signal.c: save the original sighandlers for fatal signals

On Android, a signal handler that is not SIG_DFL is set by default for
SIGSEGV.  Ruby's install_sighandler inserts Ruby's handler only when the
signal has no handler, so it does not insert Ruby's SEGV report handler,
which caused some test failures.

This changeset forces to install Ruby's handler for some fatal signals
(sigbus, sigsegv, and sigill).  They keep the original handlers, and
call them when the interpreter receives the signals.

error.c

@@ -599,7 +599,7 @@ rb_bug(const char *fmt, ...)
 }
 
 void
-rb_bug_for_fatal_signal(const void *ctx, const char *fmt, ...)
+rb_bug_for_fatal_signal(RETSIGTYPE (*default_sighandler)(int), int sig, const void *ctx, const char *fmt, ...)
 {
     const char *file = NULL;
     int line = 0;
@@ -610,6 +610,8 @@ rb_bug_for_fatal_signal(const void *ctx, const char *fmt, ...)
 
     report_bug(file, line, fmt, ctx);
 
+    if (default_sighandler) default_sighandler(sig);
+
     die();
 }
 

signal.c

@@ -918,6 +918,7 @@ NOINLINE(static void check_reserved_signal_(const char *name, size_t name_len));
 
 #ifdef SIGBUS
 
+static sighandler_t default_sigbus_handler;
 NORETURN(static ruby_sigaction_t sigbus);
 
 static RETSIGTYPE
@@ -933,25 +934,27 @@ sigbus(int sig SIGINFO_ARG)
 #if defined __APPLE__ || defined __linux__
     CHECK_STACK_OVERFLOW();
 #endif
-    rb_bug_for_fatal_signal(SIGINFO_CTX, "Bus Error" MESSAGE_FAULT_ADDRESS);
+    rb_bug_for_fatal_signal(default_sigbus_handler, sig, SIGINFO_CTX, "Bus Error" MESSAGE_FAULT_ADDRESS);
 }
 #endif
 
 #ifdef SIGSEGV
 
+static sighandler_t default_sigsegv_handler;
 NORETURN(static ruby_sigaction_t sigsegv);
 
 static RETSIGTYPE
 sigsegv(int sig SIGINFO_ARG)
 {
     check_reserved_signal("SEGV");
     CHECK_STACK_OVERFLOW();
-    rb_bug_for_fatal_signal(SIGINFO_CTX, "Segmentation fault" MESSAGE_FAULT_ADDRESS);
+    rb_bug_for_fatal_signal(default_sigsegv_handler, sig, SIGINFO_CTX, "Segmentation fault" MESSAGE_FAULT_ADDRESS);
 }
 #endif
 
 #ifdef SIGILL
 
+static sighandler_t default_sigill_handler;
 NORETURN(static ruby_sigaction_t sigill);
 
 static RETSIGTYPE
@@ -961,7 +964,7 @@ sigill(int sig SIGINFO_ARG)
 #if defined __APPLE__
     CHECK_STACK_OVERFLOW();
 #endif
-    rb_bug_for_fatal_signal(SIGINFO_CTX, "Illegal instruction" MESSAGE_FAULT_ADDRESS);
+    rb_bug_for_fatal_signal(default_sigill_handler, sig, SIGINFO_CTX, "Illegal instruction" MESSAGE_FAULT_ADDRESS);
 }
 #endif
 
@@ -1432,21 +1435,28 @@ sig_list(VALUE _)
 	perror(failed); \
     } while (0)
 static int
-install_sighandler(int signum, sighandler_t handler)
+install_sighandler_core(int signum, sighandler_t handler, sighandler_t *old_handler)
 {
     sighandler_t old;
 
     old = ruby_signal(signum, handler);
     if (old == SIG_ERR) return -1;
-    /* signal handler should be inherited during exec. */
-    if (old != SIG_DFL) {
-	ruby_signal(signum, old);
+    if (old_handler) {
+        *old_handler = (old == SIG_DFL || old == SIG_IGN) ? 0 : old;
+    }
+    else {
+        /* signal handler should be inherited during exec. */
+        if (old != SIG_DFL) {
+            ruby_signal(signum, old);
+        }
     }
     return 0;
 }
 
 #  define install_sighandler(signum, handler) \
-    INSTALL_SIGHANDLER(install_sighandler(signum, handler), #signum, signum)
+    INSTALL_SIGHANDLER(install_sighandler_core(signum, handler, NULL), #signum, signum)
+#  define force_install_sighandler(signum, handler, old_handler) \
+    INSTALL_SIGHANDLER(install_sighandler_core(signum, handler, old_handler), #signum, signum)
 
 #if RUBY_SIGCHLD
 static int
@@ -1558,14 +1568,14 @@ Init_signal(void)
 
     if (!ruby_enable_coredump) {
 #ifdef SIGBUS
-	install_sighandler(SIGBUS, (sighandler_t)sigbus);
+	force_install_sighandler(SIGBUS, (sighandler_t)sigbus, &default_sigbus_handler);
 #endif
 #ifdef SIGILL
-	install_sighandler(SIGILL, (sighandler_t)sigill);
+	force_install_sighandler(SIGILL, (sighandler_t)sigill, &default_sigill_handler);
 #endif
 #ifdef SIGSEGV
 	RB_ALTSTACK_INIT(GET_VM()->main_altstack);
-	install_sighandler(SIGSEGV, (sighandler_t)sigsegv);
+	force_install_sighandler(SIGSEGV, (sighandler_t)sigsegv, &default_sigsegv_handler);
 #endif
     }
 #ifdef SIGPIPE

vm_core.h

@@ -1624,7 +1624,7 @@ extern void rb_vmdebug_debug_print_post(const rb_execution_context_t *ec, const
 #define SDR() rb_vmdebug_stack_dump_raw(GET_EC(), GET_EC()->cfp)
 #define SDR2(cfp) rb_vmdebug_stack_dump_raw(GET_EC(), (cfp))
 void rb_vm_bugreport(const void *);
-NORETURN(void rb_bug_for_fatal_signal(const void *, const char *fmt, ...));
+NORETURN(void rb_bug_for_fatal_signal(RETSIGTYPE (*default_sighandler)(int), int sig, const void *, const char *fmt, ...));
 
 /* functions about thread/vm execution */
 RUBY_SYMBOL_EXPORT_BEGIN