fs: allow overring fs for streams

doc/api/fs.md

@@ -1668,6 +1668,10 @@ changes:
   - version: v2.3.0
     pr-url: https://github.com/nodejs/node/pull/1845
     description: The passed `options` object can be a string now.
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/REPLACEME
+    description: The `fs` options allow overriding the used `fs`
+                 implementation.
 -->
 
 * `path` {string|Buffer|URL}
@@ -1682,7 +1686,8 @@ changes:
   * `start` {integer}
   * `end` {integer} **Default:** `Infinity`
   * `highWaterMark` {integer} **Default:** `64 * 1024`
-* Returns: {fs.ReadStream}
+  * `fs` {Object|null} **Default:** `null`
+* Returns: {fs.ReadStream} See [Readable Stream][].
 
 Unlike the 16 kb default `highWaterMark` for a readable stream, the stream
 returned by this method has a default `highWaterMark` of 64 kb.
@@ -1709,6 +1714,10 @@ By default, the stream will not emit a `'close'` event after it has been
 destroyed. This is the opposite of the default for other `Readable` streams.
 Set the `emitClose` option to `true` 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,
+you must override `open`, `close` and `read`.
+
 ```js
 const fs = require('fs');
 // Create a stream from some character device.
@@ -1762,6 +1771,10 @@ changes:
   - version: v2.3.0
     pr-url: https://github.com/nodejs/node/pull/1845
     description: The passed `options` object can be a string now.
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/REPLACEME
+    description: The `fs` options allow overriding the used `fs`
+                 implementation.
 -->
 
 * `path` {string|Buffer|URL}
@@ -1774,7 +1787,8 @@ changes:
   * `autoClose` {boolean} **Default:** `true`
   * `emitClose` {boolean} **Default:** `false`
   * `start` {integer}
-* Returns: {fs.WriteStream}
+  * `fs` {Object|null} **Default:** `null`
+* Returns: {fs.WriteStream} See [Writable Stream][].
 
 `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
@@ -1793,6 +1807,12 @@ By default, the stream will not emit a `'close'` event after it has been
 destroyed. This is the opposite of the default for other `Writable` streams.
 Set the `emitClose` option to `true` 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()`
+without `writev()` can reduce performance as some optimizations (`_writev()`)
+will be disabled. When providing the `fs` option, you must override `open`,
+`close` and at least one of `write` and `writev`.
+
 Like [`ReadStream`][], if `fd` is specified, [`WriteStream`][] will ignore the
 `path` argument and will use the specified file descriptor. This means that no
 `'open'` event will be emitted. `fd` should be blocking; non-blocking `fd`s
@@ -5514,6 +5534,7 @@ the file contents.
 [`Number.MAX_SAFE_INTEGER`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/MAX_SAFE_INTEGER
 [`ReadDirectoryChangesW`]: https://docs.microsoft.com/en-us/windows/desktop/api/winbase/nf-winbase-readdirectorychangesw
 [`ReadStream`]: #fs_class_fs_readstream
+[Readable Stream]: #stream_class_stream_readable
 [`URL`]: url.html#url_the_whatwg_url_api
 [`UV_THREADPOOL_SIZE`]: cli.html#cli_uv_threadpool_size_size
 [`WriteStream`]: #fs_class_fs_writestream
@@ -5571,3 +5592,4 @@ the file contents.
 [chcp]: https://ss64.com/nt/chcp.html
 [inode]: https://en.wikipedia.org/wiki/Inode
 [support of file system `flags`]: #fs_file_system_flags
+[Writable Stream]: #stream_class_stream_writable

lib/internal/fs/streams.js

@@ -11,6 +11,7 @@ const {
 } = primordials;
 
 const {
+  ERR_INVALID_ARG_TYPE,
   ERR_OUT_OF_RANGE,
   ERR_STREAM_DESTROYED
 } = require('internal/errors').codes;
@@ -28,6 +29,7 @@ const kIoDone = Symbol('kIoDone');
 const kIsPerformingIO = Symbol('kIsPerformingIO');
 
 const kMinPoolSpace = 128;
+const kFs = Symbol('kFs');
 
 let pool;
 // It can happen that we expect to read a large chunk of data, and reserve
@@ -76,6 +78,23 @@ function ReadStream(path, options) {
     options.emitClose = false;
   }
 
+  this[kFs] = options.fs || fs;
+
+  if (typeof this[kFs].open !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.open', 'function',
+                                   this[kFs].open);
+  }
+
+  if (typeof this[kFs].read !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.read', 'function',
+                                   this[kFs].read);
+  }
+
+  if (typeof this[kFs].close !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.close', 'function',
+                                   this[kFs].close);
+  }
+
   Readable.call(this, options);
 
   // Path will be ignored when fd is specified, so it can be falsy
@@ -136,7 +155,7 @@ function _openReadFs(stream) {
     return;
   }
 
-  fs.open(stream.path, stream.flags, stream.mode, (er, fd) => {
+  stream[kFs].open(stream.path, stream.flags, stream.mode, (er, fd) => {
     if (er) {
       if (stream.autoClose) {
         stream.destroy();
