stream: skip zero-byte broadcast writes

Zero-byte writes append entries to the broadcast buffer without
increasing bufferedBytes. This allows write('') and writev([]) to
bypass backpressure and grow the buffer without bound.

Treat zero-byte batches as successful no-ops, matching push streams,
and add coverage for the synchronous and asynchronous writer methods.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: codex:gpt-5.6-sol

Author: trivikr

lib/internal/streams/iter/broadcast.js

@@ -274,6 +274,12 @@ class BroadcastImpl {
   [kWrite](chunk) {
     if (this.#ended || this.#cancelled) return false;
 
+    const batchSize = this.#batchByteSize(chunk);
+
+    // Skip empty chunks -- zero-byte writes would accumulate infinitely
+    // without ever triggering backpressure under a byte-budget model.
+    if (batchSize === 0) return true;
+
     if (this.#bufferedBytes >= this.#options.budget) {
       switch (this.#options.backpressure) {
         case 'strict':
@@ -299,7 +305,6 @@ class BroadcastImpl {
       }
     }
 
-    const batchSize = this.#batchByteSize(chunk);
     this.#buffer.push(chunk);
     this.#bufferedBytes += batchSize;
     this.#notifyConsumers();

test/parallel/test-stream-iter-broadcast-backpressure.js

@@ -147,6 +147,26 @@ async function testWritevAsync() {
   assert.strictEqual(data, 'hello world');
 }
 
+// Zero-byte writes do not consume buffer entries.
+async function testZeroByteWrites() {
+  const { writer, broadcast: bc } = broadcast({ budget: 16384 });
+  const consumer = bc.push();
+
+  for (let i = 0; i < 1000; i++) {
+    assert.strictEqual(writer.writeSync(''), true);
+    assert.strictEqual(writer.writevSync([]), true);
+  }
+  await writer.write('');
+  await writer.writev([]);
+  assert.strictEqual(writer.canWrite, true);
+  writer.endSync();
+
+  let entries = 0;
+  const iterator = consumer[Symbol.asyncIterator]();
+  while (!(await iterator.next()).done) entries++;
+  assert.strictEqual(entries, 0);
+}
+
 // endSync returns the total byte count
 async function testEndSyncReturnValue() {
   const { writer, broadcast: bc } = broadcast({ budget: 16384 });
@@ -165,5 +185,6 @@ Promise.all([
   testBlockBackpressureContent(),
   testStrictBackpressureOverflow(),
   testWritevAsync(),
+  testZeroByteWrites(),
   testEndSyncReturnValue(),
 ]).then(common.mustCall());