zlib: simplify flushing mechanism

Previously, flushing on zlib streams was implemented through
stream 'drain' handlers. This has a number of downsides; in
particular, it is complex, and could lead to unpredictable
behaviour, since it meant that in a sequence like

```js
compressor.write('abc');
compressor.flush();
waitForMoreDataAsynchronously(() => {
  compressor.write('def');
});
```

it was not fully deterministic whether the flush happens after
the second chunk is written or the first one.

This commit replaces this mechanism by one that piggy-backs
along the stream’s write queue, using a “special” `Buffer`
instance that signals that a flush is currently due.

PR-URL: #23186
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/zlib.js

@@ -311,10 +311,9 @@ function Zlib(opts, mode) {
   this._level = level;
   this._strategy = strategy;
   this._chunkSize = chunkSize;
-  this._flushFlag = flush;
-  this._scheduledFlushFlag = Z_NO_FLUSH;
-  this._origFlushFlag = flush;
+  this._defaultFlushFlag = flush;
   this._finishFlushFlag = finishFlush;
+  this._nextFlush = -1;
   this._info = opts && opts.info;
   this.once('end', this.close);
 }
@@ -398,6 +397,7 @@ function maxFlush(a, b) {
   return flushiness[a] > flushiness[b] ? a : b;
 }
 
+const flushBuffer = Buffer.alloc(0);
 Zlib.prototype.flush = function flush(kind, callback) {
   var ws = this._writableState;
 
@@ -412,21 +412,13 @@ Zlib.prototype.flush = function flush(kind, callback) {
   } else if (ws.ending) {
     if (callback)
       this.once('end', callback);
-  } else if (ws.needDrain) {
-    const alreadyHadFlushScheduled = this._scheduledFlushFlag !== Z_NO_FLUSH;
-    this._scheduledFlushFlag = maxFlush(kind, this._scheduledFlushFlag);
-
-    // If a callback was passed, always register a new `drain` + flush handler,
-    // mostly because that's simpler and flush callbacks piling up is a rare
-    // thing anyway.
-    if (!alreadyHadFlushScheduled || callback) {
-      const drainHandler = () => this.flush(this._scheduledFlushFlag, callback);
-      this.once('drain', drainHandler);
-    }
+  } else if (this._nextFlush !== -1) {
+    // This means that there is a flush currently in the write queue.
+    // We currently coalesce this flush into the pending one.
+    this._nextFlush = maxFlush(this._nextFlush, kind);
   } else {
-    this._flushFlag = kind;
-    this.write(Buffer.alloc(0), '', callback);
-    this._scheduledFlushFlag = Z_NO_FLUSH;
+    this._nextFlush = kind;
+    this.write(flushBuffer, '', callback);
   }
 };
 
@@ -436,20 +428,18 @@ Zlib.prototype.close = function close(callback) {
 };
 
 Zlib.prototype._transform = function _transform(chunk, encoding, cb) {
-  // If it's the last chunk, or a final flush, we use the Z_FINISH flush flag
-  // (or whatever flag was provided using opts.finishFlush).
-  // If it's explicitly flushing at some other time, then we use
-  // Z_FULL_FLUSH. Otherwise, use the original opts.flush flag.
-  var flushFlag;
+  var flushFlag = this._defaultFlushFlag;
+  // We use a 'fake' zero-length chunk to carry information about flushes from
+  // the public API to the actual stream implementation.
+  if (chunk === flushBuffer) {
+    flushFlag = this._nextFlush;
+    this._nextFlush = -1;
+  }
+
+  // For the last chunk, also apply `_finishFlushFlag`.
   var ws = this._writableState;
   if ((ws.ending || ws.ended) && ws.length === chunk.byteLength) {
-    flushFlag = this._finishFlushFlag;
-  } else {
-    flushFlag = this._flushFlag;
-    // once we've flushed the last of the queue, stop flushing and
-    // go back to the normal behavior.
-    if (chunk.byteLength >= ws.length)
-      this._flushFlag = this._origFlushFlag;
+    flushFlag = maxFlush(flushFlag, this._finishFlushFlag);
   }
   processChunk(this, chunk, flushFlag, cb);
 };

test/parallel/test-zlib-flush-drain.js

@@ -44,5 +44,5 @@ process.once('exit', function() {
   assert.strictEqual(
     drainCount, 1);
   assert.strictEqual(
-    flushCount, 2);
+    flushCount, 1);
 });

test/parallel/test-zlib-write-after-flush.js

@@ -35,7 +35,7 @@ gunz.setEncoding('utf8');
 gunz.on('data', (c) => output += c);
 gunz.on('end', common.mustCall(() => {
   assert.strictEqual(output, input);
-  assert.strictEqual(gzip._flushFlag, zlib.constants.Z_NO_FLUSH);
+  assert.strictEqual(gzip._nextFlush, -1);
 }));
 
 // make sure that flush/write doesn't trigger an assert failure