lib: fix blob.stream() causing hanging promises

Refs: #47993 (comment)
PR-URL: #48232
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/blob.js

@@ -329,34 +329,45 @@ class Blob {
       pull(c) {
         const { promise, resolve, reject } = createDeferredPromise();
         this.pendingPulls.push({ resolve, reject });
-        reader.pull((status, buffer) => {
-          // If pendingPulls is empty here, the stream had to have
-          // been canceled, and we don't really care about the result.
-          // we can simply exit.
-          if (this.pendingPulls.length === 0) {
-            return;
-          }
-          const pending = this.pendingPulls.shift();
-          if (status === 0) {
-            // EOS
-            c.close();
-            pending.resolve();
-            return;
-          } else if (status < 0) {
-            // The read could fail for many different reasons when reading
-            // from a non-memory resident blob part (e.g. file-backed blob).
-            // The error details the system error code.
-            const error = lazyDOMException('The blob could not be read', 'NotReadableError');
-
-            c.error(error);
-            pending.reject(error);
-            return;
-          }
-          if (buffer !== undefined) {
-            c.enqueue(new Uint8Array(buffer));
-          }
-          pending.resolve();
-        });
+        const readNext = () => {
+          reader.pull((status, buffer) => {
+            // If pendingPulls is empty here, the stream had to have
+            // been canceled, and we don't really care about the result.
+            // We can simply exit.
+            if (this.pendingPulls.length === 0) {
+              return;
+            }
+            if (status === 0) {
+              // EOS
+              c.close();
+              const pending = this.pendingPulls.shift();
+              pending.resolve();
+              return;
+            } else if (status < 0) {
+              // The read could fail for many different reasons when reading
+              // from a non-memory resident blob part (e.g. file-backed blob).
+              // The error details the system error code.
+              const error = lazyDOMException('The blob could not be read', 'NotReadableError');
+              const pending = this.pendingPulls.shift();
+              c.error(error);
+              pending.reject(error);
+              return;
+            }
+            if (buffer !== undefined) {
+              c.enqueue(new Uint8Array(buffer));
+            }
+            // We keep reading until we either reach EOS, some error, or we
+            // hit the flow rate of the stream (c.desiredSize).
+            queueMicrotask(() => {
+              if (c.desiredSize <= 0) {
+                // A manual backpressure check.
+                return;
+              }
+              readNext();
+            });
+          });
+        };
+        readNext();
         return promise;
       },
       cancel(reason) {

test/parallel/test-blob.js

@@ -1,11 +1,12 @@
-// Flags: --no-warnings
+// Flags: --no-warnings --expose-internals
 'use strict';
 
 const common = require('../common');
 const assert = require('assert');
 const { Blob } = require('buffer');
 const { inspect } = require('util');
 const { EOL } = require('os');
+const { kState } = require('internal/webstreams/util');
 
 {
   const b = new Blob();
@@ -237,6 +238,50 @@ assert.throws(() => new Blob({}), {
   assert(res.done);
 })().then(common.mustCall());
 
+(async () => {
+  const b = new Blob(Array(10).fill('hello'));
+  const reader = b.stream().getReader();
+  const chunks = [];
+  while (true) {
+    const res = await reader.read();
+    if (res.done) break;
+    assert.strictEqual(res.value.byteLength, 5);
+    chunks.push(res.value);
+  }
+  assert.strictEqual(chunks.length, 10);
+})().then(common.mustCall());
+
+(async () => {
+  const b = new Blob(Array(10).fill('hello'));
+  const reader = b.stream().getReader();
+  const chunks = [];
+  while (true) {
+    const res = await reader.read();
+    if (chunks.length === 5) {
+      reader.cancel('boom');
+      break;
+    }
+    if (res.done) break;
+    assert.strictEqual(res.value.byteLength, 5);
+    chunks.push(res.value);
+  }
+  assert.strictEqual(chunks.length, 5);
+  reader.closed.then(common.mustCall());
+})().then(common.mustCall());
+
+(async () => {
+  const b = new Blob(Array(10).fill('hello'));
+  const stream = b.stream();
+  const reader = stream.getReader();
+  assert.strictEqual(stream[kState].controller.desiredSize, 1);
+  const { value, done } = await reader.read();
+  assert.strictEqual(value.byteLength, 5);
+  assert(!done);
+  setTimeout(() => {
+    assert.strictEqual(stream[kState].controller.desiredSize, 0);
+  }, 0);
+})().then(common.mustCall());
+
 {
   const b = new Blob(['hello\n'], { endings: 'native' });
   assert.strictEqual(b.size, EOL.length + 5);