be an algorithm that returns 1.
@@ -7124,12 +7195,16 @@ reason.
      null otherwise. If this throws an exception |e|, return [=a promise rejected with=] |e|.
   1. If |result| is a {{Promise}}, then return |result|.
   1. Return [=a promise resolved with=] undefined.
+ 1. Let |cancelAlgorithmWrapper| be an algorithm that runs these steps given a value |reason|:
+  1. Let |result| be the result of running |cancelAlgorithm| given |reason|, if |cancelAlgorithm|
+     was given, or null otherwise. If this throws an exception |e|, return
+     [=a promise rejected with=] |e|.
  1. Let |startPromise| be [=a promise resolved with=] undefined.
  1. Perform ! [$InitializeTransformStream$](|stream|, |startPromise|, |writableHighWaterMark|,
     |writableSizeAlgorithm|, |readableHighWaterMark|, |readableSizeAlgorithm|).
  1. Let |controller| be a [=new=] {{TransformStreamDefaultController}}.
  1. Perform ! [$SetUpTransformStreamDefaultController$](|stream|, |controller|,
-    |transformAlgorithmWrapper|, |flushAlgorithmWrapper|).
+    |transformAlgorithmWrapper|, |flushAlgorithmWrapper|, |cancelAlgorithmWrapper|).
 
  Other specifications should be careful when constructing their
  <i>[=TransformStream/set up/transformAlgorithm=]</i> to avoid [=in parallel=] reads from the given

reference-implementation/lib/Transformer.webidl

@@ -2,10 +2,12 @@ dictionary Transformer {
   TransformerStartCallback start;
   TransformerTransformCallback transform;
   TransformerFlushCallback flush;
+  TransformerCancelCallback cancel;
   any readableType;
   any writableType;
 };
 
 callback TransformerStartCallback = any (TransformStreamDefaultController controller);
 callback TransformerFlushCallback = Promise<undefined> (TransformStreamDefaultController controller);
 callback TransformerTransformCallback = Promise<undefined> (any chunk, TransformStreamDefaultController controller);
+callback TransformerCancelCallback = Promise<undefined> (any reason, TransformStreamDefaultController controller);

reference-implementation/lib/abstract-ops/transform-streams.js

@@ -51,8 +51,7 @@ function InitializeTransformStream(
   }
 
   function cancelAlgorithm(reason) {
-    TransformStreamErrorWritableAndUnblockWrite(stream, reason);
-    return promiseResolvedWith(undefined);
+    return TransformStreamDefaultSourceCancelAlgorithm(stream, reason);
   }
 
   stream._readable = CreateReadableStream(
@@ -77,6 +76,10 @@ function TransformStreamError(stream, e) {
 function TransformStreamErrorWritableAndUnblockWrite(stream, e) {
   TransformStreamDefaultControllerClearAlgorithms(stream._controller);
   WritableStreamDefaultControllerErrorIfNeeded(stream._writable._controller, e);
+  TransformStreamUnblockWrite(stream);
+}
+
+function TransformStreamUnblockWrite(stream) {
   if (stream._backpressure === true) {
     // Pretend that pull() was called to permit any pending write() calls to complete. TransformStreamSetBackpressure()
     // cannot be called from enqueue() or pull() once the ReadableStream is errored, so this will will be the final time
@@ -102,7 +105,8 @@ function TransformStreamSetBackpressure(stream, backpressure) {
 
 // Default controllers
 
-function SetUpTransformStreamDefaultController(stream, controller, transformAlgorithm, flushAlgorithm) {
+function SetUpTransformStreamDefaultController(stream, controller, transformAlgorithm, flushAlgorithm,
+                                               cancelAlgorithm) {
   assert(TransformStream.isImpl(stream));
   assert(stream._controller === undefined);
 
@@ -111,6 +115,7 @@ function SetUpTransformStreamDefaultController(stream, controller, transformAlgo
 
   controller._transformAlgorithm = transformAlgorithm;
   controller._flushAlgorithm = flushAlgorithm;
+  controller._cancelAlgorithm = cancelAlgorithm;
 }
 
 function SetUpTransformStreamDefaultControllerFromTransformer(stream, transformer, transformerDict) {
@@ -125,21 +130,26 @@ function SetUpTransformStreamDefaultControllerFromTransformer(stream, transforme
     }
   };
 
-  let flushAlgorithm = () => promiseResolvedWith(undefined);
+  let flushAlgorithm = () => promiseResolvedWith(undefined); 
+  let cancelAlgorithm = () => promiseResolvedWith(undefined);
 
   if ('transform' in transformerDict) {
     transformAlgorithm = chunk => transformerDict.transform.call(transformer, chunk, controller);
   }
   if ('flush' in transformerDict) {
     flushAlgorithm = () => transformerDict.flush.call(transformer, controller);
   }
+  if ('cancel' in transformerDict) {
+    cancelAlgorithm = reason => transformerDict.cancel.call(transformer, reason, controller);
+  }
 
-  SetUpTransformStreamDefaultController(stream, controller, transformAlgorithm, flushAlgorithm);
+  SetUpTransformStreamDefaultController(stream, controller, transformAlgorithm, flushAlgorithm, cancelAlgorithm);
 }
 
 function TransformStreamDefaultControllerClearAlgorithms(controller) {
   controller._transformAlgorithm = undefined;
   controller._flushAlgorithm = undefined;
+  controller._cancelAlgorithm = undefined;
 }
 
 function TransformStreamDefaultControllerEnqueue(controller, chunk) {
@@ -221,10 +231,17 @@ function TransformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 }
 
 function TransformStreamDefaultSinkAbortAlgorithm(stream, reason) {
-  // abort() is not called synchronously, so it is possible for abort() to be called when the stream is already
-  // errored.
-  TransformStreamError(stream, reason);
-  return promiseResolvedWith(undefined);
+  verbose('TransformStreamDefaultSinkAbortAlgorithm()');
+
+  // stream._readable cannot change after construction, so caching it across a call to user code is safe.
+  const readable = stream._readable;
+  ReadableStreamDefaultControllerError(readable._controller, reason);
+
+  const controller = stream._controller;
+  const cancelPromise = controller._cancelAlgorithm(reason);
+  TransformStreamDefaultControllerClearAlgorithms(controller);
+
+  return cancelPromise;
 }
 
 function TransformStreamDefaultSinkCloseAlgorithm(stream) {
@@ -264,3 +281,21 @@ function TransformStreamDefaultSourcePullAlgorithm(stream) {
   // Prevent the next pull() call until there is backpressure.
   return stream._backpressureChangePromise;
 }
+
+function TransformStreamDefaultSourceCancelAlgorithm(stream, reason) {
+  verbose('TransformStreamDefaultSourceCancelAlgorithm()');
+
+  // stream._writable cannot change after construction, so caching it across a call to user code is safe.
+  const writable = stream._writable;
+  if (writable._state !== 'writable') {
+    return promiseResolvedWith(undefined);
+  }
+  WritableStreamDefaultControllerErrorIfNeeded(writable._controller, reason);
+  TransformStreamUnblockWrite(stream);
+
+  const controller = stream._controller;
+  const cancelPromise = controller._cancelAlgorithm(reason);
+  TransformStreamDefaultControllerClearAlgorithms(controller);
+
+  return cancelPromise;
+}