[merp] Use a separate process as the hang supervisor.

Fixes mono#15646

macOS does not like signals being sent from the forked supervisor process, possibly towards anywhere but definitely when sent to the parent process. The following message is currently spewed after the supervisor process attempting to send a SIGSEGV to a hanged Mono process:
"The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec()."

We follow that direction and introduce a new binary that, when available on the environment's PATH, is used to abort the parent process for us.

configure.ac

@@ -6681,6 +6681,7 @@ tools/Makefile
 tools/locale-builder/Makefile
 tools/sgen/Makefile
 tools/pedump/Makefile
+tools/mono-hang-watchdog/Makefile
 runtime/Makefile
 msvc/Makefile
 po/Makefile

mono/metadata/threads.c

@@ -6262,7 +6262,6 @@ mono_threads_summarize_one (MonoThreadSummary *out, MonoContext *ctx)
 	return success;
 }
 
-#define TIMEOUT_CRASH_REPORTER_FATAL 30
 #define MAX_NUM_THREADS 128
 typedef struct {
 	gint32 has_owner; // state of this memory
@@ -6278,7 +6277,7 @@ typedef struct {
 	gboolean silent; // print to stdout
 } SummarizerGlobalState;
 
-#if defined(HAVE_KILL) && !defined(HOST_ANDROID) && defined(HAVE_WAITPID) && ((!defined(HOST_DARWIN) && defined(SYS_fork)) || HAVE_FORK)
+#if defined(HAVE_KILL) && !defined(HOST_ANDROID) && defined(HAVE_WAITPID) && defined(HAVE_EXECVP) && ((!defined(HOST_DARWIN) && defined(SYS_fork)) || HAVE_FORK)
 #define HAVE_MONO_SUMMARIZER_SUPERVISOR 1
 #endif
 
@@ -6289,12 +6288,6 @@ typedef struct {
 } SummarizerSupervisorState;
 
 #ifndef HAVE_MONO_SUMMARIZER_SUPERVISOR
-static void
-summarizer_supervisor_wait (SummarizerSupervisorState *state)
-{
-	return;
-}
-
 static pid_t
 summarizer_supervisor_start (SummarizerSupervisorState *state)
 {
@@ -6309,23 +6302,6 @@ summarizer_supervisor_end (SummarizerSupervisorState *state)
 }
 
 #else
-static void
-summarizer_supervisor_wait (SummarizerSupervisorState *state)
-{
-	sleep (TIMEOUT_CRASH_REPORTER_FATAL);
-
-	// If we haven't been SIGKILL'ed yet, we signal our parent
-	// and then exit
-#ifdef HAVE_KILL
-	g_async_safe_printf("Crash Reporter has timed out, sending SIGSEGV\n");
-	kill (state->pid, SIGSEGV);
-#else
-	g_error ("kill () is not supported by this platform");
-#endif
-
-	exit (1);
-}
-
 static pid_t
 summarizer_supervisor_start (SummarizerSupervisorState *state)
 {
@@ -6350,8 +6326,14 @@ summarizer_supervisor_start (SummarizerSupervisorState *state)
 	g_assert_not_reached ();
 #endif
 
-	if (pid != 0)
+	if (pid != 0) {
 		state->supervisor_pid = pid;
+		char pid_str[20]; // pid is a unit64_t, 20 digits max in decimal form
+		sprintf (pid_str, "%d", state->pid);
+		const char *const args[] = { "mono-hang-watchdog", pid_str };
+		execvp (args[0], (char * const*)args); // run 'mono-hang-watchdog [pid]'
+		g_assert_not_reached ();
+	}
 
 	return pid;
 }
@@ -6665,8 +6647,6 @@ mono_threads_summarize (MonoContext *ctx, gchar **out, MonoStackHash *hashes, gb
 				g_assert (mem);
 				success = mono_threads_summarize_execute_internal (ctx, out, hashes, silent, mem, provided_size, TRUE);
 				summarizer_supervisor_end (&synch);
-			} else {
-				summarizer_supervisor_wait (&synch);
 			}
 
 			if (!already_async)

tools/Makefile.am

@@ -1,3 +1,3 @@
-SUBDIRS = locale-builder sgen pedump
+SUBDIRS = locale-builder sgen pedump mono-hang-watchdog
 
 

tools/mono-hang-watchdog/Makefile.am

@@ -0,0 +1,13 @@
+
+AM_CPPFLAGS = $(SHARED_CFLAGS)
+
+if DISABLE_EXECUTABLES
+bin_PROGRAMS =
+else
+bin_PROGRAMS = mono-hang-watchdog
+endif
+
+CFLAGS = $(filter-out @CXX_REMOVE_CFLAGS@, @CFLAGS@)
+mono_hang_watchdog_CFLAGS = @CXX_ADD_CFLAGS@
+
+mono_hang_watchdog_SOURCES = mono-hang-watchdog.c

tools/mono-hang-watchdog/mono-hang-watchdog.c

@@ -0,0 +1,47 @@
+/* Given a external process' id as argument, the program waits for a set timeout then attempts to abort that process */
+/* Used by the Mono runtime's crash reporting. */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+
+#include "config.h"
+
+#ifdef HAVE_SIGNAL_H
+#include <signal.h>
+#endif
+
+#ifndef HAVE_KILL
+#error "kill() is not supported by this platform. This tool won't be able to function."
+#endif
+
+#define TIMEOUT_CRASH_REPORTER_FATAL 30
+
+static char* program_name;
+void program_exit (int exit_code, const char* message);
+
+void program_exit (int exit_code, const char* message) {
+    if (message)
+        printf ("%s\n", message);
+    printf ("Usage: '%s [pid]'\t\t[pid]: The id for the Mono process\n", program_name);
+    exit (exit_code);
+}
+
+int main (int argc, char* argv[])
+{
+    program_name = argv[0];
+    if (argc < 2)
+        program_exit (-1, "Please provide one argument (pid)");
+    errno = 0;
+    pid_t pid = (pid_t)strtoul (argv[1], NULL, 10);
+    if (errno)
+        program_exit (-2, "Invalid pid");
+
+    sleep (TIMEOUT_CRASH_REPORTER_FATAL);
+
+    /*  Abort the process. This isn't suspect to 'pid' referring a wrong new process with the same id:
+        this process is killed by Mono if Mono doesn't hang */
+    printf ("Mono hang watchdog timed out, sending SIGSEGV to %ul\n", pid);
+    kill (pid, SIGSEGV);
+}