stream: fix merge() idle source draining

Defer pulling a merged source again until the consumer requests the
next merged item. This prevents fast sources from being drained while
the merged iterator is idle.

Fixes: #63566

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5

Author: trivikr

lib/internal/streams/iter/consumers.js

@@ -17,6 +17,7 @@ const {
   ArrayPrototypeSlice,
   Promise,
   PromisePrototypeThen,
+  PromiseResolve,
   SafePromiseAllReturnVoid,
   SymbolAsyncIterator,
   TypedArrayPrototypeGetBuffer,
@@ -412,19 +413,22 @@ function merge(...args) {
 
   return {
     __proto__: null,
-    async *[SymbolAsyncIterator]() {
+    [SymbolAsyncIterator]() {
       const signal = options?.signal;
 
       signal?.throwIfAborted();
 
-      if (normalized.length === 0) return;
+      if (normalized.length === 0) {
+        return (async function*() {})();
+      }
 
       if (normalized.length === 1) {
-        for await (const batch of normalized[0]) {
-          signal?.throwIfAborted();
-          yield batch;
-        }
-        return;
+        return (async function*() {
+          for await (const batch of normalized[0]) {
+            signal?.throwIfAborted();
+            yield batch;
+          }
+        })();
       }
 
       // Multiple sources - use a ready queue so that batches that settle
@@ -434,74 +438,63 @@ function merge(...args) {
       const ready = [];
       let activeCount = normalized.length;
       let waitResolve = null;
+      let lastIterator = null;
+      let started = false;
+      let done = false;
 
       // Called when a source's .next() settles. Pushes the result into
       // the ready queue and wakes the consumer if it's waiting.
       const onSettled = (iterator, result) => {
+        if (done) return;
         if (result.done) {
           activeCount--;
         } else {
-          ArrayPrototypePush(ready, result.value);
-          // Immediately request the next value from this source
-          // (at most one pending .next() per source)
-          PromisePrototypeThen(
-            iterator.next(),
-            (r) => onSettled(iterator, r),
-            (err) => {
-              ArrayPrototypePush(ready, { __proto__: null, error: err });
-              if (waitResolve) {
-                waitResolve();
-                waitResolve = null;
-              }
-            },
-          );
+          ArrayPrototypePush(ready, {
+            __proto__: null,
+            iterator,
+            value: result.value,
+          });
         }
         if (waitResolve) {
           waitResolve();
           waitResolve = null;
         }
       };
 
-      // Start one .next() per source
-      const iterators = [];
-      for (let i = 0; i < normalized.length; i++) {
-        const iterator = normalized[i][SymbolAsyncIterator]();
-        ArrayPrototypePush(iterators, iterator);
+      const requestNext = (iterator) => {
         PromisePrototypeThen(
           iterator.next(),
           (r) => onSettled(iterator, r),
           (err) => {
+            if (done) return;
             ArrayPrototypePush(ready, { __proto__: null, error: err });
             if (waitResolve) {
               waitResolve();
               waitResolve = null;
             }
           },
         );
-      }
+      };
 
-      try {
-        while (activeCount > 0 || ready.length > 0) {
-          signal?.throwIfAborted();
+      const iterators = [];
 
-          // Drain ready queue synchronously
-          while (ready.length > 0) {
-            const item = ArrayPrototypeShift(ready);
-            if (item?.error) {
-              throw item.error;
-            }
-            yield item;
-          }
+      const start = () => {
+        if (started) return;
+        started = true;
+        for (let i = 0; i < normalized.length; i++) {
+          const iterator = normalized[i][SymbolAsyncIterator]();
+          ArrayPrototypePush(iterators, iterator);
+          requestNext(iterator);
+        }
+      };
 
-          // If sources are still active, wait for the next settlement
-          if (activeCount > 0) {
-            await new Promise((resolve) => {
-              waitResolve = resolve;
-            });
-          }
+      const cleanup = async () => {
+        if (done) return;
+        done = true;
+        if (waitResolve) {
+          waitResolve();
+          waitResolve = null;
         }
-      } finally {
-        // Clean up: return all iterators
         await SafePromiseAllReturnVoid(iterators, async (iterator) => {
           if (iterator.return) {
             try {
@@ -511,7 +504,79 @@ function merge(...args) {
             }
           }
         });
-      }
+      };
+
+      const nextImpl = async () => {
+        try {
+          if (done) {
+            return { __proto__: null, done: true, value: undefined };
+          }
+
+          signal?.throwIfAborted();
+
+          start();
+
+          if (lastIterator !== null) {
+            requestNext(lastIterator);
+            lastIterator = null;
+          }
+
+          while (!done) {
+            signal?.throwIfAborted();
+
+            while (ready.length > 0) {
+              const item = ArrayPrototypeShift(ready);
+              if (item?.error) {
+                await cleanup();
+                throw item.error;
+              }
+              lastIterator = item.iterator;
+              return { __proto__: null, done: false, value: item.value };
+            }
+
+            if (activeCount === 0) {
+              await cleanup();
+              return { __proto__: null, done: true, value: undefined };
+            }
+
+            await new Promise((resolve) => {
+              waitResolve = resolve;
+            });
+          }
+
+          return { __proto__: null, done: true, value: undefined };
+        } catch (err) {
+          await cleanup();
+          throw err;
+        }
+      };
+
+      const returnImpl = async () => {
+        await cleanup();
+        return { __proto__: null, done: true, value: undefined };
+      };
+
+      let nextQueue = PromiseResolve();
+      const enqueue = (fn) => {
+        const result = PromisePrototypeThen(nextQueue, fn, fn);
+        nextQueue = PromisePrototypeThen(result, () => {}, () => {});
+        return result;
+      };
+
+      return {
+        __proto__: null,
+        [SymbolAsyncIterator]() {
+          return this;
+        },
+
+        next() {
+          return enqueue(nextImpl);
+        },
+
+        return() {
+          return enqueue(returnImpl);
+        },
+      };
     },
   };
 }

test/parallel/test-stream-iter-consumers-merge.js

@@ -131,6 +131,31 @@ async function testMergeConsumerBreak() {
   assert.strictEqual(source1Return && source2Return, true);
 }
 
+async function testMergeDoesNotDrainIdleSources() {
+  function source(n) {
+    return {
+      pulls: 0,
+      async *[Symbol.asyncIterator]() {
+        while (this.pulls < n) {
+          yield [Buffer.from(`${++this.pulls}`)];
+        }
+      },
+    };
+  }
+
+  const source1 = source(5);
+  const source2 = source(5);
+  const iterator = merge(source1, source2)[Symbol.asyncIterator]();
+
+  await iterator.next();
+  await new Promise((resolve) => setTimeout(resolve, 20));
+
+  assert.ok(source1.pulls <= 1);
+  assert.ok(source2.pulls <= 1);
+
+  await iterator.return?.();
+}
+
 async function testMergeSignalMidIteration() {
   const ac = new AbortController();
   async function* slowSource() {
@@ -190,6 +215,7 @@ Promise.all([
   testMergeSyncSources(),
   testMergeSourceError(),
   testMergeConsumerBreak(),
+  testMergeDoesNotDrainIdleSources(),
   testMergeSignalMidIteration(),
   testMergeStringSources(),
   testMergeObjectLikeSources(),