lib: ensure readable stream flows to end

lib/_stream_readable.js

@@ -568,16 +568,38 @@ function maybeReadMore(stream, state) {
 }
 
 function maybeReadMore_(stream, state) {
-  var len = state.length;
+  // Attempt to read more data if we should.
+  //
+  // The conditions for reading more data are (one of):
+  // - Not enough data buffered (state.length < state.highWaterMark). The loop
+  //   is responsible for filling the buffer with enough data if such data
+  //   is available. If highWaterMark is 0 and we are not in the flowing mode
+  //   we should _not_ attempt to buffer any extra data. We'll get more data
+  //   when the stream consumer calls read() instead.
+  // - No data in the buffer, and the stream is in flowing mode. In this mode
+  //   the loop below is responsible for ensuring read() is called. Failing to
+  //   call read here would abort the flow and there's no other mechanism for
+  //   continuing the flow if the stream consumer has just subscribed to the
+  //   'data' event.
+  //
+  // In addition to the above conditions to keep reading data, the following
+  // conditions prevent the data from being read:
+  // - The stream has ended (state.ended).
+  // - There is already a pending 'read' operation (state.reading). This is a
+  //   case where the the stream has called the implementation defined _read()
+  //   method, but they are processing the call asynchronously and have _not_
+  //   called push() with new data. In this case we skip performing more
+  //   read()s. The execution ends in this method again after the _read() ends
+  //   up calling push() with more data.
   while (!state.reading && !state.ended &&
-         state.length < state.highWaterMark) {
+         (state.length < state.highWaterMark ||
+          (state.flowing && state.length === 0))) {
+    const len = state.length;
     debug('maybeReadMore read 0');
     stream.read(0);
     if (len === state.length)
       // didn't get any data, stop spinning.
       break;
-    else
-      len = state.length;
   }
   state.readingMore = false;
 }

test/parallel/test-stream-readable-hwm-0-async.js

@@ -0,0 +1,27 @@
+'use strict';
+
+const common = require('../common');
+
+// This test ensures that Readable stream will call _read() for streams
+// with highWaterMark === 0 upon .read(0) instead of just trying to
+// emit 'readable' event.
+
+const { Readable } = require('stream');
+
+let count = 5;
+
+const r = new Readable({
+  // Called 6 times: First 5 return data, last one signals end of stream.
+  read: common.mustCall(() => {
+    process.nextTick(common.mustCall(() => {
+      if (count--)
+        r.push('a');
+      else
+        r.push(null);
+    }));
+  }, 6),
+  highWaterMark: 0,
+});
+
+r.on('end', common.mustCall());
+r.on('data', common.mustCall(5));