stream: resume stream on drain

Previously we would just resume "flowing" the stream without
reseting the "paused" state. Fixes this by properly using
pause/resume methods for .pipe.

Fixes: #41785

lib/internal/streams/readable.js

@@ -853,8 +853,7 @@ function pipeOnDrain(src, dest) {
 
     if ((!state.awaitDrainWriters || state.awaitDrainWriters.size === 0) &&
       EE.listenerCount(src, 'data')) {
-      state.flowing = true;
-      flow(src);
+      src.resume();
     }
   };
 }

test/parallel/test-stream-readable-pause-and-resume.js

@@ -56,3 +56,20 @@ function readAndPause() {
     assert(readable.isPaused());
   });
 }
+
+{
+  const { PassThrough } = require('stream');
+
+  const source3 = new PassThrough();
+  const target3 = new PassThrough();
+
+  const chunk = Buffer.allocUnsafe(1000);
+  let chunks = 1;
+  while (target3.write(chunk)) chunks++;
+
+  source3.pipe(target3);
+  target3.on('drain', common.mustCall(() => {
+    assert(!source3.isPaused());
+  }));
+  target3.on('data', () => {});
+}

tmp.js

@@ -0,0 +1,24 @@
+const { PassThrough } = require('stream');
+
+// THIRD EXPERIMENT
+console.info('\n********** THIRD EXPERIMENT **********');
+const source3 = new PassThrough();
+const target3 = new PassThrough();
+
+// stall target3
+const chunk = Buffer.allocUnsafe(1000);
+let chunks = 1;
+while (target3.write(chunk)) chunks++;
+console.info(`${chunks} chunks of ${chunk.length} bytes to stall target3`);
+
+// `Readable.pipe()` PAUSES the source if the target needs drain (only in
+// version >= v14.17.0) and it does not resume it after drain
+console.info(`source3 before pipe. Paused: ${source3.isPaused()}`);
+source3.pipe(target3);
+console.info(`source3 after pipe. Paused: ${source3.isPaused()}`);
+target3.on('drain', () => {
+  console.info('target3 drained');
+  console.info(`source3 after drain. Paused: ${source3.isPaused()}`);
+  console.info(source3.readableFlowing);
+});
+target3.on('data', () => {});