transport: call Protocol.connection_made ASAP

It's been observed that in certain real-life scenarios we can start
receiving data via `.pipe_data_received()` before `.subprocess_exec()`
completes.  That happens when there are enough async callbacks in flight
that the spawned subprocess starts writing to its stdout before we have
a chance to deliver the result of `.subprocess_exec()`.

This is problematic, because we were deferring calling the
`.connection_made()` of our connected Protocol until after the exec call
returned.  This gap made it possible for us to start delivering data to
the Protocol before connecting to it.

Instead of waiting for the async exec call to finish running, call
connection_made() on the Protocol as soon as our own connection_made()
is called.  There was really no reason not to do it that way in the
first place — and the assumption that it was equivalent to calling it at
the end of `.connection_made()` proved incorrect.

Author: allisonkarlitskaya

src/ferny/transport.py

@@ -153,11 +153,8 @@ def exec_completed(task: asyncio.Task) -> None:
             assert self._stdin_transport is not None
             assert self._stdout_transport is not None
 
-            # Time to go live — ask the InteractionAgent to start processing
-            # stderr and tell our protocol that we're ready to receive data.
+            # Ask the InteractionAgent to start processing stderr.
             self._agent.start()
-            logger.debug('calling connection_made(%r, %r)', self, self._protocol)
-            self._protocol.connection_made(self)
 
         self._exec_task.add_done_callback(exec_completed)
 
@@ -239,6 +236,9 @@ def connection_made(self, transport: asyncio.BaseTransport) -> None:
         stderr_transport = transport.get_pipe_transport(2)
         assert stderr_transport is None
 
+        logger.debug('calling connection_made(%r, %r)', self, self._protocol)
+        self._protocol.connection_made(self)
+
     def connection_lost(self, exc: 'Exception | None') -> None:
         logger.debug('connection_lost(%r, %r)', self, exc)
         if self._exception is None: