stream: allow using .push()/.unshift() during once('data')

addaleax · addaleax · commit d44b05b18cc2 · 2020-09-22T19:56:29.000+02:00
Previously, the `.push()` or `.unshift()` call would just have jumped straight to emitting a `'data'` event, even if there were no listeners, effectively just silently dropping the chunk. PR-URL: #34957 Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com> Reviewed-By: Luigi Pinca <luigipinca@gmail.com> Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: Rich Trott <rtrott@gmail.com> Reviewed-By: Denys Otrishko <shishugi@gmail.com> Reviewed-By: Robert Nagy <ronagy@icloud.com> Reviewed-By: Ricky Zhou <0x19951125@gmail.com>
diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
@@ -284,7 +284,8 @@ function readableAddChunk(stream, chunk, encoding, addToFront) {
 }
 
 function addChunk(stream, state, chunk, addToFront) {
-  if (state.flowing && state.length === 0 && !state.sync) {
+  if (state.flowing && state.length === 0 && !state.sync &&
+      stream.listenerCount('data') > 0) {
     // Use the guard to avoid creating `Set()` repeatedly
     // when we have multiple pipes.
     if (state.multiAwaitDrain) {
diff --git a/test/parallel/test-stream-readable-add-chunk-during-data.js b/test/parallel/test-stream-readable-add-chunk-during-data.js
@@ -0,0 +1,21 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const { Readable } = require('stream');
+
+// Verify that .push() and .unshift() can be called from 'data' listeners.
+
+for (const method of ['push', 'unshift']) {
+  const r = new Readable({ read() {} });
+  r.once('data', common.mustCall((chunk) => {
+    assert.strictEqual(r.readableLength, 0);
+    r[method](chunk);
+    assert.strictEqual(r.readableLength, chunk.length);
+
+    r.on('data', common.mustCall((chunk) => {
+      assert.strictEqual(chunk.toString(), 'Hello, world');
+    }));
+  }));
+
+  r.push('Hello, world');
+}

Original file line number	Diff line number	Diff line change
`@@ -284,7 +284,8 @@ function readableAddChunk(stream, chunk, encoding, addToFront) {`
`284`	`284`	`}`
`285`	`285`
`286`	`286`	`function addChunk(stream, state, chunk, addToFront) {`
`287`		`- if (state.flowing && state.length === 0 && !state.sync) {`
	`287`	`+ if (state.flowing && state.length === 0 && !state.sync &&`
	`288`	`+ stream.listenerCount('data') > 0) {`
`288`	`289`	// Use the guard to avoid creating `Set()` repeatedly
`289`	`290`	`// when we have multiple pipes.`
`290`	`291`	`if (state.multiAwaitDrain) {`