fs: add stream utilities to FileHandle

PR-URL: #40009
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -230,6 +230,99 @@ try {
 }
 ```
 
+#### `filehandle.createReadStream([options])`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object}
+  * `encoding` {string} **Default:** `null`
+  * `autoClose` {boolean} **Default:** `true`
+  * `emitClose` {boolean} **Default:** `true`
+  * `start` {integer}
+  * `end` {integer} **Default:** `Infinity`
+  * `highWaterMark` {integer} **Default:** `64 * 1024`
+* Returns: {fs.ReadStream}
+
+Unlike the 16 kb default `highWaterMark` for a {stream.Readable}, the stream
+returned by this method has a default `highWaterMark` of 64 kb.
+
+`options` can include `start` and `end` values to read a range of bytes from
+the file instead of the entire file. Both `start` and `end` are inclusive and
+start counting at 0, allowed values are in the
+[0, [`Number.MAX_SAFE_INTEGER`][]] range. If `start` is
+omitted or `undefined`, `filehandle.createReadStream()` reads sequentially from
+the current file position. The `encoding` can be any one of those accepted by
+{Buffer}.
+
+If the `FileHandle` points to a character device that only supports blocking
+reads (such as keyboard or sound card), read operations do not finish until data
+is available. This can prevent the process from exiting and the stream from
+closing naturally.
+
+By default, the stream will emit a `'close'` event after it has been
+destroyed.  Set the `emitClose` option to `false` to change this behavior.
+
+```mjs
+import { open } from 'fs/promises';
+
+const fd = await open('/dev/input/event0');
+// Create a stream from some character device.
+const stream = fd.createReadStream();
+setTimeout(() => {
+  stream.close(); // This may not close the stream.
+  // Artificially marking end-of-stream, as if the underlying resource had
+  // indicated end-of-file by itself, allows the stream to close.
+  // This does not cancel pending read operations, and if there is such an
+  // operation, the process may still not be able to exit successfully
+  // until it finishes.
+  stream.push(null);
+  stream.read(0);
+}, 100);
+```
+
+If `autoClose` is false, then the file descriptor won't be closed, even if
+there's an error. It is the application's responsibility to close it and make
+sure there's no file descriptor leak. If `autoClose` is set to true (default
+behavior), on `'error'` or `'end'` the file descriptor will be closed
+automatically.
+
+An example to read the last 10 bytes of a file which is 100 bytes long:
+
+```mjs
+import { open } from 'fs/promises';
+
+const fd = await open('sample.txt');
+fd.createReadStream({ start: 90, end: 99 });
+```
+
+#### `filehandle.createWriteStream([options])`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object}
+  * `encoding` {string} **Default:** `'utf8'`
+  * `autoClose` {boolean} **Default:** `true`
+  * `emitClose` {boolean} **Default:** `true`
+  * `start` {integer}
+* Returns: {fs.WriteStream}
+
+`options` may also include a `start` option to allow writing data at some
+position past the beginning of the file, allowed values are in the
+[0, [`Number.MAX_SAFE_INTEGER`][]] range. Modifying a file rather than replacing
+it may require the `flags` `open` option to be set to `r+` rather than the
+default `r`. The `encoding` can be any one of those accepted by {Buffer}.
+
+If `autoClose` is set to true (default behavior) on `'error'` or `'finish'`
+the file descriptor will be closed automatically. If `autoClose` is false,
+then the file descriptor won't be closed, even if there's an error.
+It is the application's responsibility to close it and make sure there's no
+file descriptor leak.
+
+By default, the stream will emit a `'close'` event after it has been
+destroyed.  Set the `emitClose` option to `false` to change this behavior.
+
 #### `filehandle.datasync()`
 <!-- YAML
 added: v10.0.0
@@ -1985,9 +2078,9 @@ changes:
   * `end` {integer} **Default:** `Infinity`
   * `highWaterMark` {integer} **Default:** `64 * 1024`
   * `fs` {Object|null} **Default:** `null`
-* Returns: {fs.ReadStream} See [Readable Stream][].
+* Returns: {fs.ReadStream}
 
-Unlike the 16 kb default `highWaterMark` for a readable stream, the stream
+Unlike the 16 kb default `highWaterMark` for a {stream.Readable}, the stream
 returned by this method has a default `highWaterMark` of 64 kb.
 
 `options` can include `start` and `end` values to read a range of bytes from
@@ -2009,8 +2102,7 @@ available. This can prevent the process from exiting and the stream from
 closing naturally.
 
 By default, the stream will emit a `'close'` event after it has been
-destroyed, like most `Readable` streams.  Set the `emitClose` option to
-`false` to change this behavior.
+destroyed.  Set the `emitClose` option to `false` to change this behavior.
 
 By providing the `fs` option, it is possible to override the corresponding `fs`
 implementations for `open`, `read`, and `close`. When providing the `fs` option,
@@ -2106,7 +2198,7 @@ changes:
   * `emitClose` {boolean} **Default:** `true`
   * `start` {integer}
   * `fs` {Object|null} **Default:** `null`
-* Returns: {fs.WriteStream} See [Writable Stream][].
