fs: add FileHandle.prototype.readableWebStream()

Adds an experimental `readableWebStream()` method to `FileHandle` that
returns a web `ReadableStream`

Signed-off-by: James M Snell <[email protected]>

Author: jasnell

doc/api/fs.md

@@ -302,6 +302,52 @@ 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()`
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* Returns: {ReadableStream}
+
+Returns a `ReadableStream` that may be used to read the files data.
+
+An error will be thrown if this method is called more than once or is called
+after the `FileHandle` is closed or closing.
+
+```mjs
+import {
+  open,
+} from 'node:fs/promises';
+
+const file = await open('./some/file/to/read');
+
+for await (const chunk of file.readableWebStream())
+  console.log(chunk);
+
+await file.close();
+```
+
+```cjs
+const {
+  open,
+} = require('fs/promises');
+
+(async () => {
+  const file = await open('./some/file/to/read');
+
+  for await (const chunk of file.readableWebStream())
+    console.log(chunk);
+
+  await file.close();
+})();
+```
+
+While the `ReadableStream` will read the file to completion, it will not
+close the `FileHandle` automatically. User code must still call the
+`fileHandle.close()` method.
+
 #### `filehandle.readFile(options)`
 <!-- YAML
 added: v10.0.0

lib/internal/fs/promises.js

@@ -30,6 +30,7 @@ const {
   codes: {
     ERR_FS_FILE_TOO_LARGE,
     ERR_INVALID_ARG_VALUE,
+    ERR_INVALID_STATE,
     ERR_METHOD_NOT_IMPLEMENTED,
   },
   AbortError,
@@ -90,12 +91,21 @@ const kCloseResolve = Symbol('kCloseResolve');
 const kCloseReject = Symbol('kCloseReject');
 const kRef = Symbol('kRef');
 const kUnref = Symbol('kUnref');
+const kLocked = Symbol('kLocked');
 
 const { kUsePromises } = binding;
 const {
   JSTransferable, kDeserialize, kTransfer, kTransferList
 } = require('internal/worker/js_transferable');
 
+const {
+  newReadableStreamFromStreamBase,
+} = require('internal/webstreams/adapters');
+
+const {
+  readableStreamCancel,
+} = require('internal/webstreams/readablestream');
+
 const getDirectoryEntriesPromise = promisify(getDirents);
 const validateRmOptionsPromise = promisify(validateRmOptions);
 
@@ -209,6 +219,33 @@ class FileHandle extends EventEmitterMixin(JSTransferable) {
     return this[kClosePromise];
   }
 
+  /**
+   * @typedef {import('../webstreams/readablestream').ReadableStream
+   * } ReadableStream
+   * @returns {ReadableStream}
+   */
+  readableWebStream() {
+    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 readable = newReadableStreamFromStreamBase(
+      this[kHandle],
+      undefined,
+      { ondone: () => this[kUnref]() });
+
+    this[kRef]();
+    this.once('close', () => {
+      readableStreamCancel(readable);
+    });
+
+    return readable;
+  }
+
   [kTransfer]() {
     if (this[kClosePromise] || this[kRefs] > 1) {
       throw lazyDOMException('Cannot transfer FileHandle while in use',

lib/internal/webstreams/adapters.js

@@ -859,12 +859,20 @@ function newWritableStreamFromStreamBase(streamBase, strategy) {
  * @param {QueuingStrategy} strategy
  * @returns {ReadableStream}
  */
-function newReadableStreamFromStreamBase(streamBase, strategy) {
+function newReadableStreamFromStreamBase(streamBase, strategy, options = {}) {
   validateObject(streamBase, 'streamBase');
+  validateObject(options, 'options');
+
+  const {
+    ondone = () => {},
+  } = options;
 
   if (typeof streamBase.onread === 'function')
     throw new ERR_INVALID_STATE('StreamBase already has a consumer');
 
+  if (typeof ondone !== 'function')
+    throw new ERR_INVALID_ARG_TYPE('options.ondone', 'Function', ondone);
+
   let controller;
 
   streamBase.onread = (arrayBuffer) => {
@@ -877,6 +885,11 @@ function newReadableStreamFromStreamBase(streamBase, strategy) {
       if (nread === UV_EOF) {
         controller.close();
         streamBase.readStop();
+        try {
+          ondone();
+        } catch (error) {
+          controller.error(error);
+        }
         return;
       }
 
@@ -899,6 +912,12 @@ function newReadableStreamFromStreamBase(streamBase, strategy) {
 
     cancel() {
       const promise = createDeferredPromise();
+      try {
+        ondone();
+      } catch (error) {
+        promise.reject(error);
+        return promise.promise;
+      }
       const req = new ShutdownWrap();
       req.oncomplete = () => promise.resolve();
       const err = streamBase.shutdown(req);

test/parallel/test-filehandle-readablestream.js

@@ -0,0 +1,87 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+const {
+  readFileSync,
+} = require('fs');
+
+const {
+  open,
+} = require('fs/promises');
+
+const check = readFileSync(__filename, { encoding: 'utf8' });
+
+// Make sure the ReadableStream works...
+(async () => {
+  const dec = new TextDecoder();
+  const file = await open(__filename);
+  let data = '';
+  for await (const chunk of file.readableWebStream())
+    data += dec.decode(chunk);
+
+  assert.strictEqual(check, data);
+
+  assert.throws(() => file.readableWebStream(), {
+    code: 'ERR_INVALID_STATE',
+  });
+
+  await file.close();
+})().then(common.mustCall());
+
+// Make sure that acquiring a ReadableStream fails if the
+// FileHandle is already closed.
+(async () => {
+  const file = await open(__filename);
+  await file.close();
+
+  assert.throws(() => file.readableWebStream(), {
+    code: 'ERR_INVALID_STATE',
+  });
+})().then(common.mustCall());
+
+// Make sure that acquiring a ReadableStream fails if the
+// FileHandle is already closing.
+(async () => {
+  const file = await open(__filename);
+  file.close();
+
+  assert.throws(() => file.readableWebStream(), {
+    code: 'ERR_INVALID_STATE',
+  });
+})().then(common.mustCall());
+
+// Make sure the ReadableStream is closed when the underlying
+// FileHandle is closed.
+(async () => {
+  const file = await open(__filename);
+  const readable = file.readableWebStream();
+  const reader = readable.getReader();
+  file.close();
+  await reader.closed;
+})().then(common.mustCall());
+
+// Make sure the ReadableStream is closed when the underlying
+// FileHandle is closed.
+(async () => {
+  const file = await open(__filename);
+  const readable = file.readableWebStream();
+  file.close();
+  const reader = readable.getReader();
+  await reader.closed;
+})().then(common.mustCall());
+
+// Make sure that the FileHandle is properly marked "in use"
+// when a ReadableStream has been acquired for it.
+(async () => {
+  const file = await open(__filename);
+  file.readableWebStream();
+  const mc = new MessageChannel();
+  mc.port1.onmessage = common.mustNotCall();
+  assert.throws(() => mc.port2.postMessage(file, [file]), {
+    code: 25  // DataCloneError
+  });
+  mc.port1.close();
+  await file.close();
+})().then(common.mustCall());