_signals.py: allow calling signal handler from non-main threads

* This modifies the signal terminator so that it can be called from any
thread.

This checks that either:

- The signal handler is already in place
- Or the caller is in the main thread, allowing to set the signal
  handler.

This also removes the exact callback that was added instead of removing
the last one, and fixes the `suspend_handler` to do the same.

This is required, as we don't know which interleaving of calls will be
done, and we can't guarantee that the last one is the right one to
remove

2 files changed, 18 insertions, 8 deletions

diff --git a/src/buildstream/_cas/casserver.py b/src/buildstream/_cas/casserver.py
index 013fb07dd..04c5eb836 100644
--- a/src/buildstream/_cas/casserver.py
+++ b/src/buildstream/_cas/casserver.py
@@ -30,6 +30,7 @@ import grpc
 import click
 
 from .._protos.build.bazel.remote.asset.v1 import remote_asset_pb2_grpc
+from .. import _signals
 from .._protos.build.bazel.remote.execution.v2 import (
     remote_execution_pb2,
     remote_execution_pb2_grpc,
@@ -137,7 +138,11 @@ def create_server(repo, *, enable_push, quota, index_only, log_level=LogLevel.Le
             _ReferenceStorageServicer(casd_channel, root, enable_push=enable_push), server
         )
 
-        yield server
+        # Ensure we have the signal handler set for SIGTERM
+        # This allows threads from GRPC to call our methods that do register
+        # handlers at exit.
+        with _signals.terminator(lambda: None):
+            yield server
 
     finally:
         casd_channel.close()
diff --git a/src/buildstream/_signals.py b/src/buildstream/_signals.py
index 03b55b052..8032752a8 100644
--- a/src/buildstream/_signals.py
+++ b/src/buildstream/_signals.py
@@ -33,6 +33,9 @@ from typing import Callable, Deque
 terminator_stack: Deque[Callable] = deque()
 suspendable_stack: Deque[Callable] = deque()
 
+terminator_lock = threading.Lock()
+suspendable_lock = threading.Lock()
+
 
 # Per process SIGTERM handler
 def terminator_handler(signal_, frame):
@@ -80,13 +83,10 @@ def terminator_handler(signal_, frame):
 def terminator(terminate_func):
     global terminator_stack  # pylint: disable=global-statement
 
-    # Signal handling only works in the main thread
-    if threading.current_thread() != threading.main_thread():
-        yield
-        return
-
     outermost = bool(not terminator_stack)
 
+    assert threading.current_thread() == threading.main_thread() or not outermost
+
     terminator_stack.append(terminate_func)
     if outermost:
         original_handler = signal.signal(signal.SIGTERM, terminator_handler)
@@ -96,7 +96,9 @@ def terminator(terminate_func):
     finally:
         if outermost:
             signal.signal(signal.SIGTERM, original_handler)
-        terminator_stack.pop()
+
+        with terminator_lock:
+            terminator_stack.remove(terminate_func)
 
 
 # Just a simple object for holding on to two callbacks
@@ -146,6 +148,8 @@ def suspendable(suspend_callback, resume_callback):
     global suspendable_stack  # pylint: disable=global-statement
 
     outermost = bool(not suspendable_stack)
+    assert threading.current_thread() == threading.main_thread() or not outermost
+
     suspender = Suspender(suspend_callback, resume_callback)
     suspendable_stack.append(suspender)
 
@@ -158,7 +162,8 @@ def suspendable(suspend_callback, resume_callback):
         if outermost:
             signal.signal(signal.SIGTSTP, original_stop)
 
-        suspendable_stack.pop()
+        with suspendable_lock:
+            suspendable_stack.remove(suspender)
 
 
 # blocked()