fs: implement byob mode for readableWebStream()

Fixes: #45853
PR-URL: #46933
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/fs.md

@@ -446,14 +446,22 @@ Reads data from the file and stores that in the given buffer.
 If the file is not modified concurrently, the end-of-file is reached when the
 number of bytes read is zero.
 
-#### `filehandle.readableWebStream()`
+#### `filehandle.readableWebStream(options)`
 
 <!-- YAML
 added: v17.0.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/46933
+    description: Added option to create a 'bytes' stream.
 -->
 
 > Stability: 1 - Experimental
 
+* `options` {Object}
+  * `type` {string|undefined} Whether to open a normal or a `'bytes'` stream.
+    **Default:** `undefined`
+
 * Returns: {ReadableStream}
 
 Returns a `ReadableStream` that may be used to read the files data.

lib/internal/fs/promises.js

@@ -14,6 +14,7 @@ const {
   SafePromisePrototypeFinally,
   Symbol,
   Uint8Array,
+  FunctionPrototypeBind,
 } = primordials;
 
 const { fs: constants } = internalBinding('constants');
@@ -249,29 +250,73 @@ class FileHandle extends EventEmitterMixin(JSTransferable) {
    * } ReadableStream
    * @returns {ReadableStream}
    */
-  readableWebStream() {
+  readableWebStream(options = kEmptyObject) {
     if (this[kFd] === -1)
       throw new ERR_INVALID_STATE('The FileHandle is closed');
     if (this[kClosePromise])
       throw new ERR_INVALID_STATE('The FileHandle is closing');
     if (this[kLocked])
       throw new ERR_INVALID_STATE('The FileHandle is locked');
     this[kLocked] = true;
-    const {
-      newReadableStreamFromStreamBase,
-    } = require('internal/webstreams/adapters');
-    const readable = newReadableStreamFromStreamBase(
-      this[kHandle],
-      undefined,
-      { ondone: () => this[kUnref]() });
-
-    const {
-      readableStreamCancel,
-    } = require('internal/webstreams/readablestream');
-    this[kRef]();
-    this.once('close', () => {
-      readableStreamCancel(readable);
-    });
+
+    if (options.type !== undefined) {
+      validateString(options.type, 'options.type');
+    }
+
+    let readable;
+
+    if (options.type !== 'bytes') {
+      const {
+        newReadableStreamFromStreamBase,
+      } = require('internal/webstreams/adapters');
+      readable = newReadableStreamFromStreamBase(
+        this[kHandle],
+        undefined,
+        { ondone: () => this[kUnref]() });
+
+      const {
+        readableStreamCancel,
+      } = require('internal/webstreams/readablestream');
+      this[kRef]();
+      this.once('close', () => {
+        readableStreamCancel(readable);
+      });
+    } else {
+      const {
+        readableStreamCancel,
+        ReadableStream,
+      } = require('internal/webstreams/readablestream');
+
+      const readFn = FunctionPrototypeBind(this.read, this);
+      const ondone = FunctionPrototypeBind(this[kUnref], this);
+
+      readable = new ReadableStream({
+        type: 'bytes',
+        autoAllocateChunkSize: 16384,
+
+        async pull(controller) {
+          const view = controller.byobRequest.view;
+          const { bytesRead } = await readFn(view, view.byteOffset, view.byteLength);
+
+          if (bytesRead === 0) {
+            ondone();
+            controller.close();
+          }
+
+          controller.byobRequest.respond(bytesRead);
+        },
+
+        cancel() {
+          ondone();
+        },
+      });
+
+      this[kRef]();
+
+      this.once('close', () => {
+        readableStreamCancel(readable);
+      });
+    }
 
     return readable;
   }

test/parallel/test-filehandle-readablestream.js

@@ -86,3 +86,87 @@ const check = readFileSync(__filename, { encoding: 'utf8' });
   mc.port1.close();
   await file.close();
 })().then(common.mustCall());
+
+// Make sure 'bytes' stream works
+(async () => {
+  const file = await open(__filename);
+  const dec = new TextDecoder();
+  const readable = file.readableWebStream({ type: 'bytes' });
+  const reader = readable.getReader({ mode: 'byob' });
+
+  let data = '';
+  let result;
+  do {
+    const buff = new ArrayBuffer(100);
+    result = await reader.read(new DataView(buff));
+    if (result.value !== undefined) {
+      data += dec.decode(result.value);
+      assert.ok(result.value.byteLength <= 100);
+    }
+  } while (!result.done);
+
+  assert.strictEqual(check, data);
+
+  assert.throws(() => file.readableWebStream(), {
+    code: 'ERR_INVALID_STATE',
+  });
+
+  await file.close();
+})().then(common.mustCall());
+
+// Make sure that acquiring a ReadableStream 'bytes' stream
+// fails if the FileHandle is already closed.
+(async () => {
+  const file = await open(__filename);
+  await file.close();
+
+  assert.throws(() => file.readableWebStream({ type: 'bytes' }), {
+    code: 'ERR_INVALID_STATE',
+  });
+})().then(common.mustCall());
+
+// Make sure that acquiring a ReadableStream 'bytes' stream
+// fails if the FileHandle is already closing.
+(async () => {
+  const file = await open(__filename);
+  file.close();
+
+  assert.throws(() => file.readableWebStream({ type: 'bytes' }), {
+    code: 'ERR_INVALID_STATE',
+  });
+})().then(common.mustCall());
+
+// Make sure the 'bytes' ReadableStream is closed when the underlying
+// FileHandle is closed.
+(async () => {
+  const file = await open(__filename);
+  const readable = file.readableWebStream({ type: 'bytes' });
+  const reader = readable.getReader({ mode: 'byob' });
+  file.close();
+  await reader.closed;
+})().then(common.mustCall());
+
+// Make sure the 'bytes' ReadableStream is closed when the underlying
+// FileHandle is closed.
+(async () => {
+  const file = await open(__filename);
+  const readable = file.readableWebStream({ type: 'bytes' });
+  file.close();
+  const reader = readable.getReader({ mode: 'byob' });
+  await reader.closed;
+})().then(common.mustCall());
+
+// Make sure that the FileHandle is properly marked "in use"
+// when a 'bytes' ReadableStream has been acquired for it.
+(async () => {
+  const file = await open(__filename);
+  file.readableWebStream({ type: 'bytes' });
+  const mc = new MessageChannel();
+  mc.port1.onmessage = common.mustNotCall();
+  assert.throws(() => mc.port2.postMessage(file, [file]), {
+    code: 25,
+    name: 'DataCloneError',
+  });
+  mc.port1.close();
+  await file.close();
+})().then(common.mustCall());