init: actually report which signal is causing the reboot.

It wasn't clear to me why init was rebooting until I saw that it was
SIGABRT, which then made me read through earlier log spam to work out
what was actually unhappy (the SELinux compiler, in my case).

Test: worked out why init was rebooting my device
Change-Id: I605d8956213c4c23711073fd4b0ff99562b7f351

init/host_init_stubs.h

@@ -55,7 +55,7 @@ inline uint32_t HandlePropertySet(const std::string&, const std::string&, const
 
 // reboot_utils.h
 inline void SetFatalRebootTarget() {}
-inline void __attribute__((noreturn)) InitFatalReboot() {
+inline void __attribute__((noreturn)) InitFatalReboot(int signal_number) {
     abort();
 }
 

init/reboot_utils.cpp

@@ -109,7 +109,7 @@ void __attribute__((noreturn)) RebootSystem(unsigned int cmd, const std::string&
     abort();
 }
 
-void __attribute__((noreturn)) InitFatalReboot() {
+void __attribute__((noreturn)) InitFatalReboot(int signal_number) {
     auto pid = fork();
 
     if (pid == -1) {
@@ -124,6 +124,7 @@ void __attribute__((noreturn)) InitFatalReboot() {
     }
 
     // In the parent, let's try to get a backtrace then shutdown.
+    LOG(ERROR) << __FUNCTION__ << ": signal " << signal_number;
     std::unique_ptr<Backtrace> backtrace(
             Backtrace::Create(BACKTRACE_CURRENT_PROCESS, BACKTRACE_CURRENT_THREAD));
     if (!backtrace->Unwind(0)) {
@@ -154,7 +155,7 @@ void InstallRebootSignalHandlers() {
         // RebootSystem uses syscall() which isn't actually async-signal-safe, but our only option
         // and probably good enough given this is already an error case and only enabled for
         // development builds.
-        InitFatalReboot();
+        InitFatalReboot(signal);
     };
     action.sa_flags = SA_RESTART;
     sigaction(SIGABRT, &action, nullptr);

init/reboot_utils.h

@@ -27,7 +27,7 @@ void SetFatalRebootTarget();
 bool IsRebootCapable();
 // This is a wrapper around the actual reboot calls.
 void __attribute__((noreturn)) RebootSystem(unsigned int cmd, const std::string& reboot_target);
-void __attribute__((noreturn)) InitFatalReboot();
+void __attribute__((noreturn)) InitFatalReboot(int signal_number);
 void InstallRebootSignalHandlers();
 
 }  // namespace init

init/util.cpp

@@ -20,6 +20,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <pwd.h>
+#include <signal.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -481,7 +482,7 @@ static void InitAborter(const char* abort_message) {
         return;
     }
 
-    InitFatalReboot();
+    InitFatalReboot(SIGABRT);
 }
 
 // The kernel opens /dev/console and uses that fd for stdin/stdout/stderr if there is a serial