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: https://github.com/nodejs/node/pull/23186
Reviewed-By: James M Snell <jasnell@gmail.com>

2 files changed, 2 insertions, 2 deletions

diff --git a/test/parallel/test-zlib-flush-drain.js b/test/parallel/test-zlib-flush-drain.js
index 0619eecf3c..a470e32090 100644
--- a/test/parallel/test-zlib-flush-drain.js
+++ b/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);
 });
diff --git a/test/parallel/test-zlib-write-after-flush.js b/test/parallel/test-zlib-write-after-flush.js
index 2ba6ba4550..6d8d787343 100644
--- a/test/parallel/test-zlib-write-after-flush.js
+++ b/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