Reset _unwindInfoMissing when info is found eventually

[dim]

Summary

As reported in https://github.com/llvm/llvm-project/issues/54799, when
using the C APIs unw_init_local(), unw_set_reg(), unw_get_reg()
and unw_step(), sometimes libunwind can erroneously conclude there is
no unwind info available.

This is because the initial call to co->setInfoBasedOnIPRegister() in
unw_init_local() can set _unwindInfoMissing to true, as at that
point there may not yet be unwind info available.

Subsequent calls to unw_set_reg(), with the register parameter set to
UNW_REG_IP, might still be able to find unwind info based on the new
IP value, but it does *not* reset _unwindInfoMissing to false in
those cases, so follow-up calls to unw_step() will erroneously return
UNW_STEP_END immediately.

To fix this, add explicit resets of _unwindInfoMissing to false, in
the various places where setInfoBasedOnIPRegister() actually does find
unwind info.

[MaskRay]

See https://github.com/llvm/llvm-project/issues/54799#issuecomment-1094405136 , the test is incorrect and the original report doesn't indicate a bug to me.

libunwind/src/UnwindCursor.hpp

#if defined(_LIBUNWIND_SUPPORT_COMPACT_UNWIND)
     if (sects.compact_unwind_section != 0) {
       if (this->getInfoFromCompactEncodingSection(pc, sects)) {
   #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
         // Found info in table, done unless encoding says to use dwarf.
         uint32_t dwarfOffset;
         if ((sects.dwarf_section != 0) && compactSaysUseDwarf(&dwarfOffset)) {
           if (this->getInfoFromDwarfSection(pc, sects, dwarfOffset)) {
             // found info in dwarf, done
+            _unwindInfoMissing = false;
             return;
           }
         }
   #endif
         // If unwind table has entry, but entry says there is no unwind info,
         // record that we have no unwind info.
         if (_info.format == 0)
           _unwindInfoMissing = true;
         return;
       }
     }
 #endif // defined(_LIBUNWIND_SUPPORT_COMPACT_UNWIND)
 
 #if defined(_LIBUNWIND_SUPPORT_SEH_UNWIND)
     // If there is SEH unwind info, look there next.
-    if (this->getInfoFromSEH(pc))
+    if (this->getInfoFromSEH(pc)) {
+      _unwindInfoMissing = false;
       return;
+    }
 #endif
 
 #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
     // If there is dwarf unwind info, look there next.
     if (sects.dwarf_section != 0) {
       if (this->getInfoFromDwarfSection(pc, sects)) {
         // found info in dwarf, done
+        _unwindInfoMissing = false;
         return;
       }
     }
 #endif
 
 #if defined(_LIBUNWIND_ARM_EHABI)
     // If there is ARM EHABI unwind info, look there next.
-    if (sects.arm_section != 0 && this->getInfoFromEHABISection(pc, sects))
+    if (sects.arm_section != 0 && this->getInfoFromEHABISection(pc, sects)) {
+      _unwindInfoMissing = false;
       return;
+    }
 #endif
   }
 
 #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
   // There is no static unwind info for this pc. Look to see if an FDE was
   // dynamically registered for it.
   pint_t cachedFDE = DwarfFDECache<A>::findFDE(DwarfFDECache<A>::kSearchAll,
                                                pc);
   if (cachedFDE != 0) {
     typename CFI_Parser<A>::FDE_Info fdeInfo;
     typename CFI_Parser<A>::CIE_Info cieInfo;
     if (!CFI_Parser<A>::decodeFDE(_addressSpace, cachedFDE, &fdeInfo, &cieInfo))
-      if (getInfoFromFdeCie(fdeInfo, cieInfo, pc, 0))
+      if (getInfoFromFdeCie(fdeInfo, cieInfo, pc, 0)) {
+        _unwindInfoMissing = false;
         return;
       }
+  }
 
   // Lastly, ask AddressSpace object about platform specific ways to locate
   // other FDEs.
   pint_t fde;
   if (_addressSpace.findOtherFDE(pc, fde)) {
     typename CFI_Parser<A>::FDE_Info fdeInfo;
     typename CFI_Parser<A>::CIE_Info cieInfo;
     if (!CFI_Parser<A>::decodeFDE(_addressSpace, fde, &fdeInfo, &cieInfo)) {
       // Double check this FDE is for a function that includes the pc.
       if ((fdeInfo.pcStart <= pc) && (pc < fdeInfo.pcEnd))
-        if (getInfoFromFdeCie(fdeInfo, cieInfo, pc, 0))
+        if (getInfoFromFdeCie(fdeInfo, cieInfo, pc, 0)) {
+          _unwindInfoMissing = false;
           return;
         }
     }
+  }
 #endif // #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
 
 #if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
   if (setInfoForSigReturn())
     return;
 #endif
 
   // no unwind info, flag that we can't reliably unwind