Allow nested CrashRecoveryContexts.

[thakis]

Summary

libclang uses a CrashRecoveryContext, and building a module does too. If a module gets built through libclang, nested CrashRecoveryContexts are used. They work fine with threads as things are stored in ThreadLocal variables, but in LLVM_ENABLE_THREADS=OFF builds the two recovery contexts would write to the same globals.

To fix, keep active CrashRecoveryContextImpls in a list and have the global point to the innermost one, and do something similar for tlIsRecoveringFromCrash.

Necessary (but not sufficient) for PR11974 and PR20325

[thakis]

akyrtzi: Ping. (I'm asking you for review because your r184380 from two years ago is the most recent related change in this area.)

[akyrtzi]

LGTM.

[thakis]

r244251, thanks!

include/llvm/Support/CrashRecoveryContext.h

 ///      CrashRecoveryContext CRC;
 ///
 ///      if (!CRC.RunSafely(actual_work, 0)) {
 ///         ... a crash was detected, report error to user ...
 ///      }
 ///
 ///      ... no crash was detected ...
 ///    }
-///
-/// Crash recovery contexts may not be nested.
 class CrashRecoveryContext {
   void *Impl;
   CrashRecoveryContextCleanup *head;
 
 public:
   CrashRecoveryContext() : Impl(nullptr), head(nullptr) {}
   ~CrashRecoveryContext();
 

lib/Support/CrashRecoveryContext.cpp

 namespace {
 
 struct CrashRecoveryContextImpl;
 
 static ManagedStatic<
     sys::ThreadLocal<const CrashRecoveryContextImpl> > CurrentContext;
 
 struct CrashRecoveryContextImpl {
+  const CrashRecoveryContextImpl *Next;
+
   CrashRecoveryContext *CRC;
   std::string Backtrace;
   ::jmp_buf JumpBuffer;
   volatile unsigned Failed : 1;
   unsigned SwitchedThread : 1;
 
 public:
   CrashRecoveryContextImpl(CrashRecoveryContext *CRC) : CRC(CRC),
                                                         Failed(false),
                                                         SwitchedThread(false) {
+    Next = CurrentContext->get();
     CurrentContext->set(this);
   }
   ~CrashRecoveryContextImpl() {
     if (!SwitchedThread)
-      CurrentContext->erase();
+      CurrentContext->set(Next);
   }
 
   /// \brief Called when the separate crash-recovery thread was finished, to
   /// indicate that we don't need to clear the thread-local CurrentContext.
-  void setSwitchedThread() { SwitchedThread = true; }
+  void setSwitchedThread() { 
+#if defined(LLVM_ENABLE_THREADS) && LLVM_ENABLE_THREADS != 0
+    SwitchedThread = true;
+#endif
+  }
 
   void HandleCrash() {
     // Eliminate the current context entry, to avoid re-entering in case the
     // cleanup code crashes.
-    CurrentContext->erase();
+    CurrentContext->set(Next);
 
     assert(!Failed && "Crash recovery context already failed!");
     Failed = true;
 
     // FIXME: Stash the backtrace.
 
     // Jump back to the RunSafely we were called under.
     longjmp(JumpBuffer, 1);
   }
 };
 
 }
 
 static ManagedStatic<sys::Mutex> gCrashRecoveryContextMutex;
 static bool gCrashRecoveryEnabled = false;
 
-static ManagedStatic<sys::ThreadLocal<const CrashRecoveryContextCleanup> >
+static ManagedStatic<sys::ThreadLocal<const CrashRecoveryContext>>
        tlIsRecoveringFromCrash;
 
 CrashRecoveryContextCleanup::~CrashRecoveryContextCleanup() {}
 
 CrashRecoveryContext::~CrashRecoveryContext() {
   // Reclaim registered resources.
   CrashRecoveryContextCleanup *i = head;
-  tlIsRecoveringFromCrash->set(head);
+  const CrashRecoveryContext *PC = tlIsRecoveringFromCrash->get();
+  tlIsRecoveringFromCrash->set(this);
   while (i) {
     CrashRecoveryContextCleanup *tmp = i;
     i = tmp->next;
     tmp->cleanupFired = true;
     tmp->recoverResources();
     delete tmp;
   }
-  tlIsRecoveringFromCrash->erase();
+  tlIsRecoveringFromCrash->set(PC);
   
   CrashRecoveryContextImpl *CRCI = (CrashRecoveryContextImpl *) Impl;
   delete CRCI;
 }
 
 bool CrashRecoveryContext::isRecoveringFromCrash() {
   return tlIsRecoveringFromCrash->get() != nullptr;
 }