Provide fix for dotnet#81093

"Mono does not emit ProcessExit event on SIGTERM"

	* src/mono/mono/mini/mini-posix.c
	  - Add signal handler for SIGTERM
	  - SIGTERM handler will set a global variable that may be
	    monitored by the GC finalizer thread

	* src/mono/mono/metadata/gc.c
	  - Monitor for sigterm and kick off the shutdown process when
	    encountered by calling mono_runtime_try_shutdown().
	  - Exit with either the user set exitcode
	    (System.Environment.ExitCode) or SIGTERM + 128.

src/mono/mono/metadata/gc.c

@@ -43,6 +43,7 @@
 #include <mono/utils/mono-os-wait.h>
 #include <mono/utils/mono-lazy-init.h>
 #include <mono/utils/mono-threads-wasm.h>
+#include <mono/metadata/runtime.h>
 #ifndef HOST_WIN32
 #include <pthread.h>
 #endif
@@ -63,6 +64,8 @@ static gboolean gc_disabled;
 
 static gboolean finalizing_root_domain;
 
+extern volatile gboolean term_signaled;
+
 gboolean mono_log_finalizers;
 gboolean mono_do_not_finalize;
 static volatile gboolean suspend_finalizers;
@@ -879,6 +882,17 @@ finalizer_thread (gpointer unused)
 	mono_hazard_pointer_install_free_queue_size_callback (hazard_free_queue_is_too_big);
 
 	while (!finished) {
+
+		/* Just in case we've received a SIGTERM */
+		if (term_signaled) {
+			int ec = mono_environment_exitcode_get();
+			mono_runtime_try_shutdown();
+			if (ec == 0)
+				exit(128+SIGTERM);
+			else
+				exit(ec);
+		}
+
 		/* Wait to be notified that there's at least one
 		 * finaliser to run
 		 */

src/mono/mono/mini/mini-posix.c

@@ -97,6 +97,8 @@
 #endif
 #include "mono/utils/mono-tls-inline.h"
 
+gboolean term_signaled = FALSE;
+
 #if defined(HOST_WATCHOS)
 
 void
@@ -291,6 +293,15 @@ MONO_SIG_HANDLER_FUNC (static, sigquit_signal_handler)
 	mono_chain_signal (MONO_SIG_HANDLER_PARAMS);
 }
 
+MONO_SIG_HANDLER_FUNC (static, sigterm_signal_handler)
+{
+	term_signaled = TRUE;
+
+	mono_gc_finalize_notify ();
+
+	mono_chain_signal (MONO_SIG_HANDLER_PARAMS);
+}
+
 MONO_SIG_HANDLER_FUNC (static, sigusr2_signal_handler)
 {
 	gboolean enabled = mono_trace_is_enabled ();
@@ -390,6 +401,8 @@ mono_runtime_posix_install_handlers (void)
 	sigaddset (&signal_set, SIGFPE);
 	add_signal_handler (SIGQUIT, sigquit_signal_handler, SA_RESTART);
 	sigaddset (&signal_set, SIGQUIT);
+	add_signal_handler (SIGTERM, sigterm_signal_handler, SA_RESTART);
+	sigaddset (&signal_set, SIGTERM);
 	add_signal_handler (SIGILL, mono_crashing_signal_handler, 0);
 	sigaddset (&signal_set, SIGILL);
 	add_signal_handler (SIGBUS, mono_sigsegv_signal_handler, 0);