@@ -186,7 +205,7 @@ ReadStream.prototype._read = function(n) {
 
   // the actual read.
   this[kIsPerformingIO] = true;
-  fs.read(this.fd, pool, pool.used, toRead, this.pos, (er, bytesRead) => {
+  this[kFs].read(this.fd, pool, pool.used, toRead, this.pos, (er, bytesRead) => {
     this[kIsPerformingIO] = false;
     // Tell ._destroy() that it's safe to close the fd now.
     if (this.destroyed) return this.emit(kIoDone, er);
@@ -245,7 +264,7 @@ ReadStream.prototype._destroy = function(err, cb) {
 };
 
 function closeFsStream(stream, cb, err) {
-  fs.close(stream.fd, (er) => {
+  stream[kFs].close(stream.fd, (er) => {
     er = er || err;
     cb(er);
     stream.closed = true;
@@ -279,6 +298,40 @@ function WriteStream(path, options) {
     options.emitClose = false;
   }
 
+  this[kFs] = options.fs || fs;
+  if (typeof this[kFs].open !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.open', 'function',
+                                   this[kFs].open);
+  }
+
+  if (!this[kFs].write && !this[kFs].writev) {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.write', 'function',
+                                   this[kFs].write);
+  }
+
+  if (this[kFs].write && typeof this[kFs].write !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.write', 'function',
+                                   this[kFs].write);
+  }
+
+  if (this[kFs].writev && typeof this[kFs].writev !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.writev', 'function',
+                                   this[kFs].writev);
+  }
+
+  if (typeof this[kFs].close !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE('options.fs.close', 'function',
+                                   this[kFs].close);
+  }
+
+  // It's enough to override either, in which case only one will be used.
+  if (!this[kFs].write) {
+    this._write = null;
+  }
+  if (!this[kFs].writev) {
+    this._writev = null;
+  }
+
   Writable.call(this, options);
 
   // Path will be ignored when fd is specified, so it can be falsy
@@ -335,7 +388,7 @@ function _openWriteFs(stream) {
     return;
   }
 
-  fs.open(stream.path, stream.flags, stream.mode, (er, fd) => {
+  stream[kFs].open(stream.path, stream.flags, stream.mode, (er, fd) => {
     if (er) {
       if (stream.autoClose) {
         stream.destroy();
@@ -361,7 +414,7 @@ WriteStream.prototype._write = function(data, encoding, cb) {
   if (this.destroyed) return cb(new ERR_STREAM_DESTROYED('write'));
 
   this[kIsPerformingIO] = true;
-  fs.write(this.fd, data, 0, data.length, this.pos, (er, bytes) => {
+  this[kFs].write(this.fd, data, 0, data.length, this.pos, (er, bytes) => {
     this[kIsPerformingIO] = false;
     // Tell ._destroy() that it's safe to close the fd now.
     if (this.destroyed) {
@@ -405,7 +458,7 @@ WriteStream.prototype._writev = function(data, cb) {
   }
 
   this[kIsPerformingIO] = true;
-  fs.writev(this.fd, chunks, this.pos, (er, bytes) => {
+  this[kFs].writev(this.fd, chunks, this.pos, (er, bytes) => {
     this[kIsPerformingIO] = false;
     // Tell ._destroy() that it's safe to close the fd now.
     if (this.destroyed) {

test/parallel/test-fs-read-stream.js

@@ -31,11 +31,11 @@ const fixtures = require('../common/fixtures');
 const fn = fixtures.path('elipses.txt');
 const rangeFile = fixtures.path('x.txt');
 
-{
+function test1(options) {
   let paused = false;
   let bytesRead = 0;
 
-  const file = fs.createReadStream(fn);
+  const file = fs.createReadStream(fn, options);
   const fileSize = fs.statSync(fn).size;
 
   assert.strictEqual(file.bytesRead, 0);
@@ -88,6 +88,15 @@ const rangeFile = fixtures.path('x.txt');
   });
 }
 
+test1({});
+test1({
+  fs: {
+    open: common.mustCall(fs.open),
+    read: common.mustCallAtLeast(fs.read, 1),
+    close: common.mustCall(fs.close),
+  }
+});
+
 {
   const file = fs.createReadStream(fn, { encoding: 'utf8' });
   file.length = 0;

test/parallel/test-fs-write-stream-fs.js

@@ -0,0 +1,38 @@
+'use strict';
+const common = require('../common');
+const path = require('path');
+const fs = require('fs');
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+{
+  const file = path.join(tmpdir.path, 'write-end-test0.txt');
+  const stream = fs.createWriteStream(file, {
+    fs: {
+      open: common.mustCall(fs.open),
+      write: common.mustCallAtLeast(fs.write, 1),
+      close: common.mustCall(fs.close),
+    }
+  });
+  stream.end('asd');
+  stream.on('close', common.mustCall());
+}
+
+
+{
+  const file = path.join(tmpdir.path, 'write-end-test1.txt');
+  const stream = fs.createWriteStream(file, {
+    fs: {
+      open: common.mustCall(fs.open),
+      write: fs.write,
+      writev: common.mustCallAtLeast(fs.writev, 1),
+      close: common.mustCall(fs.close),
+    }
+  });
+  stream.write('asd');
+  stream.write('asd');
+  stream.write('asd');
+  stream.end();
+  stream.on('close', common.mustCall());
+}