port retry (#761)

* WIP

* import find_available_port method

* move method to utils

so I can add to eventually add to gui

* add fixed-port flag to tests

* Update server/utils/utils.py

Co-Authored-By: Tony Tung <tonytung@merly.org>

* pr review suggestions

* pr review suggestions

* fix outdated package.json

* update error message

* simplify find_available_port function

* Auto scan for ports unless port is specified.

* fix tests

* fix comment for find_available_port

* lint error

* differentiate port error from generic os error

* add errno to OSerror

* pr review fixes

* raise e -> raise

* oserror -> socket error

server/utils/utils.py

@@ -0,0 +1,19 @@
+import contextlib
+import errno
+import socket
+
+
+def find_available_port(host, port=5005):
+    """
+    Helper method to find open port on host. Tries 5000 ports incremented from the specified port
+    """
+    # Takes approx 2 seconds to do a scan of 5000 ports on my laptop
+    num_ports_to_try = 5000
+    for port_to_try in range(port, port + num_ports_to_try):
+        with contextlib.closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as s:
+            try:
+                s.bind((host, port_to_try))
+                return port_to_try
+            except socket.error:
+                pass
+    raise socket.error(errno.EADDRINUSE, f"No port in range {port} - {port + num_ports_to_try - 1} available.")