zlib: fix interaction of flushing and needDrain

Fixes: nodejs#14523
PR-URL: nodejs#14527
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/zlib.js

@@ -188,6 +188,7 @@ function Zlib(opts, mode) {
   this._flushFlag = opts.flush || constants.Z_NO_FLUSH;
   this._finishFlushFlag = opts.finishFlush !== undefined ?
     opts.finishFlush : constants.Z_FINISH;
+  this._scheduledFlushFlag = constants.Z_NO_FLUSH;
 
   if (opts.chunkSize !== undefined) {
     if (opts.chunkSize < constants.Z_MIN_CHUNK) {
@@ -300,6 +301,23 @@ Zlib.prototype._flush = function _flush(callback) {
   this._transform(Buffer.alloc(0), '', callback);
 };
 
+// If a flush is scheduled while another flush is still pending, a way to figure
+// out which one is the "stronger" flush is needed.
+// Roughly, the following holds:
+// Z_NO_FLUSH (< Z_TREES) < Z_BLOCK < Z_PARTIAL_FLUSH <
+//     Z_SYNC_FLUSH < Z_FULL_FLUSH < Z_FINISH
+const flushiness = [];
+let i = 0;
+for (const flushFlag of [constants.Z_NO_FLUSH, constants.Z_BLOCK,
+                         constants.Z_PARTIAL_FLUSH, constants.Z_SYNC_FLUSH,
+                         constants.Z_FULL_FLUSH, constants.Z_FINISH]) {
+  flushiness[flushFlag] = i++;
+}
+
+function maxFlush(a, b) {
+  return flushiness[a] > flushiness[b] ? a : b;
+}
+
 Zlib.prototype.flush = function flush(kind, callback) {
   var ws = this._writableState;
 
@@ -315,13 +333,21 @@ Zlib.prototype.flush = function flush(kind, callback) {
     if (callback)
       this.once('end', callback);
   } else if (ws.needDrain) {
-    if (callback) {
-      const drainHandler = () => this.flush(kind, callback);
+    const alreadyHadFlushScheduled =
+        this._scheduledFlushFlag !== constants.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 {
     this._flushFlag = kind;
     this.write(Buffer.alloc(0), '', callback);
+    this._scheduledFlushFlag = constants.Z_NO_FLUSH;
   }
 };
 

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

@@ -0,0 +1,27 @@
+'use strict';
+
+// Regression test for https://github.com/nodejs/node/issues/14523.
+// Checks that flushes interact properly with writableState.needDrain,
+// even if no flush callback was passed.
+
+const common = require('../common');
+const assert = require('assert');
+const zlib = require('zlib');
+
+const zipper = zlib.createGzip({ highWaterMark: 16384 });
+const unzipper = zlib.createGunzip();
+zipper.pipe(unzipper);
+
+zipper.write('A'.repeat(17000));
+zipper.flush();
+
+let received = 0;
+unzipper.on('data', common.mustCall((d) => {
+  received += d.length;
+}, 2));
+
+// Properly `.end()`ing the streams would interfere with checking that
+// `.flush()` works.
+process.on('exit', () => {
+  assert.strictEqual(received, 17000);
+});

test/parallel/test-zlib-flush-multiple-scheduled.js

@@ -0,0 +1,41 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const zlib = require('zlib');
+
+const {
+  Z_PARTIAL_FLUSH, Z_SYNC_FLUSH, Z_FULL_FLUSH, Z_FINISH
+} = zlib.constants;
+
+common.crashOnUnhandledRejection();
+
+async function getOutput(...sequenceOfFlushes) {
+  const zipper = zlib.createGzip({ highWaterMark: 16384 });
+
+  zipper.write('A'.repeat(17000));
+  for (const flush of sequenceOfFlushes) {
+    zipper.flush(flush);
+  }
+
+  const data = [];
+
+  return new Promise((resolve) => {
+    zipper.on('data', common.mustCall((d) => {
+      data.push(d);
+      if (data.length === 2) resolve(Buffer.concat(data));
+    }, 2));
+  });
+}
+
+(async function() {
+  assert.deepStrictEqual(await getOutput(Z_SYNC_FLUSH),
+                         await getOutput(Z_SYNC_FLUSH, Z_PARTIAL_FLUSH));
+  assert.deepStrictEqual(await getOutput(Z_SYNC_FLUSH),
+                         await getOutput(Z_PARTIAL_FLUSH, Z_SYNC_FLUSH));
+
+  assert.deepStrictEqual(await getOutput(Z_FINISH),
+                         await getOutput(Z_FULL_FLUSH, Z_FINISH));
+  assert.deepStrictEqual(await getOutput(Z_FINISH),
+                         await getOutput(Z_SYNC_FLUSH, Z_FINISH));
+})();