debuggerd_handler: restore errno.

Bug: http://b/31448909
Test: mma
Change-Id: I737d66e8bed5fb31c2558f68608d3df460fa73c9

debuggerd/handler/debuggerd_handler.cpp

@@ -62,6 +62,19 @@
 
 #define CRASH_DUMP_PATH "/system/bin/" CRASH_DUMP_NAME
 
+class ErrnoRestorer {
+ public:
+  ErrnoRestorer() : saved_errno_(errno) {
+  }
+
+  ~ErrnoRestorer() {
+    errno = saved_errno_;
+  }
+
+ private:
+  int saved_errno_;
+};
+
 extern "C" void debuggerd_fallback_handler(siginfo_t*, ucontext_t*, void*);
 
 static debuggerd_callbacks_t g_callbacks;
@@ -328,6 +341,10 @@ static void resend_signal(siginfo_t* info, bool crash_dump_started) {
 // Handler that does crash dumping by forking and doing the processing in the child.
 // Do this by ptracing the relevant thread, and then execing debuggerd to do the actual dump.
 static void debuggerd_signal_handler(int signal_number, siginfo_t* info, void* context) {
+  // Make sure we don't change the value of errno, in case a signal comes in between the process
+  // making a syscall and checking errno.
+  ErrnoRestorer restorer;
+
   // It's possible somebody cleared the SA_SIGINFO flag, which would mean
   // our "info" arg holds an undefined value.
   if (!have_siginfo(signal_number)) {