sanitizer_common: use 0 for empty stack id

[dvyukov]

Summary

We use 0 for empty stack id from stack depot.
Deadlock detector 1 is the only place that uses -1
as a special case. Use 0 because there is a number
of checks of the form "if (stack id) ...".

[melver]

How did this not result in errors before?

[dvyukov]

In D105776#2870659, @melver wrote:

How did this not result in errors before?

I don't know exactly, this code is not particularly maintained.
One option is that findEdge always assigns something to stk_from/stk_to so that the initial values are never used.
I found it when I experimented with introduction of strong typedefs for some types including stack IDs. It did not forgive such sloppiness as assigning random values to these types.

compiler-rt/lib/sanitizer_common/sanitizer_deadlock_detector1.cpp

void DD::ReportDeadlock(DDCallback *cb, DDMutex *m) {
   DDReport *rep = &lt->rep;
   rep->n = len;
   for (uptr i = 0; i < len; i++) {
     uptr from = path[i];
     uptr to = path[(i + 1) % len];
     DDMutex *m0 = (DDMutex*)dd.getData(from);
     DDMutex *m1 = (DDMutex*)dd.getData(to);
 
-    u32 stk_from = -1U, stk_to = -1U;
+    u32 stk_from = 0, stk_to = 0;
     int unique_tid = 0;
     dd.findEdge(from, to, &stk_from, &stk_to, &unique_tid);
     // Printf("Edge: %zd=>%zd: %u/%u T%d\n", from, to, stk_from, stk_to,
     //    unique_tid);
     rep->loop[i].thr_ctx = unique_tid;
     rep->loop[i].mtx_ctx0 = m0->ctx;
     rep->loop[i].mtx_ctx1 = m1->ctx;
     rep->loop[i].stk[0] = stk_to;