test: add tests for clearBuffer state machine

This checks to see that clearBuffer appropriately decrements the
correct values in _writableState when clearBuffer is invoked in
end.
Fixes: #8687
PR-URL: #9922
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

Author: captainsafia

test/parallel/test-stream-writableState-uncorked-bufferedRequestCount.js

@@ -0,0 +1,57 @@
+'use strict';
+
+constcommon=require('../common');
+constassert=require('assert');
+conststream=require('stream');
+
+constwritable=newstream.Writable();
+
+writable._writev=common.mustCall((chunks,cb)=>{
+assert.equal(chunks.length,2,'two chunks to write');
+cb();
+},1);
+
+writable._write=common.mustCall((chunk,encoding,cb)=>{
+cb();
+},1);
+
+// first cork
+writable.cork();
+assert.strictEqual(writable._writableState.corked,1);
+assert.strictEqual(writable._writableState.bufferedRequestCount,0);
+
+// cork again
+writable.cork();
+assert.strictEqual(writable._writableState.corked,2);
+
+// the first chunk is buffered
+writable.write('first chunk');
+assert.strictEqual(writable._writableState.bufferedRequestCount,1);
+
+// first uncork does nothing
+writable.uncork();
+assert.strictEqual(writable._writableState.corked,1);
+assert.strictEqual(writable._writableState.bufferedRequestCount,1);
+
+process.nextTick(uncork);
+
+// the second chunk is buffered, because we uncork at the end of tick
+writable.write('second chunk');
+assert.strictEqual(writable._writableState.corked,1);
+assert.strictEqual(writable._writableState.bufferedRequestCount,2);
+
+functionuncork(){
+// second uncork flushes the buffer
+writable.uncork();
+assert.strictEqual(writable._writableState.corked,0);
+assert.strictEqual(writable._writableState.bufferedRequestCount,0);
+
+// verify that end() uncorks correctly
+writable.cork();
+writable.write('third chunk');
+writable.end();
+
+// end causes an uncork() as well
+assert.strictEqual(writable._writableState.corked,0);
+assert.strictEqual(writable._writableState.bufferedRequestCount,0);
+}