Support GATEWAY_API_URL env var

Author: threema-eduard

README.rst

@@ -69,6 +69,18 @@ the Threema gateway. Run the following command to see usage information:
 
     $ threema-gateway --help
 
+Setting a different API url
+---------------------------
+
+The default MsgApi URL points to https://msgapi.threema.ch/.
+
+If you are a Threema OnPrem customer or have another reason
+to use a different MsgApi endpoint, you may set an environment variable as follows:
+
+.. code-block:: bash
+
+    $ GATEWAY_API_URL=https://onprem.myinstance.tld/msgapi threema-gateway ...
+
 Examples
 ********
 

threema/gateway/bin/gateway_client.py

@@ -36,12 +36,21 @@
 
 # Apply mock URL when starting CLI in debug mode
 _test_port = os.environ.get('THREEMA_TEST_API')
+_api_url = os.environ.get('GATEWAY_API_URL')
 if _test_port is not None:
+    if _api_url is not None:
+        raise RuntimeError('GATEWAY_API_URL cannot be set alongside THREEMA_TEST_API')
     _mock_url = 'http://{}:{}'.format('127.0.0.1', _test_port)
     Connection.urls = {key: value.replace('https://msgapi.threema.ch', _mock_url)
                        for key, value in Connection.urls.items()}
     click.echo(('WARNING: Currently running in test mode!'
                 'The Threema Gateway Server will not be contacted!'), err=True)
+else:
+    if _api_url is not None:
+        if not _api_url.startswith('https://'):
+            raise RuntimeError('GATEWAY_API_URL must begin with "https://"')
+        Connection.urls = {key: value.replace('https://msgapi.threema.ch', _api_url.rstrip('/'))
+                           for key, value in Connection.urls.items()}
 
 
 class _MockConnection(AioRunMixin):