stream: simplify howMuchToRead()

This slightly refactors read by moving side effects out of
howMuchToRead().

We don't actually have to set state.needReadable = true; in
howMuchToRead() since read handles 0 return as needReadable.

PR-URL: #29155
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/_stream_readable.js

@@ -377,17 +377,9 @@ function howMuchToRead(n, state) {
     else
       return state.length;
   }
-  // If we're asking for more than the current hwm, then raise the hwm.
-  if (n > state.highWaterMark)
-    state.highWaterMark = computeNewHighWaterMark(n);
   if (n <= state.length)
     return n;
-  // Don't have enough
-  if (!state.ended) {
-    state.needReadable = true;
-    return 0;
-  }
-  return state.length;
+  return state.ended ? state.length : 0;
 }
 
 // You can override either this method, or the async _read(n) below.
@@ -403,6 +395,10 @@ Readable.prototype.read = function(n) {
   const state = this._readableState;
   const nOrig = n;
 
+  // If we're asking for more than the current hwm, then raise the hwm.
+  if (n > state.highWaterMark)
+    state.highWaterMark = computeNewHighWaterMark(n);
+
   if (n !== 0)
     state.emittedReadable = false;