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

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>

Author: addaleax

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) {

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');
+}