report: catch internal segfaults

`--diagnostic-report-on-fatalerror` does not capture internal
segfaults at present, instead limited to only faults that are
detected by the Node / v8. libuv's signal handler deliveres
result asynchronously which does not help for SIGSEGV as:
i) the synchronous sequence restarts the failing code,
ii) due to i) the callback is never delivered to the consumer.

Install a SIGSEGV handler in Node and follow the sequence of
fatalerror handling, as of now at least under report.

Fixes: nodejs#25762

Author: gireeshpunathil

src/node_errors.cc

@@ -306,6 +306,21 @@ void PrintErrorString(const char* format, ...) {
   ABORT();
 }
 
+#ifndef _WIN32
+void OnSIGSEGV(int signo) {
+  uv_tty_reset_mode();
+#ifdef __FreeBSD__
+  // FreeBSD has a nasty bug, see RegisterSignalHandler for details
+  struct sigaction sa;
+  memset(&sa, 0, sizeof(sa));
+  sa.sa_handler = SIG_DFL;
+  CHECK_EQ(sigaction(signo, &sa, nullptr), 0);
+#endif
+  CHECK_EQ(signo, SIGSEGV);
+  OnFatalError(__func__, "caught internal error.");
+}
+#endif  //  _WIN32
+
 void OnFatalError(const char* location, const char* message) {
   if (location) {
     PrintErrorString("FATAL ERROR: %s %s\n", location, message);

src/node_errors.h

@@ -22,6 +22,9 @@ void AppendExceptionLine(Environment* env,
 
 [[noreturn]] void FatalError(const char* location, const char* message);
 void OnFatalError(const char* location, const char* message);
+#ifndef _WIN32
+void OnSIGSEGV(int signo);
+#endif  //  _WIN32
 
 // Like a `TryCatch` but exits the process if an exception was caught.
 class FatalTryCatch : public v8::TryCatch {

src/node_report_module.cc

@@ -1,4 +1,5 @@
 #include "env.h"
+#include "node.h"
 #include "node_errors.h"
 #include "node_internals.h"
 #include "node_options.h"
@@ -262,6 +263,20 @@ static void Initialize(Local<Object> exports,
   env->SetMethod(exports, "onUserSignal", OnUserSignal);
   env->SetMethod(exports, "syncConfig", SyncConfig);
 
+  // Libuv based signal handling does not help internal segfaults,
+  // as synchronously delivered signals are handled asynchronously
+  // in libuv at present, which Node.js is unable to effectively
+  // consume. So install one of our own; make sure reset tty mode
+  // and exit; do not return gracefully that causes fault context
+  // to be invoked again.
+  // TODO(gireeshpunathil) decide what to do with asynchronous
+  // sigsegv handling - handle separately, or merge with this one.
+#ifndef _WIN32
+  if (options->report_on_fatalerror) {
+    node::RegisterSignalHandler(SIGSEGV, node::OnSIGSEGV, true);
+  }
+#endif  //  _WIN32
+
   // TODO(gireeshpunathil) if we are retaining this flag,
   // insert more verbose information at vital control flow
   // points. Right now, it is only this one.