Warn about practically unfixable TSAN warnings in stack trace (#7723)

Summary:
TSAN reports that our stack trace handler makes unsafe calls
during a signal handler. I just tried fixing some of them and I don't
think it's fixable unless we can get away from using FILE stdio. Even if
we can use lower level functions only, I'm not sure it's fixed.

I also tried suppressing the reports with function and file level TSAN
suppression, but that doesn't seem to work, perhaps because the
violation is reported on the callee, not the caller.

So I added a warning to be printed whenever these violations would be
reported that they are practically ignorable.

Internal ref: T77844138

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7723

Test Plan:
run external_sst_file_test with seeded abort(), with TSAN
(TSAN warnings + new warning) and without TSAN (no warning, just stack
trace).

Reviewed By: akankshamahajan15

Differential Revision: D25228011

Pulled By: pdillinger

fbshipit-source-id: 3eda1d6e7ca3cdc64076cf99ae954168837d2818

port/stack_trace.cc

@@ -144,6 +144,22 @@ static void StackTraceHandler(int sig) {
   fprintf(stderr, "Received signal %d (%s)\n", sig, strsignal(sig));
   // skip the top three signal handler related frames
   PrintStack(3);
+
+  // Efforts to fix or suppress TSAN warnings "signal-unsafe call inside of
+  // a signal" have failed, so just warn the user about them.
+#if defined(__clang__) && defined(__has_feature)
+#if __has_feature(thread_sanitizer)
+  fprintf(stderr,
+          "==> NOTE: any above warnings about \"signal-unsafe call\" are\n"
+          "==> ignorable, as they are expected when generating a stack\n"
+          "==> trace because of a signal under TSAN. Consider why the\n"
+          "==> signal was generated to begin with, and the stack trace\n"
+          "==> in the TSAN warning can be useful for that. (The stack\n"
+          "==> trace printed by the signal handler is likely obscured\n"
+          "==> by TSAN output.)\n");
+#endif
+#endif
+
   // re-signal to default handler (so we still get core dump if needed...)
   raise(sig);
 }