Add close() method to WritableStream

Fixes #1007.

Author: MattiasBuelens

index.bs

@@ -3171,6 +3171,7 @@ like
     get <a href="#ws-locked">locked</a>()
 
     <a href="#ws-abort">abort</a>(reason)
+    <a href="#ws-close">close</a>()
     <a href="#ws-get-writer">getWriter</a>()
   }
 </code></pre>
@@ -3385,6 +3386,25 @@ bridging the gap with non-promise-based APIs, as seen for example in [[#example-
   1. Return ! WritableStreamAbort(*this*, _reason_).
 </emu-alg>
 
+<h5 id="ws-close" method for="WritableStream">close()</h5>
+
+<div class="note">
+  The <code>close</code> method closes the stream. The <a>underlying sink</a> will finish processing any
+  previously-written <a>chunks</a>, before invoking its close behavior. During this time any further attempts to write
+  will fail (without erroring the stream).
+
+  The method returns a promise that is fulfilled with <emu-val>undefined</emu-val> if all remaining <a>chunks</a> are
+  successfully written and the stream successfully closes, or rejects if an error is encountered during this process.
+</div>
+
+<emu-alg>
+  1. If ! IsWritableStream(*this*) is *false*, return <a>a promise rejected with</a> a *TypeError* exception.
+  1. If ! IsWritableStreamLocked(*this*) is *true*, return <a>a promise rejected with</a> a *TypeError* exception.
+  1. If ! WritableStreamCloseQueuedOrInFlight(*this*) is *true*, return <a>a promise rejected with</a> a *TypeError*
+  exception.
+  1. Return ! WritableStreamClose(*this*).
+</emu-alg>
+
 <h5 id="ws-get-writer" method for="WritableStream">getWriter()</h5>
 
 <div class="note">
@@ -3490,6 +3510,22 @@ writable stream is <a>locked to a writer</a>.
   1. Return _promise_.
 </emu-alg>
 
+<h4 id="writable-stream-close" aoid="WritableStreamClose" nothrow>WritableStreamClose ( <var>stream</var> )</h4>
+
+<emu-alg>
+  1. Let _state_ be _stream_.[[state]].
+  1. If _state_ is `"closed"` or `"errored"`, return <a>a promise rejected with</a> a *TypeError* exception.
+  1. Assert: _state_ is `"writable"` or `"erroring"`.
+  1. Assert: ! WritableStreamCloseQueuedOrInFlight(_stream_) is *false*.
+  1. Let _promise_ be <a>a new promise</a>.
+  1. Set _stream_.[[closeRequest]] to _promise_.
+  1. Let _writer_ be _stream_.[[writer]].
+  1. If _writer_ is not *undefined*, and _stream_.[[backpressure]] is *true*, and _state_ is `"writable"`,
+     <a>resolve</a> _writer_.[[readyPromise]] with *undefined*.
+  1. Perform ! WritableStreamDefaultControllerClose(_stream_.[[writableStreamController]]).
+  1. Return _promise_.
+</emu-alg>
+
 <h3 id="ws-abstract-ops-used-by-controllers">Writable stream abstract operations used by controllers</h3>
 
 To allow future flexibility to add different writable stream behaviors (similar to the distinction between default
@@ -3853,12 +3889,8 @@ lt="WritableStreamDefaultWriter(stream)">new WritableStreamDefaultWriter(<var>st
 <h5 id="default-writer-close" method for="WritableStreamDefaultWriter">close()</h5>
 
 <div class="note">
-  The <code>close</code> method will close the associated writable stream. The <a>underlying sink</a> will finish
-  processing any previously-written <a>chunks</a>, before invoking its close behavior. During this time any further
-  attempts to write will fail (without erroring the stream).
-
-  The method returns a promise that is fulfilled with <emu-val>undefined</emu-val> if all remaining <a>chunks</a> are
-  successfully written and the stream successfully closes, or rejects if an error is encountered during this process.
+  If the writer is <a lt="active writer">active</a>, the <code>close</code> method behaves the same as that for the
+  associated stream. (Otherwise, it returns a rejected promise.)
 </div>
 