+* Returns: {fs.WriteStream}
 
 `options` may also include a `start` option to allow writing data at some
 position past the beginning of the file, allowed values are in the
@@ -2121,8 +2213,7 @@ It is the application's responsibility to close it and make sure there's no
 file descriptor leak.
 
 By default, the stream will emit a `'close'` event after it has been
-destroyed, like most `Writable` streams.  Set the `emitClose` option to
-`false` to change this behavior.
+destroyed.  Set the `emitClose` option to `false` to change this behavior.
 
 By providing the `fs` option it is possible to override the corresponding `fs`
 implementations for `open`, `write`, `writev` and `close`. Overriding `write()`
@@ -6921,8 +7012,6 @@ the file contents.
 [MSDN-Rel-Path]: https://docs.microsoft.com/en-us/windows/desktop/FileIO/naming-a-file#fully-qualified-vs-relative-paths
 [MSDN-Using-Streams]: https://docs.microsoft.com/en-us/windows/desktop/FileIO/using-streams
 [Naming Files, Paths, and Namespaces]: https://docs.microsoft.com/en-us/windows/desktop/FileIO/naming-a-file
-[Readable Stream]: stream.md#class-streamreadable
-[Writable Stream]: stream.md#class-streamwritable
 [`AHAFS`]: https://developer.ibm.com/articles/au-aix_event_infrastructure/
 [`Buffer.byteLength`]: buffer.md#static-method-bufferbytelengthstring-encoding
 [`FSEvents`]: https://developer.apple.com/documentation/coreservices/file_system_events

lib/internal/fs/promises.js

@@ -115,6 +115,12 @@ function lazyLoadCpPromises() {
   return cpPromises ??= require('internal/fs/cp/cp').cpFn;
 }
 
+// Lazy loaded to avoid circular dependency.
+let fsStreams;
+function lazyFsStreams() {
+  return fsStreams ??= require('internal/fs/streams');
+}
+
 class FileHandle extends EventEmitterMixin(JSTransferable) {
   /**
    * @param {InternalFSBinding.FileHandle | undefined} filehandle
@@ -252,6 +258,40 @@ class FileHandle extends EventEmitterMixin(JSTransferable) {
     return readable;
   }
 
+  /**
+   * @typedef {import('./streams').ReadStream
+   * } ReadStream
+   * @param {{
+   *   encoding?: string;
+   *   autoClose?: boolean;
+   *   emitClose?: boolean;
+   *   start: number;
+   *   end?: number;
+   *   highWaterMark?: number;
+   *   }} [options]
+   * @returns {ReadStream}
+   */
+  createReadStream(options = undefined) {
+    const { ReadStream } = lazyFsStreams();
+    return new ReadStream(undefined, { ...options, fd: this });
+  }
+
+  /**
+   * @typedef {import('./streams').WriteStream
+   * } WriteStream
+   * @param {{
+   *   encoding?: string;
+   *   autoClose?: boolean;
+   *   emitClose?: boolean;
+   *   start: number;
+   *   }} [options]
+   * @returns {WriteStream}
+   */
+  createWriteStream(options = undefined) {
+    const { WriteStream } = lazyFsStreams();
+    return new WriteStream(undefined, { ...options, fd: this });
+  }
+
   [kTransfer]() {
     if (this[kClosePromise] || this[kRefs] > 1) {
       throw lazyDOMException('Cannot transfer FileHandle while in use',

test/parallel/test-fs-promises-file-handle-stream.js

@@ -0,0 +1,48 @@
+'use strict';
+
+const common = require('../common');
+
+// The following tests validate base functionality for the fs.promises
+// FileHandle.write method.
+
+const fs = require('fs');
+const { open } = fs.promises;
+const path = require('path');
+const tmpdir = require('../common/tmpdir');
+const assert = require('assert');
+const { finished } = require('stream/promises');
+const { buffer } = require('stream/consumers');
+const tmpDir = tmpdir.path;
+
+tmpdir.refresh();
+
+async function validateWrite() {
+  const filePathForHandle = path.resolve(tmpDir, 'tmp-write.txt');
+  const fileHandle = await open(filePathForHandle, 'w');
+  const buffer = Buffer.from('Hello world'.repeat(100), 'utf8');
+
+  const stream = fileHandle.createWriteStream();
+  stream.end(buffer);
+  await finished(stream);
+
+  const readFileData = fs.readFileSync(filePathForHandle);
+  assert.deepStrictEqual(buffer, readFileData);
+}
+
+async function validateRead() {
+  const filePathForHandle = path.resolve(tmpDir, 'tmp-read.txt');
+  const buf = Buffer.from('Hello world'.repeat(100), 'utf8');
+
+  fs.writeFileSync(filePathForHandle, buf);
+
+  const fileHandle = await open(filePathForHandle);
+  assert.deepStrictEqual(
+    await buffer(fileHandle.createReadStream()),
+    buf
+  );
+}
+
+Promise.all([
+  validateWrite(),
+  validateRead(),
+]).then(common.mustCall());