lib: ensure readable stream flows to end

If a readable stream was set up with `highWaterMark 0`, the while-loop
in `maybeReadMore_` function would never execute.

The while loop now has an extra or-condition for the case where the
stream is flowing and there are no items. The or-condition is adapted
from the emit-condition of the `addChunk` function.

The `addChunk` also contains a check for `state.sync`. However that part
of the check was omitted here because the `maybeReadMore_` is executed
using `process.nextTick`. `state.sync` is set and then unset  within the
`read()` function so it should never be in effect in `maybeReadMore_`.

Fixes: https://github.com/nodejs/node/issues/24915

PR-URL: https://github.com/nodejs/node/pull/24918
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

1 files changed, 104 insertions, 0 deletions

diff --git a/test/parallel/test-stream-readable-hwm-0-no-flow-data.js b/test/parallel/test-stream-readable-hwm-0-no-flow-data.js
new file mode 100644
index 0000000000..5f0186d720
--- /dev/null
+++ b/test/parallel/test-stream-readable-hwm-0-no-flow-data.js
@@ -0,0 +1,104 @@
+'use strict';
+
+const common = require('../common');
+
+// Ensure that subscribing the 'data' event will not make the stream flow.
+// The 'data' event will require calling read() by hand.
+//
+// The test is written for the (somewhat rare) highWaterMark: 0 streams to
+// specifically catch any regressions that might occur with these streams.
+
+const assert = require('assert');
+const { Readable } = require('stream');
+
+const streamData = [ 'a', null ];
+
+// Track the calls so we can assert their order later.
+const calls = [];
+const r = new Readable({
+  read: common.mustCall(() => {
+    calls.push('_read:' + streamData[0]);
+    process.nextTick(() => {
+      calls.push('push:' + streamData[0]);
+      r.push(streamData.shift());
+    });
+  }, streamData.length),
+  highWaterMark: 0,
+
+  // Object mode is used here just for testing convenience. It really
+  // shouldn't affect the order of events. Just the data and its format.
+  objectMode: true,
+});
+
+assert.strictEqual(r.readableFlowing, null);
+r.on('readable', common.mustCall(() => {
+  calls.push('readable');
+}, 2));
+assert.strictEqual(r.readableFlowing, false);
+r.on('data', common.mustCall((data) => {
+  calls.push('data:' + data);
+}, 1));
+r.on('end', common.mustCall(() => {
+  calls.push('end');
+}));
+assert.strictEqual(r.readableFlowing, false);
+
+// The stream emits the events asynchronously but that's not guaranteed to
+// happen on the next tick (especially since the _read implementation above
+// uses process.nextTick).
+//
+// We use setImmediate here to give the stream enough time to emit all the
+// events it's about to emit.
+setImmediate(() => {
+
+  // Only the _read, push, readable calls have happened. No data must be
+  // emitted yet.
+  assert.deepStrictEqual(calls, ['_read:a', 'push:a', 'readable']);
+
+  // Calling 'r.read()' should trigger the data event.
+  assert.strictEqual(r.read(), 'a');
+  assert.deepStrictEqual(
+    calls,
+    ['_read:a', 'push:a', 'readable', 'data:a']);
+
+  // The next 'read()' will return null because hwm: 0 does not buffer any
+  // data and the _read implementation above does the push() asynchronously.
+  //
+  // Note: This 'null' signals "no data available". It isn't the end-of-stream
+  // null value as the stream doesn't know yet that it is about to reach the
+  // end.
+  //
+  // Using setImmediate again to give the stream enough time to emit all the
+  // events it wants to emit.
+  assert.strictEqual(r.read(), null);
+  setImmediate(() => {
+
+    // There's a new 'readable' event after the data has been pushed.
+    // The 'end' event will be emitted only after a 'read()'.
+    //
+    // This is somewhat special for the case where the '_read' implementation
+    // calls 'push' asynchronously. If 'push' was synchronous, the 'end' event
+    // would be emitted here _before_ we call read().
+    assert.deepStrictEqual(
+      calls,
+      ['_read:a', 'push:a', 'readable', 'data:a',
+       '_read:null', 'push:null', 'readable']);
+
+    assert.strictEqual(r.read(), null);
+
+    // While it isn't really specified whether the 'end' event should happen
+    // synchronously with read() or not, we'll assert the current behavior
+    // ('end' event happening on the next tick after read()) so any changes
+    // to it are noted and acknowledged in the future.
+    assert.deepStrictEqual(
+      calls,
+      ['_read:a', 'push:a', 'readable', 'data:a',
+       '_read:null', 'push:null', 'readable']);
+    process.nextTick(() => {
+      assert.deepStrictEqual(
+        calls,
+        ['_read:a', 'push:a', 'readable', 'data:a',
+         '_read:null', 'push:null', 'readable', 'end']);
+    });
+  });
+});