 <emu-alg>
@@ -3936,16 +3968,7 @@ nothrow>WritableStreamDefaultWriterClose ( <var>writer</var> )</h4>
 <emu-alg>
   1. Let _stream_ be _writer_.[[ownerWritableStream]].
   1. Assert: _stream_ is not *undefined*.
-  1. Let _state_ be _stream_.[[state]].
-  1. If _state_ is `"closed"` or `"errored"`, return <a>a promise rejected with</a> a *TypeError* exception.
-  1. Assert: _state_ is `"writable"` or `"erroring"`.
-  1. Assert: ! WritableStreamCloseQueuedOrInFlight(_stream_) is *false*.
-  1. Let _promise_ be <a>a new promise</a>.
-  1. Set _stream_.[[closeRequest]] to _promise_.
-  1. If _stream_.[[backpressure]] is *true* and _state_ is `"writable"`, <a>resolve</a> _writer_.[[readyPromise]] with
-     *undefined*.
-  1. Perform ! WritableStreamDefaultControllerClose(_stream_.[[writableStreamController]]).
-  1. Return _promise_.
+  1. Return ! WritableStreamClose(_stream_).
 </emu-alg>
 
 <h4 id="writable-stream-default-writer-close-with-error-propagation" aoid="WritableStreamDefaultWriterCloseWithErrorPropagation"

reference-implementation/lib/writable-stream.js

@@ -55,6 +55,22 @@ class WritableStream {
     return WritableStreamAbort(this, reason);
   }
 
+  close() {
+    if (IsWritableStream(this) === false) {
+      return promiseRejectedWith(streamBrandCheckException('close'));
+    }
+
+    if (IsWritableStreamLocked(this) === true) {
+      return promiseRejectedWith(new TypeError('Cannot close a stream that already has a writer'));
+    }
+
+    if (WritableStreamCloseQueuedOrInFlight(this) === true) {
+      return promiseRejectedWith(new TypeError('Cannot close an already-closing stream'));
+    }
+
+    return WritableStreamClose(this);
+  }
+
   getWriter() {
     if (IsWritableStream(this) === false) {
       throw streamBrandCheckException('getWriter');
@@ -71,6 +87,7 @@ module.exports = {
   IsWritableStreamLocked,
   WritableStream,
   WritableStreamAbort,
+  WritableStreamClose,
   WritableStreamDefaultControllerErrorIfNeeded,
   WritableStreamDefaultWriterCloseWithErrorPropagation,
   WritableStreamDefaultWriterRelease,
@@ -192,6 +209,35 @@ function WritableStreamAbort(stream, reason) {
   return promise;
 }
 
+function WritableStreamClose(stream) {
+  const state = stream._state;
+  if (state === 'closed' || state === 'errored') {
+    return promiseRejectedWith(new TypeError(
+      `The stream (in ${state} state) is not in the writable state and cannot be closed`));
+  }
+
+  assert(state === 'writable' || state === 'erroring');
+  assert(WritableStreamCloseQueuedOrInFlight(stream) === false);
+
+  const promise = newPromise((resolve, reject) => {
+    const closeRequest = {
+      _resolve: resolve,
+      _reject: reject
+    };
+
+    stream._closeRequest = closeRequest;
+  });
+
+  const writer = stream._writer;
+  if (writer !== undefined && stream._backpressure === true && state === 'writable') {
+    defaultWriterReadyPromiseResolve(writer);
+  }
+
+  WritableStreamDefaultControllerClose(stream._writableStreamController);
+
+  return promise;
+}
+
 // WritableStream API exposed for controllers.
 
 function WritableStreamAddWriteRequest(stream) {
@@ -501,7 +547,7 @@ class WritableStreamDefaultWriter {
     }
 
     if (WritableStreamCloseQueuedOrInFlight(stream) === true) {
-      return promiseRejectedWith(new TypeError('cannot close an already-closing stream'));
+      return promiseRejectedWith(new TypeError('Cannot close an already-closing stream'));
     }
 
     return WritableStreamDefaultWriterClose(this);
@@ -565,34 +611,9 @@ function WritableStreamDefaultWriterClose(writer) {
 
   assert(stream !== undefined);
 
-  const state = stream._state;
-  if (state === 'closed' || state === 'errored') {
-    return promiseRejectedWith(new TypeError(
-      `The stream (in ${state} state) is not in the writable state and cannot be closed`));
-  }
-
-  assert(state === 'writable' || state === 'erroring');
-  assert(WritableStreamCloseQueuedOrInFlight(stream) === false);
-
-  const promise = newPromise((resolve, reject) => {
-    const closeRequest = {
-      _resolve: resolve,
-      _reject: reject
-    };
-
-    stream._closeRequest = closeRequest;
-  });
-
-  if (stream._backpressure === true && state === 'writable') {
-    defaultWriterReadyPromiseResolve(writer);
-  }
-
-  WritableStreamDefaultControllerClose(stream._writableStreamController);
-
-  return promise;
+  return WritableStreamClose(stream);
 }
 
-
 function WritableStreamDefaultWriterCloseWithErrorPropagation(writer) {
   const stream = writer._ownerWritableStream;