fix: improve EOFError exception when remote nvim crashes #589

Problem: When the remote Nvim instance is aborted or terminates, the
pynvim client will face an exception `OSError: EOF`. It is not very
clear what is wrong and why EOF is received.

Solution: This happens when the remote the nvim process terminates
unexpectedly (or a tcp/file socket is broken or gets disconnected). We
can provide a bit more detailed information about why the asyncio
session is stopping, through asyncio Protocol. An `EOFError` will be
raised instead of OSError.

For example, during pynvim's unit tests we may see:
```
    EOFError: process_exited: pid = 40000, return_code = -6
```
which means that the Nvim subprocess (pid = 40000) exited unexpectedly
after getting SIGABRT (signal 6).  Other error messages (different
signals such as SIGSEGV/segfault) or connection lost (when connected
through socket, etc.) are also possible.

pynvim/msgpack_rpc/event_loop/asyncio.py

@@ -47,13 +47,14 @@ def __init__(self, on_data, on_error):
     @override
     def connection_made(self, transport):
         """Used to signal `asyncio.Protocol` of a successful connection."""
-        del transport  # no-op
+        self._transport = transport
 
     @override
     def connection_lost(self, exc: Optional[Exception]) -> None:
         """Used to signal `asyncio.Protocol` of a lost connection."""
-        debug(f"connection_lost: exc = {exc}")
-        self._on_error(exc if exc else EOFError())
+        warn(f"connection_lost: exc = {exc}")
+
+        self._on_error(exc if exc else EOFError("connection_lost"))
 
     @override
     def data_received(self, data: bytes) -> None:
@@ -63,11 +64,19 @@ def data_received(self, data: bytes) -> None:
     @override
     def pipe_connection_lost(self, fd: int, exc: Optional[Exception]) -> None:
         """Used to signal `asyncio.SubprocessProtocol` of a lost connection."""
-        debug("pipe_connection_lost: fd = %s, exc = %s", fd, exc)
+
+        assert isinstance(self._transport, asyncio.SubprocessTransport)
+        debug_info = {'fd': fd, 'exc': exc, 'pid': self._transport.get_pid()}
+        warn(f"pipe_connection_lost {debug_info}")
+
         if os.name == 'nt' and fd == 2:  # stderr
             # On windows, ignore piped stderr being closed immediately (#505)
             return
-        self._on_error(exc if exc else EOFError())
+
+        # pipe_connection_lost() *may* be called before process_exited() is
+        # called, when a Nvim subprocess crashes (SIGABRT). Do not handle
+        # errors here, as errors will be handled somewhere else
+        # self._on_error(exc if exc else EOFError("pipe_connection_lost"))
 
     @override
     def pipe_data_received(self, fd, data):
@@ -81,8 +90,13 @@ def pipe_data_received(self, fd, data):
     @override
     def process_exited(self) -> None:
         """Used to signal `asyncio.SubprocessProtocol` when the child exits."""
-        debug("process_exited")
-        self._on_error(EOFError())
+        assert isinstance(self._transport, asyncio.SubprocessTransport)
+        pid = self._transport.get_pid()
+        return_code = self._transport.get_returncode()
+
+        warn("process_exited, pid = %s, return_code = %s", pid, return_code)
+        err = EOFError(f"process_exited: pid = {pid}, return_code = {return_code}")
+        self._on_error(err)
 
 
 class AsyncioEventLoop(BaseEventLoop):

pynvim/msgpack_rpc/event_loop/base.py

@@ -194,6 +194,13 @@ def run(self, data_cb: Callable[[bytes], None]) -> None:
             signal.signal(signal.SIGINT, default_int_handler)
         self._on_data = None
 
+        # eventloop was stopped due to an error, re-raise it
+        # (e.g. connection lost when subprocess nvim dies)
+        if self._error:
+            # Note: traceback is not preserved and attached for some reason,
+            # should be somewhere from msgpack_rpc.event_loop.asyncio.Protocol
+            raise self._error
+
     @abstractmethod
     def _run(self) -> None:
         raise NotImplementedError()
@@ -234,8 +241,11 @@ def _on_signal(self, signum: signal.Signals) -> None:
         self.stop()
 
     def _on_error(self, exc: Exception) -> None:
-        debug(str(exc))
-        self._error = exc
+        warn('on_error: %s', repr(exc))
+        if self._error is None:
+            # ignore subsequent exceptions, it's enough to raise only
+            # the first exception arrived
+            self._error = exc
         self.stop()
 
     def _on_interrupt(self) -> None: