stream: fix fd is null when calling clearBuffer

Author: kylo5aby

lib/internal/streams/writable.js

@@ -733,7 +733,8 @@ function errorBuffer(state) {
 
 // If there's something in the buffer waiting, then process it.
 function clearBuffer(stream, state) {
-  if ((state[kState] & (kDestroyed | kBufferProcessing | kCorked | kBuffered)) !== kBuffered) {
+  if ((state[kState] & (kDestroyed | kBufferProcessing | kCorked | kBuffered)) !== kBuffered ||
+    (state[kState] & (kDestroyed | kBufferProcessing | kCorked | kConstructed)) !== kConstructed) {
     return;
   }
 

test/parallel/test-file-write-stream5.js

@@ -0,0 +1,25 @@
+'use strict';
+
+// Test 'uncork' for WritableStream.
+// Refs: https://github.com/nodejs/node/issues/50979
+
+const common = require('../common');
+const fs = require('fs');
+const assert = require('assert');
+const tmpdir = require('../common/tmpdir');
+
+const filepath = tmpdir.resolve('write_stream.txt');
+tmpdir.refresh();
+
+const data = 'data';
+
+const fileWriteStream = fs.createWriteStream(filepath);
+
+fileWriteStream.on('finish', () => {
+  const writtenData = fs.readFileSync(filepath, 'utf8');
+  assert.strictEqual(writtenData, data);
+});
+fileWriteStream.cork();
+fileWriteStream.write(data, common.mustCall());
+fileWriteStream.uncork();
+fileWriteStream.end();