stream: add final method

Adds the ability to for write streams to have an _final method which acts
similarly to the _flush method that transform streams have but is called before
the finish event is emitted and if asynchronous delays the stream from
finishing.  The `final` option may also be passed in order to set it.

PR-URL: #12828
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

doc/api/stream.md

@@ -1198,7 +1198,8 @@ on the type of stream being created, as detailed in the chart below:
       <p>[Writable](#stream_class_stream_writable)</p>
     </td>
     <td>
-      <p><code>[_write][stream-_write]</code>, <code>[_writev][stream-_writev]</code></p>
+      <p><code>[_write][stream-_write]</code>, <code>[_writev][stream-_writev]</code>,
+      <code>[_final][stream-_final]</code></p>
     </td>
   </tr>
   <tr>
@@ -1209,7 +1210,8 @@ on the type of stream being created, as detailed in the chart below:
       <p>[Duplex](#stream_class_stream_duplex)</p>
     </td>
     <td>
-      <p><code>[_read][stream-_read]</code>, <code>[_write][stream-_write]</code>, <code>[_writev][stream-_writev]</code></p>
+      <p><code>[_read][stream-_read]</code>, <code>[_write][stream-_write]</code>, <code>[_writev][stream-_writev]</code>,
+      <code>[_final][stream-_final]</code></p>
     </td>
   </tr>
   <tr>
@@ -1220,7 +1222,8 @@ on the type of stream being created, as detailed in the chart below:
       <p>[Transform](#stream_class_stream_transform)</p>
     </td>
     <td>
-      <p><code>[_transform][stream-_transform]</code>, <code>[_flush][stream-_flush]</code></p>
+      <p><code>[_transform][stream-_transform]</code>, <code>[_flush][stream-_flush]</code>,
+      <code>[_final][stream-_final]</code></p>
     </td>
   </tr>
 </table>
@@ -1279,6 +1282,8 @@ constructor and implement the `writable._write()` method. The
     [`stream._writev()`][stream-_writev] method.
   * `destroy` {Function} Implementation for the
     [`stream._destroy()`][writable-_destroy] method.
+  * `final` {Function} Implementation for the
+    [`stream._final()`][stream-_final] method.
 
 For example:
 
@@ -1398,6 +1403,22 @@ added: REPLACEME
 * `callback` {Function} A callback function that takes an optional error argument
   which is invoked when the writable is destroyed.
 
+#### writable.\_final(callback)
+<!-- YAML
+added: REPLACEME
+-->
+
+* `callback` {Function} Call this function (optionally with an error
+  argument) when you are done writing any remaining data.
+
+Note: `_final()` **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 before the stream closes, delaying the
+`finish` event until `callback` is called. This is useful to close resources
+or write buffered data before a stream ends.
+
 #### Errors While Writing
 
 It is recommended that errors occurring during the processing of the
@@ -2115,6 +2136,7 @@ readable buffer so there is nothing for a user to consume.
 [stream-_transform]: #stream_transform_transform_chunk_encoding_callback
 [stream-_write]: #stream_writable_write_chunk_encoding_callback_1
 [stream-_writev]: #stream_writable_writev_chunks_callback
+[stream-_final]: #stream_writable_final_callback
 [stream-end]: #stream_writable_end_chunk_encoding_callback
 [stream-pause]: #stream_readable_pause
 [stream-push]: #stream_readable_push_chunk_encoding

lib/_stream_writable.js

@@ -58,6 +58,12 @@ function WritableState(options, stream) {
   // cast to ints.
   this.highWaterMark = Math.floor(this.highWaterMark);
 
+  // if _final has been called
+  this.finalCalled = false;
+
+  // if _final has been called
+  this.finalCalled = false;
+
   // drain event flag.
   this.needDrain = false;
   // at the start of calling end()
@@ -199,6 +205,9 @@ function Writable(options) {
 
     if (typeof options.destroy === 'function')
       this._destroy = options.destroy;
+
+    if (typeof options.final === 'function')
+      this._final = options.final;
   }
 
   Stream.call(this);
@@ -520,23 +529,37 @@ function needFinish(state) {
           !state.finished &&
           !state.writing);
 }
-
-function prefinish(stream, state) {
-  if (!state.prefinished) {
+function callFinal(stream, state) {
+  stream._final((err) => {
+    state.pendingcb--;
+    if (err) {
+      stream.emit('error', err);
+    }
     state.prefinished = true;
     stream.emit('prefinish');
+    finishMaybe(stream, state);
+  });
+}
+function prefinish(stream, state) {
+  if (!state.prefinished && !state.finalCalled) {
+    if (typeof stream._final === 'function') {
+      state.pendingcb++;
+      state.finalCalled = true;
+      process.nextTick(callFinal, stream, state);
+    } else {
+      state.prefinished = true;
+      stream.emit('prefinish');
+    }
   }
 }
 
 function finishMaybe(stream, state) {
   var need = needFinish(state);
   if (need) {
+    prefinish(stream, state);
     if (state.pendingcb === 0) {
-      prefinish(stream, state);
       state.finished = true;
       stream.emit('finish');
-    } else {
-      prefinish(stream, state);
     }
   }
   return need;

test/parallel/test-stream-readable-constructor-set-methods.js

@@ -1,19 +1,11 @@
 'use strict';
-require('../common');
-const assert = require('assert');
+const common = require('../common');
 
 const Readable = require('stream').Readable;
 
