stream: simplify Writable.end()

Simplifies Writable.end() by inlining and
de-duplicating code.

PR-URL: #32882
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/_stream_writable.js

@@ -588,21 +588,26 @@ Writable.prototype.end = function(chunk, encoding, cb) {
     this.uncork();
   }
 
-  if (typeof cb !== 'function')
-    cb = nop;
-
   // This is forgiving in terms of unnecessary calls to end() and can hide
   // logic errors. However, usually such errors are harmless and causing a
   // hard error can be disproportionately destructive. It is not always
   // trivial for the user to determine whether end() needs to be called or not.
+  let err;
   if (!state.errored && !state.ending) {
-    endWritable(this, state, cb);
+    state.ending = true;
+    finishMaybe(this, state, true);
+    state.ended = true;
   } else if (state.finished) {
-    process.nextTick(cb, new ERR_STREAM_ALREADY_FINISHED('end'));
+    err = new ERR_STREAM_ALREADY_FINISHED('end');
   } else if (state.destroyed) {
-    process.nextTick(cb, new ERR_STREAM_DESTROYED('end'));
-  } else if (cb !== nop) {
-    onFinished(this, state, cb);
+    err = new ERR_STREAM_DESTROYED('end');
+  }
+
+  if (typeof cb === 'function') {
+    if (err || state.finished)
+      process.nextTick(cb, err);
+    else
+      onFinished(this, state, cb);
   }
 
   return this;
@@ -683,18 +688,6 @@ function finish(stream, state) {
   }
 }
 
-function endWritable(stream, state, cb) {
-  state.ending = true;
-  finishMaybe(stream, state, true);
-  if (cb !== nop) {
-    if (state.finished)
-      process.nextTick(cb);
-    else
-      onFinished(stream, state, cb);
-  }
-  state.ended = true;
-}
-
 function onCorkedFinish(corkReq, state, err) {
   let entry = corkReq.entry;
   corkReq.entry = null;