Do not crash with missing symbolication when running in DEDUP mode

[george.karpenkov]

Summary

Printing stacktrace from ASAN crashes with a segfault in DEDUP mode when symbolication is missing.
Needless to say, this leads to very confusing crashes and behaviors, exacerbated by the fact that libFuzzer forks the process with a triggered bug, hiding the error out of sight.

This commit does not resolve the issue fully, as when libFuzzer forks the process the output is redirected into a logfile and will not be seen by the user. Yet at least it would be possible to diagnoze the issue by looking at that logfile.
Perhaps libFuzzer could check for existance of "ERROR: " string, and display it to stderr as well, but that would be tying two repositories two closely.

[kubamracek]

lib/sanitizer_common/sanitizer_stacktrace_libcdep.cc

48–51

The "if" looks good, but the error message doesn't belong here: 1) it would be printed several times in this loop, 2) info.function isn't necessarily an indication that a symbolizer is not present/working (just one frame might be un-symbolizable). Either remove the error message, or move it to some other place (perhaps some init code where we initialize the symbolizers).

[george.karpenkov]

Moving error message to initialization is tricky, as that itself would require iterating over stack frames. Removing it entirely.

[kubamracek]

LGTM.

In D34914#801506, @george.karpenkov wrote:

Moving error message to initialization is tricky, as that itself would require iterating over stack frames. Removing it entirely.

I meant initialization of ASan. No need to walk any frames, just "is dedup_token set and there is no symbolizer available? abort." kind of thing.

lib/sanitizer_common/sanitizer_stacktrace_libcdep.cc

for (SymbolizedStack *cur = frames; cur; cur = cur->next) {
       frame_desc.clear();
       RenderFrame(&frame_desc, common_flags()->stack_trace_format, frame_num++,
                   cur->info, common_flags()->symbolize_vs_style,
                   common_flags()->strip_path_prefix);
       Printf("%s\n", frame_desc.data());
       if (dedup_frames-- > 0) {
         if (dedup_token.length())
           dedup_token.append("--");
+        if (cur->info.function != nullptr)
           dedup_token.append(cur->info.function);
       }
     }
     frames->ClearAll();
   }
   // Always print a trailing empty line after stack trace.
   Printf("\n");
   if (dedup_token.length())
     Printf("DEDUP_TOKEN: %s\n", dedup_token.data());
 }
 
 void BufferedStackTrace::Unwind(u32 max_depth, uptr pc, uptr bp, void *context,
                                 uptr stack_top, uptr stack_bottom,