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
PR-URL: #64772Fixes: #64771
Reviewed-By: Aviv Keller <me@aviv.sh>

Author: trivikr

lib/internal/streams/iter/broadcast.js

@@ -274,6 +274,12 @@ class BroadcastImpl {
 [kWrite](chunk){
 if(this.#ended ||this.#cancelled)returnfalse;
 
+constbatchSize=this.#batchByteSize(chunk);
+
+// Skip empty chunks -- zero-byte writes would accumulate infinitely
+// without ever triggering backpressure under a byte-budget model.
+if(batchSize===0)returntrue;
+
 if(this.#bufferedBytes >=this.#options.budget){
 switch(this.#options.backpressure){
 case'strict':
@@ -299,7 +305,6 @@ class BroadcastImpl {
 }
 }
 
-constbatchSize=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.
+asyncfunctiontestZeroByteWrites(){
+const{ writer,broadcast: bc}=broadcast({budget: 16384});
+constconsumer=bc.push();
+
+for(leti=0;i<1000;i++){
+assert.strictEqual(writer.writeSync(''),true);
+assert.strictEqual(writer.writevSync([]),true);
+}
+awaitwriter.write('');
+awaitwriter.writev([]);
+assert.strictEqual(writer.canWrite,true);
+writer.endSync();
+
+letentries=0;
+constiterator=consumer[Symbol.asyncIterator]();
+while(!(awaititerator.next()).done)entries++;
+assert.strictEqual(entries,0);
+}
+
 // endSync returns the total byte count
 asyncfunctiontestEndSyncReturnValue(){
 const{ writer,broadcast: bc}=broadcast({budget: 16384});
@@ -165,5 +185,6 @@ Promise.all([
 testBlockBackpressureContent(),
 testStrictBackpressureOverflow(),
 testWritevAsync(),
+testZeroByteWrites(),
 testEndSyncReturnValue(),
 ]).then(common.mustCall());