[PATCH 4/6] server/process: Avoid hanging if a parser process is terminated

[Richard Purdie <richard.purdie@linuxfoundation.org>]

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.

Use signal masks to avoid this.

Some ideas from Peter Kjellerstedt <peter.kjellerstedt@axis.com>

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
---
 lib/bb/server/process.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/bb/server/process.py b/lib/bb/server/process.py
index efc3f04b4c..dc331b3957 100644
--- a/lib/bb/server/process.py
+++ b/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
@@ -739,8 +740,12 @@ class ConnectionWriter(object):
 
     def send(self, obj):
         obj = multiprocessing.reduction.ForkingPickler.dumps(obj)
+        # We must not terminate holding this lock. For SIGTERM, raising afterwards avoids this.
+        # For SIGINT, we don't want to have written partial data to the pipe.
+        signal.pthread_sigmask(signal.SIG_BLOCK, (signal.SIGINT, signal.SIGTERM))
         with self.wlock:
             self.writer.send_bytes(obj)
+        signal.pthread_sigmask(signal.SIG_UNBLOCK, (signal.SIGINT, signal.SIGTERM))
 
     def fileno(self):
         return self.writer.fileno()
-- 
2.32.0