-let _readCalled = false;
-function _read(n) {
-  _readCalled = true;
+const _read = common.mustCall(function _read(n) {
   this.push(null);
-}
+});
 
 const r = new Readable({ read: _read });
 r.resume();
-
-process.on('exit', function() {
-  assert.strictEqual(r._read, _read);
-  assert(_readCalled);
-});

test/parallel/test-stream-transform-constructor-set-methods.js

@@ -1,24 +1,25 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 
 const Transform = require('stream').Transform;
 
-let _transformCalled = false;
-function _transform(d, e, n) {
-  _transformCalled = true;
+const _transform = common.mustCall(function _transform(d, e, n) {
   n();
-}
+});
 
-let _flushCalled = false;
-function _flush(n) {
-  _flushCalled = true;
+const _final = common.mustCall(function _final(n) {
   n();
-}
+});
+
+const _flush = common.mustCall(function _flush(n) {
+  n();
+});
 
 const t = new Transform({
   transform: _transform,
-  flush: _flush
+  flush: _flush,
+  final: _final
 });
 
 const t2 = new Transform({});
@@ -34,6 +35,5 @@ assert.throws(() => {
 process.on('exit', () => {
   assert.strictEqual(t._transform, _transform);
   assert.strictEqual(t._flush, _flush);
-  assert.strictEqual(_transformCalled, true);
-  assert.strictEqual(_flushCalled, true);
+  assert.strictEqual(t._final, _final);
 });

test/parallel/test-stream-transform-final-sync.js

@@ -0,0 +1,100 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+const stream = require('stream');
+let state = 0;
+
+/*
+What you do
+var stream = new tream.Transform({
+  transform: function transformCallback(chunk, _, next) {
+    // part 1
+    this.push(chunk);
+    //part 2
+    next();
+  },
+  final: function endCallback(done) {
+    // part 1
+    process.nextTick(function () {
+      // part 2
+      done();
+    });
+  },
+  flush: function flushCallback(done) {
+    // part 1
+    process.nextTick(function () {
+      // part 2
+      done();
+    });
+  }
+});
+t.on('data', dataListener);
+t.on('end', endListener);
+t.on('finish', finishListener);
+t.write(1);
+t.write(4);
+t.end(7, endMethodCallback);
+
+The order things are called
+
+1. transformCallback part 1
+2. dataListener
+3. transformCallback part 2
+4. transformCallback part 1
+5. dataListener
+6. transformCallback part 2
+7. transformCallback part 1
+8. dataListener
+9. transformCallback part 2
+10. finalCallback part 1
+11. finalCallback part 2
+12. flushCallback part 1
+13. finishListener
+14. endMethodCallback
+15. flushCallback part 2
+16. endListener
+*/
+
+const t = new stream.Transform({
+  objectMode: true,
+  transform: common.mustCall(function(chunk, _, next) {
+    assert.strictEqual(++state, chunk, 'transformCallback part 1');
+    this.push(state);
+    assert.strictEqual(++state, chunk + 2, 'transformCallback part 2');
+    process.nextTick(next);
+  }, 3),
+  final: common.mustCall(function(done) {
+    state++;
+    assert.strictEqual(state, 10, 'finalCallback part 1');
+    state++;
+    assert.strictEqual(state, 11, 'finalCallback part 2');
+    done();
+  }, 1),
+  flush: common.mustCall(function(done) {
+    state++;
+    assert.strictEqual(state, 12, 'flushCallback part 1');
+    process.nextTick(function() {
+      state++;
+      assert.strictEqual(state, 15, 'flushCallback part 2');
+      done();
+    });
+  }, 1)
+});
+t.on('finish', common.mustCall(function() {
+  state++;
+  assert.strictEqual(state, 13, 'finishListener');
+}, 1));
+t.on('end', common.mustCall(function() {
+  state++;
+  assert.strictEqual(state, 16, 'end event');
+}, 1));
+t.on('data', common.mustCall(function(d) {
+  assert.strictEqual(++state, d + 1, 'dataListener');
+}, 3));
+t.write(1);
+t.write(4);
+t.end(7, common.mustCall(function() {
+  state++;
+  assert.strictEqual(state, 14, 'endMethodCallback');
+}, 1));