Accept an unset command to proxy to an already started process (unmanaged process)

docs/source/server-process.rst

@@ -36,7 +36,8 @@ pairs.
    * A callable that takes any :ref:`callable arguments <server-process/callable-arguments>`,
      and returns a list of strings that are used & treated same as above.
 
-   This key is required.
+   If the command is not specified or is an empty list, it is assumed by the
+   proxy that the process is already running and available on ``{port}``.
 
 ``timeout``
 ^^^^^^^^^^^

jupyter_server_proxy/config.py

@@ -26,6 +26,7 @@ class _Proxy(SuperviseAndProxyHandler):
         def __init__(self, *args, **kwargs):
             super().__init__(*args, **kwargs)
             self.name = name
+            self.command = command
             self.proxy_base = name
             self.absolute_url = absolute_url
             self.requested_port = port
@@ -62,7 +63,7 @@ def _realize_rendered_template(self, attribute):
             return self._render_template(attribute)
 
         def get_cmd(self):
-            return self._realize_rendered_template(command)
+            return self._realize_rendered_template(self.command)
 
         def get_env(self):
             return self._realize_rendered_template(environment)
@@ -121,7 +122,7 @@ def make_server_process(name, server_process_config, serverproxy_config):
     le = server_process_config.get('launcher_entry', {})
     return ServerProcess(
         name=name,
-        command=server_process_config['command'],
+        command=server_process_config.get('command', list()),
         environment=server_process_config.get('environment', {}),
         timeout=server_process_config.get('timeout', 5),
         absolute_url=server_process_config.get('absolute_url', False),
@@ -152,12 +153,16 @@ class ServerProxy(Configurable):
 
         Value should be a dictionary with the following keys:
           command
-            A list of strings that should be the full command to be executed.
+            An optional list of strings that should be the full command to be executed.
             The optional template arguments {{port}} and {{base_url}} will be substituted with the
             port the process should listen on and the base-url of the notebook.
 
             Could also be a callable. It should return a list.
 
+            If the command is not specified or is an empty list, it is assumed the process is already
+            running, therefore the port checking is skipped and the proxy is set up on the specified
+            port.
+
           environment
             A dictionary of environment variable mappings. As with the command
             traitlet, {{port}} and {{base_url}} will be substituted.

jupyter_server_proxy/handlers.py

@@ -573,6 +573,7 @@ class SuperviseAndProxyHandler(LocalProxyHandler):
     def __init__(self, *args, **kwargs):
         self.requested_port = 0
         self.mappath = {}
+        self.command = list()
         super().__init__(*args, **kwargs)
 
     def initialize(self, state):
@@ -588,11 +589,14 @@ def port(self):
         Allocate either the requested port or a random empty port for use by
         application
         """
-        if 'port' not in self.state:
+        if 'port' not in self.state and self.command:
             sock = socket.socket()
             sock.bind(('', self.requested_port))
             self.state['port'] = sock.getsockname()[1]
             sock.close()
+        elif 'port' not in self.state:
+            self.state['port'] = self.requested_port
+
         return self.state['port']
 
     def get_cwd(self):
@@ -650,6 +654,9 @@ async def ensure_process(self):
                 proc = SupervisedProcess(self.name, *cmd, env=server_env, ready_func=self._http_ready_func, ready_timeout=timeout, log=self.log)
                 self.state['proc'] = proc
 
+                if not cmd:
+                    return
+
                 try:
                     await proc.start()
 

tests/resources/jupyter_server_config.py

@@ -82,6 +82,9 @@ def cats_only(response, path):
         'command': ['python3', './tests/resources/httpinfo.py', '{port}'],
         'rewrite_response': [cats_only, dog_to_cat],
     },
+    'python-proxyto54321-no-command': {
+        'port': 54321
+    }
 }
 
 c.ServerProxy.non_service_rewrite_response = hello_to_foo

tests/test_proxies.py

@@ -150,6 +150,18 @@ def test_server_proxy_requested_port():
     assert direct.code == 200
 
 
+def test_server_proxy_on_requested_port_no_command():
+    r = request_get(PORT, '/python-proxyto54321-no-command/ghi', TOKEN)
+    assert r.code == 200
+    s = r.read().decode('ascii')
+    assert s.startswith('GET /ghi?token=')
+    assert 'X-Forwarded-Context: /python-proxyto54321-no-command\n' in s
+    assert 'X-Proxycontextpath: /python-proxyto54321-no-command\n' in s
+
+    direct = request_get(54321, '/ghi', TOKEN)
+    assert direct.code == 200
+
+
 def test_server_proxy_port_non_absolute():
     r = request_get(PORT, '/proxy/54321/jkl', TOKEN)
     assert r.code == 200