Bug 1721148 - [remote] Report WebSocket handshake failures to the cli…

…ent. r=webdriver-reviewers,jgraham

When a WebSocket upgrade fails we currently do not
send any details of the error. As such it makes
it hard to figure out the real underlying issue.

Further some clients like curl are hanging and do
not immediately exit when the response has been
received. Specifying the Content-Length header
within the response fixes it.

Differential Revision: https://phabricator.services.mozilla.com/D120575

remote/server/WebSocketHandshake.jsm

@@ -63,9 +63,14 @@ function writeString(output, data) {
   });
 }
 
-/** Write HTTP response (array of strings) to async output stream. */
-function writeHttpResponse(output, response) {
-  const s = response.join("\r\n") + "\r\n\r\n";
+/**
+ * Write HTTP response with headers (array of strings) and body
+ * to async output stream.
+ */
+function writeHttpResponse(output, headers, body = "") {
+  headers.push(`Content-Length: ${body.length}`);
+
+  const s = headers.join("\r\n") + `\r\n\r\n${body}`;
   return writeString(output, s);
 }
 
@@ -135,13 +140,23 @@ async function serverHandshake(request, output) {
     // Send response headers
     await writeHttpResponse(output, [
       "HTTP/1.1 101 Switching Protocols",
+      "Server: httpd.js",
       "Upgrade: websocket",
       "Connection: Upgrade",
       `Sec-WebSocket-Accept: ${acceptKey}`,
     ]);
   } catch (error) {
     // Send error response in case of error
-    await writeHttpResponse(output, ["HTTP/1.1 400 Bad Request"]);
+    await writeHttpResponse(
+      output,
+      [
+        "HTTP/1.1 400 Bad Request",
+        "Server: httpd.js",
+        "Content-Type: text/plain",
+      ],
+      error.message
+    );
+
     throw error;
   }
 }