stream: construct

Provide a standardized way of asynchronously creating and
initializing resources before performing any work.

Refs: #29314

PR-URL: #29656
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>

doc/api/stream.md

@@ -550,8 +550,7 @@ added: v9.3.0
 
 * {number}
 
-Return the value of `highWaterMark` passed when constructing this
-`Writable`.
+Return the value of `highWaterMark` passed when creating this `Writable`.
 
 ##### `writable.writableLength`
 <!-- YAML
@@ -1193,8 +1192,7 @@ added: v9.3.0
 
 * {number}
 
-Returns the value of `highWaterMark` passed when constructing this
-`Readable`.
+Returns the value of `highWaterMark` passed when creating this `Readable`.
 
 ##### `readable.readableLength`
 <!-- YAML
@@ -1792,7 +1790,7 @@ expectations.
 added: v1.2.0
 -->
 
-For many simple cases, it is possible to construct a stream without relying on
+For many simple cases, it is possible to create a stream without relying on
 inheritance. This can be accomplished by directly creating instances of the
 `stream.Writable`, `stream.Readable`, `stream.Duplex` or `stream.Transform`
 objects and passing appropriate methods as constructor options.
@@ -1801,8 +1799,14 @@ objects and passing appropriate methods as constructor options.
 const { Writable } = require('stream');
 
 const myWritable = new Writable({
+  construct(callback) {
+    // Initialize state and load resources...
+  },
   write(chunk, encoding, callback) {
     // ...
+  },
+  destroy() {
+    // Free resources...
   }
 });
 ```
@@ -1861,6 +1865,8 @@ changes:
     [`stream._destroy()`][writable-_destroy] method.
   * `final` {Function} Implementation for the
     [`stream._final()`][stream-_final] method.
+  * `construct` {Function} Implementation for the
+    [`stream._construct()`][writable-_construct] method.
   * `autoDestroy` {boolean} Whether this stream should automatically call
     `.destroy()` on itself after ending. **Default:** `true`.
 
@@ -1906,6 +1912,56 @@ const myWritable = new Writable({
 });
 ```
 
+#### `writable._construct(callback)`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `callback` {Function} Call this function (optionally with an error
+  argument) when the stream has finished initializing.
+
+The `_construct()` method MUST NOT be called directly. It may be implemented
+by child classes, and if so, will be called by the internal `Writable`
+class methods only.
+
+This optional function will be called in a tick after the stream constructor
+has returned, delaying any `_write`, `_final` and `_destroy` calls until
+`callback` is called. This is useful to initialize state or asynchronously
+initialize resources before the stream can be used.
+
+```js
+const { Writable } = require('stream');
+const fs = require('fs');
+
+class WriteStream extends Writable {
+  constructor(filename) {
+    super();
+    this.filename = filename;
+    this.fd = fd;
+  }
+  _construct(callback) {
+    fs.open(this.filename, (fd, err) => {
+      if (err) {
+        callback(err);
+      } else {
+        this.fd = fd;
+        callback();
+      }
+    });
+  }
+  _write(chunk, encoding, callback) {
+    fs.write(this.fd, chunk, callback);
+  }
+  _destroy(err, callback) {
+    if (this.fd) {
+      fs.close(this.fd, (er) => callback(er || err));
+    } else {
+      callback(err);
+    }
+  }
+}
+```
+
 #### `writable._write(chunk, encoding, callback)`
 <!-- YAML
 changes:
@@ -2130,6 +2186,8 @@ changes:
     method.
   * `destroy` {Function} Implementation for the
     [`stream._destroy()`][readable-_destroy] method.
+  * `construct` {Function} Implementation for the
+    [`stream._construct()`][readable-_construct] method.
   * `autoDestroy` {boolean} Whether this stream should automatically call
     `.destroy()` on itself after ending. **Default:** `true`.
 
