bitbake: cooker/process: Fix signal handling lockups

If a parser process is terminated while holding a write lock, then it
will lead to a deadlock (see
https://docs.python.org/3/library/multiprocessing.html#multiprocessing.Process.terminate).

With SIGTERM, we don't want to terminate holding the lock. We also don't
want a SIGINT to cause a partial write to the event stream.

I tried using signal masks to avoid this but it doesn't work, see
https://bugs.python.org/issue47139

Instead, add a signal handler and catch the calls around the critical section.
We also need a thread lock to ensure other threads in the same process don't
handle the signal until all the threads are not in the lock.

(Bitbake rev: a40efaa5556a188dfe46c8d060adde37dc400dcd)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/server/process.py

@@ -20,6 +20,7 @@ import os
 import sys
 import time
 import select
+import signal
 import socket
 import subprocess
 import errno
@@ -737,11 +738,28 @@ class ConnectionWriter(object):
         # Why bb.event needs this I have no idea
         self.event = self
 
-    def send(self, obj):
-        obj = multiprocessing.reduction.ForkingPickler.dumps(obj)
+    def _send(self, obj):
         with self.wlock:
             self.writer.send_bytes(obj)
 
+    def send(self, obj):
+        obj = multiprocessing.reduction.ForkingPickler.dumps(obj)
+        # See notes/code in CookerParser
+        # We must not terminate holding this lock else processes will hang.
+        # For SIGTERM, raising afterwards avoids this.
+        # For SIGINT, we don't want to have written partial data to the pipe.
+        # pthread_sigmask block/unblock would be nice but doesn't work, https://bugs.python.org/issue47139
+        process = multiprocessing.current_process()
+        if process and hasattr(process, "queue_signals"):
+            with process.signal_threadlock:
+                process.queue_signals = True
+                self._send(obj)
+                process.queue_signals = False
+                for sig in process.signal_received.pop():
+                    process.handle_sig(sig, None)
+        else:
+            self._send(obj)
+
     def fileno(self):
         return self.writer.fileno()