Fix gdb backtraces on OSX.

We can't redirect output from gdb: since gdb will suspend us we won't
be able to read from the pipe we've redirected output to, and we'll
end up deadlocking if gdb outputs a lot of data (see bug mono#2548).

mono/mini/mini-darwin.c

@@ -226,16 +226,15 @@ mono_runtime_syscall_fork ()
 	return (pid_t) fork ();
 }
 
-gboolean
+void
 mono_gdb_render_native_backtraces (pid_t crashed_pid)
 {
-	return FALSE; // disable for now due to #2548.
 	const char *argv [5];
 	char gdb_template [] = "/tmp/mono-gdb-commands.XXXXXX";
 
 	argv [0] = g_find_program_in_path ("gdb");
 	if (argv [0] == NULL) {
-		return FALSE;
+		return;
 	}
 
 	if (mkstemp (gdb_template) != -1) {
@@ -257,6 +256,4 @@ mono_gdb_render_native_backtraces (pid_t crashed_pid)
 
 		unlink (gdb_template);
 	}
-
-	return TRUE;
 }

mono/mini/mini-exceptions.c

@@ -2170,16 +2170,10 @@ mono_handle_native_sigsegv (int signal, void *ctx)
 #if !defined(HOST_WIN32) && defined(HAVE_SYS_SYSCALL_H) && defined(SYS_fork)
 	if (!mini_get_debug_options ()->no_gdb_backtrace && !mono_debug_using_mono_debugger ()) {
 		/* From g_spawn_command_line_sync () in eglib */
-		int res;
-		int stdout_pipe [2] = { -1, -1 };
 		pid_t pid;
 		int status;
-		char buffer [1024];
 		pid_t crashed_pid = getpid ();
 
-		res = pipe (stdout_pipe);
-		g_assert (res != -1);
-
 		//pid = fork ();
 		/*
 		 * glibc fork acquires some locks, so if the crash happened inside malloc/free,
@@ -2188,31 +2182,14 @@ mono_handle_native_sigsegv (int signal, void *ctx)
 		pid = mono_runtime_syscall_fork ();
 
 		if (pid == 0) {
-			close (stdout_pipe [0]);
-			dup2 (stdout_pipe [1], STDOUT_FILENO);
-
-			for (i = getdtablesize () - 1; i >= 3; i--)
-				close (i);
-
-			if (!mono_gdb_render_native_backtraces (crashed_pid))
-				close (STDOUT_FILENO);
+			dup2 (STDERR_FILENO, STDOUT_FILENO);
 
+			mono_gdb_render_native_backtraces (crashed_pid);
 			exit (1);
 		}
 
-		close (stdout_pipe [1]);
-
 		fprintf (stderr, "\nDebug info from gdb:\n\n");
-
-		while (1) {
-			int nread = read (stdout_pipe [0], buffer, 1024);
-
-			if (nread <= 0)
-				break;
-			write (STDERR_FILENO, buffer, nread);
-		}		
-
-		waitpid (pid, &status, WNOHANG);
+		waitpid (pid, &status, 0);
 	}
 #endif
 	/*

mono/mini/mini-posix.c

@@ -635,15 +635,15 @@ mono_runtime_syscall_fork ()
 #endif
 }
 
-gboolean
+void
 mono_gdb_render_native_backtraces (pid_t crashed_pid)
 {
 	const char *argv [9];
 	char buf1 [128];
 
 	argv [0] = g_find_program_in_path ("gdb");
 	if (argv [0] == NULL) {
-		return FALSE;
+		return;
 	}
 
 	argv [1] = "-ex";
@@ -657,8 +657,6 @@ mono_gdb_render_native_backtraces (pid_t crashed_pid)
 	argv [8] = 0;
 
 	execv (argv [0], (char**)argv);
-
-	return TRUE;
 }
 #endif
 #endif /* __native_client__ */

mono/mini/mini.h

@@ -2400,7 +2400,7 @@ void mono_runtime_setup_stat_profiler (void) MONO_INTERNAL;
 void mono_runtime_shutdown_stat_profiler (void) MONO_INTERNAL;
 void mono_runtime_posix_install_handlers (void) MONO_INTERNAL;
 pid_t mono_runtime_syscall_fork (void) MONO_INTERNAL;
-gboolean mono_gdb_render_native_backtraces (pid_t crashed_pid) MONO_INTERNAL;
+void mono_gdb_render_native_backtraces (pid_t crashed_pid) MONO_INTERNAL;
 
 /*
  * Signal handling