Bug 1857193 - Make passing --port=0 select a free port, r=webdriver-reviewers,jdescottes

Previously this bound to a free port, but we weren't allowing
connections because we checked the Host header against the specified
port (i.e. 0) rather than the bound port.

Differential Revision: https://phabricator.services.mozilla.com/D189925

Author: jgraham

testing/webdriver/src/server.rs

@@ -203,7 +203,7 @@ impl Drop for Listener {
 }
 
 pub fn start<T, U>(
-    address: SocketAddr,
+    mut address: SocketAddr,
     allow_hosts: Vec<Host>,
     allow_origins: Vec<Url>,
     handler: T,
@@ -216,6 +216,11 @@ where
     let listener = StdTcpListener::bind(address)?;
     listener.set_nonblocking(true)?;
     let addr = listener.local_addr()?;
+    if address.port() == 0 {
+        // If we passed in 0 as the port number the OS will assign an unused port;
+        // we want to update the address to the actual used port
+        address.set_port(addr.port())
+    }
     let (msg_send, msg_recv) = channel();
 
     let builder = thread::Builder::new().name("webdriver server".to_string());