@@ -2172,6 +2230,63 @@ const myReadable = new Readable({
 });
 ```
 
+#### `readable._construct(callback)`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `callback` {Function} Call this function (optionally with an error
+  argument) when the stream has finished initializing.
+
+The `_construct()` method MUST NOT be called directly. It may be implemented
+by child classes, and if so, will be called by the internal `Readable`
+class methods only.
+
+This optional function will be called by the stream constructor,
+delaying any `_read` and `_destroy` calls until `callback` is called. This is
+useful to initialize state or asynchronously initialize resources before the
+stream can be used.
+
+```js
+const { Readable } = require('stream');
+const fs = require('fs');
+
+class ReadStream extends Readable {
+  constructor(filename) {
+    super();
+    this.filename = filename;
+    this.fd = null;
+  }
+  _construct(callback) {
+    fs.open(this.filename, (fd, err) => {
+      if (err) {
+        callback(err);
+      } else {
+        this.fd = fd;
+        callback();
+      }
+    });
+  }
+  _read(n) {
+    const buf = Buffer.alloc(n);
+    fs.read(this.fd, buf, 0, n, null, (err, bytesRead) => {
+      if (err) {
+        this.destroy(err);
+      } else {
+        this.push(bytesRead > 0 ? buf.slice(0, bytesRead) : null);
+      }
+    });
+  }
+  _destroy(err, callback) {
+    if (this.fd) {
+      fs.close(this.fd, (er) => callback(er || err));
+    } else {
+      callback(err);
+    }
+  }
+}
+```
+
 #### `readable._read(size)`
 <!-- YAML
 added: v0.9.4
@@ -2427,6 +2542,46 @@ const myDuplex = new Duplex({
 });
 ```
 
+When using pipeline:
+
+```js
+const { Transform, pipeline } = require('stream');
+const fs = require('fs');
+
+pipeline(
+  fs.createReadStream('object.json')
+    .setEncoding('utf-8'),
+  new Transform({
+    decodeStrings: false, // Accept string input rather than Buffers
+    construct(callback) {
+      this.data = '';
+      callback();
+    },
+    transform(chunk, encoding, callback) {
+      this.data += chunk;
+      callback();
+    },
+    flush(callback) {
+      try {
+        // Make sure is valid json.
+        JSON.parse(this.data);
+        this.push(this.data);
+      } catch (err) {
+        callback(err);
+      }
+    }
+  }),
+  fs.createWriteStream('valid-object.json'),
+  (err) => {
+    if (err) {
+      console.error('failed', err);
+    } else {
+      console.log('completed');
+    }
+  }
+);
+```
+
 #### An Example Duplex Stream
 
 The following illustrates a simple example of a `Duplex` stream that wraps a
@@ -2706,8 +2861,8 @@ unhandled post-destroy errors.
 
