test: add test-zlib-flush-drain

This test assures that if flush is called while the zlib object needs
to be drained that it will defer the callback until after the drain.

PR-URL: #3534
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

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

@@ -0,0 +1,49 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const zlib = require('zlib');
+const path = require('path');
+
+const bigData = new Buffer(10240).fill('x');
+
+const opts = {
+  level: 0,
+  highWaterMark: 16
+};
+
+const deflater = zlib.createDeflate(opts);
+
+// shim deflater.flush so we can count times executed
+var flushCount = 0;
+var drainCount = 0;
+
+const flush = deflater.flush;
+deflater.flush = function(kind, callback) {
+  flushCount++;
+  flush.call(this, kind, callback);
+};
+
+deflater.write(bigData);
+
+const ws = deflater._writableState;
+const beforeFlush = ws.needDrain;
+var afterFlush = ws.needDrain;
+
+deflater.flush(function(err) {
+  afterFlush = ws.needDrain;
+});
+
+deflater.on('drain', function() {
+  drainCount++;;
+});
+
+process.once('exit', function() {
+  assert.equal(beforeFlush, true,
+    'before calling flush the writable stream should need to drain');
+  assert.equal(afterFlush, false,
+    'after calling flush the writable stream should not need to drain');
+  assert.equal(drainCount, 1,
+    'the deflater should have emitted a single drain event');
+  assert.equal(flushCount, 2,
+    'flush should be called twice');
+});