stream: reset awaitDrain after manual .resume()

Reset the `readableState.awaitDrain` counter after manual calls to
`.resume()`.

What might happen otherwise is that a slow consumer at the end of the
pipe could end up stalling the piping in the following scenario:

1. The writable stream indicates that its buffer is full.
2. This leads the readable stream to `pause()` and increase its
   `awaitDrain` counter, which will be decreased by the writable’s next
   `drain` event.
3. Something calls `.resume()` manually.
4. The readable continues to pipe to the writable, but once again
   the writable stream indicates that the buffer is full.
5. The `awaitDrain` counter is thus increased again, but since it has
   now been increased twice for a single piping destination, the next
   `drain` event will not be able to reset `awaitDrain` to zero.
6. The pipe is stalled and no data is passed along anymore.

The solution in this commit is to reset the `awaitDrain` counter to
zero when `resume()` is called.

Fixes: #7159
PR-URL: #7160
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_readable.js

@@ -727,6 +727,7 @@ function resume_(stream, state) {
   }
 
   state.resumeScheduled = false;
+  state.awaitDrain = 0;
   stream.emit('resume');
   flow(stream);
   if (state.flowing && !state.reading)

test/parallel/test-stream-pipe-await-drain-manual-resume.js

@@ -0,0 +1,54 @@
+'use strict';
+const common = require('../common');
+const stream = require('stream');
+
+// A consumer stream with a very low highWaterMark, which starts in a state
+// where it buffers the chunk it receives rather than indicating that they
+// have been consumed.
+const writable = new stream.Writable({
+  highWaterMark: 5
+});
+
+let isCurrentlyBufferingWrites = true;
+const queue = [];
+
+writable._write = (chunk, encoding, cb) => {
+  if (isCurrentlyBufferingWrites)
+    queue.push({chunk, cb});
+  else
+    cb();
+};
+
+const readable = new stream.Readable({
+  read() {}
+});
+
+readable.pipe(writable);
+
+readable.once('pause', common.mustCall(() => {
+  // First pause, resume manually. The next write() to writable will still
+  // return false, because chunks are still being buffered, so it will increase
+  // the awaitDrain counter again.
+  process.nextTick(common.mustCall(() => {
+    readable.resume();
+  }));
+
+  readable.once('pause', common.mustCall(() => {
+    // Second pause, handle all chunks from now on. Once all callbacks that
+    // are currently queued up are handled, the awaitDrain drain counter should
+    // fall back to 0 and all chunks that are pending on the readable side
+    // should be flushed.
+    isCurrentlyBufferingWrites = false;
+    for (const queued of queue)
+      queued.cb();
+  }));
+}));
+
+readable.push(Buffer(100));  // Fill the writable HWM, first 'pause'.
+readable.push(Buffer(100));  // Second 'pause'.
+readable.push(Buffer(100));  // Should get through to the writable.
+readable.push(null);
+
+writable.on('finish', common.mustCall(() => {
+  // Everything okay, all chunks were written.
+}));