[PATCH] Restore previous sigmask in gdb.block_signals

[Tom Tromey <tromey@adacore.com>]

Tom de Vries found a bug where, sometimes, a SIGCHLD would be
delivered to a non-main thread, wreaking havoc.

The problem is that gdb.block_signals was unblocking the signals it
blocked -- but it was being called from the DAP thread, leading to
SIGCHLD being unblocked there.

This patch fixes the problem by restoring the previous set of signals
instead.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30680
---
 gdb/python/lib/gdb/__init__.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/gdb/python/lib/gdb/__init__.py b/gdb/python/lib/gdb/__init__.py
index 98aadb1dfea..b3124369fe8 100644
--- a/gdb/python/lib/gdb/__init__.py
+++ b/gdb/python/lib/gdb/__init__.py
@@ -271,11 +271,11 @@ def blocked_signals():
         return
 
     to_block = {signal.SIGCHLD, signal.SIGINT, signal.SIGALRM, signal.SIGWINCH}
-    signal.pthread_sigmask(signal.SIG_BLOCK, to_block)
+    old_mask = signal.pthread_sigmask(signal.SIG_BLOCK, to_block)
     try:
         yield None
     finally:
-        signal.pthread_sigmask(signal.SIG_UNBLOCK, to_block)
+        signal.pthread_sigmask(signal.SIG_SETMASK, old_mask)
 
 
 class Thread(threading.Thread):
-- 
2.40.1