bridge: Report PID for a spawn stream channel

Add the spawned PID to the channel's "ready" message. This will be
useful for #21021

doc/protocol.md

@@ -853,6 +853,9 @@ following options can be specified:
    size of the terminal window. Values must be integers between 0 and 0xffff.
    This option is only valid if "pty" is true.
 
+For type "spawn", the `ready` message contains a "pid" field with the spawned
+process ID.
+
 If an "done" is sent to the bridge on this channel, then the socket and/or pipe
 input is shutdown. The channel will send an "done" when the output of the socket
 or pipe is done.

src/cockpit/channel.py

@@ -370,6 +370,7 @@ class ProtocolChannel(Channel, asyncio.Protocol):
     _send_pongs: bool = True
     _last_ping: 'JsonObject | None' = None
     _create_transport_task: 'asyncio.Task[asyncio.Transport] | None' = None
+    _ready_info: 'JsonObject | None' = None
 
     # read-side EOF handling
     _close_on_eof: bool = False
@@ -400,7 +401,10 @@ def create_transport_done(self, task: 'asyncio.Task[asyncio.Transport]') -> None
             return
 
         self.connection_made(transport)
-        self.ready()
+        if self._ready_info is not None:
+            self.ready(**self._ready_info)
+        else:
+            self.ready()
 
     def connection_made(self, transport: asyncio.BaseTransport) -> None:
         assert isinstance(transport, asyncio.Transport)

src/cockpit/channels/stream.py

@@ -109,7 +109,9 @@ async def create_transport(self, loop: asyncio.AbstractEventLoop, options: JsonO
 
         try:
             transport = SubprocessTransport(loop, self, args, pty=pty, window=window, env=env, cwd=cwd, stderr=stderr)
-            logger.debug('Spawned process args=%s pid=%i', args, transport.get_pid())
+            pid = transport.get_pid()
+            self._ready_info = {'pid': pid}
+            logger.debug('Spawned process args=%s pid=%i', args, pid)
             return transport
         except FileNotFoundError as error:
             raise ChannelError('not-found') from error

test/pytest/test_bridge.py

@@ -770,6 +770,9 @@ async def serve_page(reader, writer):
             assert control['channel'] == ch
             command = control['command']
             if command == 'ready':
+                if 'spawn' in args:
+                    assert isinstance(control['pid'], int)
+                    assert os.readlink(f"/proc/{control['pid']}/exe").endswith("/cat")
                 # If we get ready, it's our turn to send data first.
                 # Hopefully we didn't receive any before.
                 assert not saw_data