[library_sockfs.js] Cleanup buffer handling in sendmsg

Extracting the `buffer.slice` expression made the code cleanup and
showed the double `new Uint8Array` to be redundant (AFAICT).

Author: sbc100

src/library_sockfs.js

@@ -619,16 +619,13 @@ addToLibrary({
           buffer = buffer.buffer;
         }
 
-        var data;
-#if PTHREADS
-        // WebSockets .send() does not allow passing a SharedArrayBuffer, so clone the portion of the SharedArrayBuffer as a regular
-        // ArrayBuffer that we want to send.
-        if (buffer instanceof SharedArrayBuffer) {
-          data = new Uint8Array(new Uint8Array(buffer.slice(offset, offset + length))).buffer;
-        } else {
-#endif
-          data = buffer.slice(offset, offset + length);
+        var data = buffer.slice(offset, offset + length);
 #if PTHREADS
+        // WebSockets .send() does not allow passing a SharedArrayBuffer, so
+        // clone the the SharedArrayBuffer as regular ArrayBuffer before
+        // sending.
+        if (data instanceof SharedArrayBuffer) {
+          data = new Uint8Array(new Uint8Array(data)).buffer;
         }
 #endif
 

test/test_sockets.py

@@ -285,6 +285,7 @@ def test_enet(self):
     'native': [WebsockifyServerHarness, 59160, ['-DTEST_DGRAM=0']],
     'tcp': [CompiledServerHarness, 59162, ['-DTEST_DGRAM=0']],
     'udp': [CompiledServerHarness, 59164, ['-DTEST_DGRAM=1']],
+    'pthread': [CompiledServerHarness, 59165, ['-pthread', '-sPROXY_TO_PTHREAD']],
   })
   def test_nodejs_sockets_echo(self, harness_class, port, args):
     if harness_class == WebsockifyServerHarness and common.EMTEST_LACKS_NATIVE_CLANG: