net,http2: refactor _write and _writev

Refactor writable part (the _write and _writev functions) in net.Socket
and http2.Http2Stream classes.
Also involves adding a generic "WriteGeneric" method to the Http2Stream
class based on net.Socket._writeGeneric, but behind a symbol.

PR-URL: nodejs#20643
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/internal/http2/core.js

@@ -95,6 +95,7 @@ const kSession = Symbol('session');
 const kState = Symbol('state');
 const kType = Symbol('type');
 const kUpdateTimer = Symbol('update-timer');
+const kWriteGeneric = Symbol('write-generic');
 
 const kDefaultSocketTimeout = 2 * 60 * 1000;
 
@@ -1635,13 +1636,16 @@ class Http2Stream extends Duplex {
                 'bug in Node.js');
   }
 
-  _write(data, encoding, cb) {
+  [kWriteGeneric](writev, data, encoding, cb) {
     // When the Http2Stream is first created, it is corked until the
     // handle and the stream ID is assigned. However, if the user calls
     // uncork() before that happens, the Duplex will attempt to pass
     // writes through. Those need to be queued up here.
     if (this.pending) {
-      this.once('ready', this._write.bind(this, data, encoding, cb));
+      this.once(
+        'ready',
+        this[kWriteGeneric].bind(this, writev, data, encoding, cb)
+      );
       return;
     }
 
@@ -1665,53 +1669,30 @@ class Http2Stream extends Duplex {
     req.callback = cb;
     req.oncomplete = afterDoStreamWrite;
     req.async = false;
-    const err = createWriteReq(req, handle, data, encoding);
+
+    let err;
+    if (writev) {
+      const chunks = new Array(data.length << 1);
+      for (var i = 0; i < data.length; i++) {
+        const entry = data[i];
+        chunks[i * 2] = entry.chunk;
+        chunks[i * 2 + 1] = entry.encoding;
+      }
+      err = handle.writev(req, chunks);
+    } else {
+      err = createWriteReq(req, handle, data, encoding);
+    }
     if (err)
       return this.destroy(errors.errnoException(err, 'write', req.error), cb);
     trackWriteState(this, req.bytes);
   }
 
-  _writev(data, cb) {
-    // When the Http2Stream is first created, it is corked until the
-    // handle and the stream ID is assigned. However, if the user calls
-    // uncork() before that happens, the Duplex will attempt to pass
-    // writes through. Those need to be queued up here.
-    if (this.pending) {
-      this.once('ready', this._writev.bind(this, data, cb));
-      return;
-    }
-
-    // If the stream has been destroyed, there's nothing else we can do
-    // because the handle has been destroyed. This should only be an
-    // issue if a write occurs before the 'ready' event in the case where
-    // the duplex is uncorked before the stream is ready to go. In that
-    // case, drop the data on the floor. An error should have already been
-    // emitted.
-    if (this.destroyed)
-      return;
-
-    this[kUpdateTimer]();
-
-    if (!this.headersSent)
-      this[kProceed]();
+  _write(data, encoding, cb) {
+    this[kWriteGeneric](false, data, encoding, cb);
+  }
 
-    const handle = this[kHandle];
-    const req = new WriteWrap();
-    req.stream = this[kID];
-    req.handle = handle;
-    req.callback = cb;
-    req.oncomplete = afterDoStreamWrite;
-    req.async = false;
-    const chunks = new Array(data.length << 1);
-    for (var i = 0; i < data.length; i++) {
-      const entry = data[i];
-      chunks[i * 2] = entry.chunk;
-      chunks[i * 2 + 1] = entry.encoding;
-    }
-    const err = handle.writev(req, chunks);
-    if (err)
-      return this.destroy(errors.errnoException(err, 'write', req.error), cb);
-    trackWriteState(this, req.bytes);
+  _writev(data, cb) {
+    this[kWriteGeneric](true, data, '', cb);
   }
 
   _final(cb) {