pw_emu: Better handling for startup errors

When using qemu with an invalid argument the command fails with a JSON
decode error because the qemu process terminates before the qmp
handshake is completed.

Catch these kind of errors and surface them to the user as start
errors. Also provide additional context by dumping the emulator logs.

Bug: 315868463
Change-Id: I07248dc3f27b7b6096fed829a71c0079eae46616
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/184859
Reviewed-by: Jonathon Reinhart <[email protected]>
Pigweed-Auto-Submit: Octavian Purdila <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

pw_emu/py/pw_emu/__main__.py

@@ -441,7 +441,7 @@ def main() -> int:
         )
 
     try:
-        emu = Emulator(args.working_dir, args.config)
+        emu = Emulator(Path(args.working_dir), args.config)
         args.func(emu, args)
     except Error as err:
         print(err)

pw_emu/py/pw_emu/core.py

@@ -143,6 +143,8 @@ class RunError(Error):
 
     def __init__(self, proc: str, msg: str) -> None:
         super().__init__(f'error running `{proc}`: {msg}')
+        self.proc = proc
+        self.msg = msg
 
 
 class InvalidPropertyPath(Error):
@@ -582,6 +584,12 @@ def get_channel_stream(
     def get_channels(self) -> List[str]:
         return self._handles.channels.keys()
 
+    def get_logs(self) -> str:
+        """Returns the emulator logs."""
+
+        log_path = self._wdir / f'{self._handles.emu}.log'
+        return log_path.read_text()
+
     def stop(self) -> None:
         """Stops the emulator."""
 
@@ -949,7 +957,15 @@ def start(
             self._stop_procs()
             raise err
 
-        self._post_start()
+        try:
+            self._post_start()
+        except RunError as err:
+            self._handles.save(wdir)
+            connector = self._get_connector(self._wdir)
+            if not connector.running():
+                msg = err.msg + '; dumping logs:\n' + connector.get_logs()
+                raise RunError(err.proc, msg)
+            raise err
         self._handles.save(wdir)
 
         if proc:

pw_emu/py/pw_emu/qemu.py

@@ -31,6 +31,7 @@
     Launcher,
     Error,
     InvalidChannelType,
+    RunError,
     WrongEmulator,
 )
 
@@ -290,9 +291,15 @@ def _pre_start(
 
     def _post_start(self) -> None:
         assert self._qmp_init_sock is not None
-        conn, _ = self._qmp_init_sock.accept()
+        try:
+            conn, _ = self._qmp_init_sock.accept()
+        except (KeyboardInterrupt, socket.timeout):
+            raise RunError('qemu', 'qmp connection failed')
         self._qmp_init_sock.close()
-        qmp = QmpClient(conn.makefile('rwb', buffering=0))
+        try:
+            qmp = QmpClient(conn.makefile('rwb', buffering=0))
+        except json.decoder.JSONDecodeError:
+            raise RunError('qemu', 'qmp handshake failed')
         conn.close()
 
         resp = qmp.request('query-chardev')

pw_emu/py/pw_emu/renode.py

@@ -22,10 +22,11 @@
 
 from pw_emu.core import (
     Connector,
+    Error,
     Handles,
     InvalidChannelType,
     Launcher,
-    Error,
+    RunError,
     WrongEmulator,
 )
 
@@ -155,7 +156,7 @@ def _post_start(self) -> None:
         if not connected:
             msg = 'failed to connect to robot channel'
             msg += f'({robot.host}:{robot.port}): {err}'
-            raise RenodeRobotError(msg)
+            raise RunError('renode', msg)
 
         sock.close()