[Sanitizer] Only unwind when it is safe to do so

[yln]

Summary

ScopedUnwinding and AsanThread::isUnwinding were introduced to

"Fix deadlock in stack unwinder on android/x86" on platforms where
libc unwinder calls malloc internally.

https://github.com/llvm/llvm-project/commit/58dbe0623051f1bb49256693e54b4e29f7db5faf

The current state of the code only protects the fast unwinder, which is
not affected by this. Only SlowUnwind calls out to _Unwind_Backtrace.
So we are in a situation where the code that should be protected isn't,
and code that doesn't need to be protected is.

Since this hasn't been causing issues, we don't even know if it strictly
required anymore. But let's be conservative and make sure at least also
SlowUnwind is protected.

[vitalybuka]

@eugenis WDYT?

[vitalybuka]

compiler-rt/lib/asan/asan_stack.cc

37

We can't assume that there is no issues here so quickly after commit.
I will take time to get Android.

I believe I've made mistake proposing the current code D58861

[yln]

Just looked at your changes. I think this is obsolete now.

compiler-rt/lib/asan/asan_stack.cc

 }  // namespace __asan
 
 void __sanitizer::BufferedStackTrace::UnwindImpl(
     uptr pc, uptr bp, void *context, bool request_fast, u32 max_depth) {
   using namespace __asan;
   size = 0;
   if (UNLIKELY(!asan_inited)) return;
 
   AsanThread *t = GetCurrentThread();
-  if (t && !t->isUnwinding() && WillUseFastUnwind(request_fast)) {
+  if (!t || t->isUnwinding()) return;
+
+  ScopedUnwinding unwind_scope(t);
+  if (WillUseFastUnwind(request_fast)) {
     uptr top = t->stack_top();
     uptr bottom = t->stack_bottom();
-    ScopedUnwinding unwind_scope(t);
     if (!SANITIZER_MIPS || IsValidFrame(bp, top, bottom)) {
       UnwindFast(pc, bp, top, bottom, max_depth);
       return;
     }
   }
 
 #if SANITIZER_CAN_SLOW_UNWIND
     UnwindSlowWithOptionalContext(pc, context, max_depth);