 #### Creating Readable Streams with Async Generators
 
-We can construct a Node.js Readable Stream from an asynchronous generator
-using the `Readable.from()` utility method:
+A Node.js Readable Stream can be created from an asynchronous generator using
+the `Readable.from()` utility method:
 
 ```js
 const { Readable } = require('stream');
@@ -2960,6 +3115,7 @@ contain multi-byte characters.
 [http-incoming-message]: http.html#http_class_http_incomingmessage
 [hwm-gotcha]: #stream_highwatermark_discrepancy_after_calling_readable_setencoding
 [object-mode]: #stream_object_mode
+[readable-_construct]: #stream_readable_construct_callback
 [readable-_destroy]: #stream_readable_destroy_err_callback
 [readable-destroy]: #stream_readable_destroy_error
 [stream-_final]: #stream_writable_final_callback
@@ -2976,6 +3132,7 @@ contain multi-byte characters.
 [stream-uncork]: #stream_writable_uncork
 [stream-write]: #stream_writable_write_chunk_encoding_callback
 [Stream Three States]: #stream_three_states
+[writable-_construct]: #stream_writable_construct_callback
 [writable-_destroy]: #stream_writable_destroy_err_callback
 [writable-destroy]: #stream_writable_destroy_error
 [writable-new]: #stream_constructor_new_stream_writable_options

lib/_stream_readable.js

@@ -118,6 +118,12 @@ function ReadableState(options, stream, isDuplex) {
   this.endEmitted = false;
   this.reading = false;
 
+  // Stream is still being constructed and cannot be
+  // destroyed until construction finished or failed.
+  // Async construction is opt in, therefore we start as
+  // constructed.
+  this.constructed = true;
+
   // A flag to be able to tell if the event 'readable'/'data' is emitted
   // immediately, or on a later tick.  We set this to true at first, because
   // any actions that shouldn't happen until "later" should generally also
@@ -197,9 +203,16 @@ function Readable(options) {
 
     if (typeof options.destroy === 'function')
       this._destroy = options.destroy;
+
+    if (typeof options.construct === 'function')
+      this._construct = options.construct;
   }
 
   Stream.call(this, options);
+
+  destroyImpl.construct(this, () => {
+    maybeReadMore(this, this._readableState);
+  });
 }
 
 Readable.prototype.destroy = destroyImpl.destroy;
@@ -461,11 +474,12 @@ Readable.prototype.read = function(n) {
   }
 
   // However, if we've ended, then there's no point, if we're already
-  // reading, then it's unnecessary, and if we're destroyed or errored,
-  // then it's not allowed.
-  if (state.ended || state.reading || state.destroyed || state.errored) {
+  // reading, then it's unnecessary, if we're constructing we have to wait,
+  // and if we're destroyed or errored, then it's not allowed,
+  if (state.ended || state.reading || state.destroyed || state.errored ||
+      !state.constructed) {
     doRead = false;
-    debug('reading or ended', doRead);
+    debug('reading, ended or constructing', doRead);
   } else if (doRead) {
     debug('do read');
     state.reading = true;
@@ -587,7 +601,7 @@ function emitReadable_(stream) {
 // However, if we're not ended, or reading, and the length < hwm,
 // then go ahead and try to read some more preemptively.
 function maybeReadMore(stream, state) {
-  if (!state.readingMore) {
+  if (!state.readingMore && state.constructed) {
     state.readingMore = true;
     process.nextTick(maybeReadMore_, stream, state);
   }

lib/_stream_writable.js

@@ -155,6 +155,12 @@ function WritableState(options, stream, isDuplex) {
   // this must be 0 before 'finish' can be emitted.
   this.pendingcb = 0;
 
+  // Stream is still being constructed and cannot be
+  // destroyed until construction finished or failed.
+  // Async construction is opt in, therefore we start as
+  // constructed.
+  this.constructed = true;
+
   // Emit prefinish if the only thing we're waiting for is _write cbs
   // This is relevant for synchronous Transform streams.
   this.prefinished = false;
@@ -249,9 +255,22 @@ function Writable(options) {
 
     if (typeof options.final === 'function')
       this._final = options.final;
+
+    if (typeof options.construct === 'function')
+      this._construct = options.construct;
   }
 
   Stream.call(this, options);
+
+  destroyImpl.construct(this, () => {
+    const state = this._writableState;
+
+    if (!state.writing) {
+      clearBuffer(this, state);
+    }
+
+    finishMaybe(this, state);
+  });
 }
 
 // Otherwise people can pipe Writable streams, which is just wrong.
@@ -342,7 +361,7 @@ function writeOrBuffer(stream, state, chunk, encoding, callback) {
 
   state.length += len;
 
-  if (state.writing || state.corked || state.errored) {
+  if (state.writing || state.corked || state.errored || !state.constructed) {
     state.buffered.push({ chunk, encoding, callback });
     if (state.allBuffers && encoding !== 'buffer') {
       state.allBuffers = false;
@@ -492,7 +511,10 @@ function errorBuffer(state, err) {
 
 // If there's something in the buffer waiting, then process it.
 function clearBuffer(stream, state) {
-  if (state.corked || state.bufferProcessing || state.destroyed) {
+  if (state.corked ||
+      state.bufferProcessing ||
+      state.destroyed ||
+      !state.constructed) {
     return;
   }
 
@@ -600,6 +622,7 @@ Writable.prototype.end = function(chunk, encoding, cb) {
 
 function needFinish(state) {
   return (state.ending &&
+          state.constructed &&
           state.length === 0 &&
           !state.errored &&
           state.buffered.length === 0 &&