fix: end async iteration when output streams are disabled

src/stream.ts

@@ -14,6 +14,9 @@ export const waitForEvent = (
 export const combineStreams = (streams: Readable[]): Readable => {
   let streamCount = streams.length;
   const combined = new PassThrough();
+  if (streamCount === 0) {
+    combined.end();
+  }
   const maybeEmitEnd = () => {
     if (--streamCount === 0) {
       combined.end();

src/test/main_test.ts

@@ -101,6 +101,32 @@ describe.for(variants)('exec ($name)', ({x, isAsync}) => {
 });
 
 describe('exec (async)', () => {
+  test.each(['ignore', 'inherit'] as const)(
+    'async iteration completes with stdio=%s',
+    async (stdio) => {
+      const proc = x(process.execPath, ['-e', ''], {
+        nodeOptions: {stdio}
+      });
+      const lines = [];
+      for await (const line of proc) lines.push(line);
+      expect(lines).toEqual([]);
+      expect(proc.exitCode).toBe(0);
+    },
+    2000
+  );
+
+  test('async iteration reports a nonzero exit without output streams', async () => {
+    const proc = x(process.execPath, ['-e', 'process.exit(1)'], {
+      nodeOptions: {stdio: 'ignore'},
+      throwOnError: true
+    });
+    await expect(async () => {
+      for await (const _line of proc) {
+        // no output streams
+      }
+    }).rejects.toThrow(NonZeroExitError);
+  }, 2000);
+
   test('non-zero exitCode throws when throwOnError=true', async () => {
     const proc = x('node', ['-e', 'process.exit(1);'], {throwOnError: true});
     try {

src/test/stream_test.ts

@@ -13,6 +13,14 @@ describe('waitForEvent', async () => {
 });
 
 describe('combineStreams', async () => {
+  test('ends without input streams', async () => {
+    const chunks = [];
+    for await (const chunk of combineStreams([])) {
+      chunks.push(chunk);
+    }
+    expect(chunks).toEqual([]);
+  }, 1000);
+
   test('works with a single stream', async () => {
     const stream = Readable.from(['foo', 'bar']);
     const combined = combineStreams([stream]);