Make G__FastAllocString cache thread safe

When using C++11, the cache used by G__FastAllocString utilizes
a non-locking thread safe circular buffer.

cint/cint/src/FastAllocString.cxx

@@ -19,6 +19,9 @@
 #include "stdio.h"
 #include "string.h"
 #include <map>
+#if __cplusplus >= 201103L
+#include <atomic>
+#endif
 
 namespace Cint {
    namespace Internal {
@@ -52,6 +55,97 @@ namespace Cint {
       // 8: 0
       class G__BufferReservoir {
       public:
+#if __cplusplus >= 201103L
+	 //Thread safe circular buffer
+	 // elements within the buffer (fBuffers) are either a nullptr or a valid Buffer_t
+	 // fNumAvailableBuffers is an approximate count of the number of valid Buffer_t.
+	 // It is only an approximate count since during a push or a pop the value can 
+	 // temporarily be off. However, given sufficient time after all push or pops have stopped,
+	 // the value of fNumAvailableBuffers will be correct. fNumAvailableBuffers can be negative
+	 // if push happens at the same time as a pop. If the push adds a valid Buffer_t to the buffer
+	 // but then the pop pulls that value out and decrements fNumAvailableBuffers before the push
+	 // has a chance to increment it. Similarly fNumAvailableBuffers can temporarily exceed fNumBuffers.
+	 // Similarly, fWatermark is only a good place to start searching for either an invalid or valid Buffer_t
+	 // since a push or pop operation may not have had a chance to move the value before the next
+         // call is made. Given the imperfect nature of fWatermark, the use of a circular buffer allows
+	 // for a better chance to find an availble element during a concurrenct call.
+         class Bucket {
+	 public:
+	    Bucket():
+	      fBuffers(0), fWatermark(0), fNumBuffers(0), fNumAvailableBuffers(0)
+	   {}
+	   ~Bucket() {
+	      // delete all buffers
+	      for(auto it = fBuffers; it != fBuffers+fNumBuffers; ++it) {
+		delete [] *it;
+	      }
+	      delete [] fBuffers;
+	   }
+
+	   void init(size_t numBuffers) {
+	      fNumBuffers = numBuffers;
+	      fBuffers = new std::atomic<Buffer_t>[numBuffers];
+	      bzero(fBuffers,sizeof(Buffer_t)*numBuffers);
+	      fWatermark.store( fBuffers + numBuffers);
+	   }
+
+	   bool push(char* buf) {
+	      if(fNumAvailableBuffers >= static_cast<long>(fNumBuffers)) {
+		return false;
+	      }
+	      std::atomic<Buffer_t>* temp = fWatermark.load();
+	      std::atomic<Buffer_t>* startValue = temp;
+	      std::atomic<Buffer_t>* loopEndCondition = fBuffers;
+	      for(int mode =0; mode != 2; ++mode) {
+		 while(temp != loopEndCondition) {
+		   --temp;
+		   Buffer_t previous = nullptr;
+		   if(temp->compare_exchange_strong(previous,buf)) {
+		     ++fNumAvailableBuffers;
+		     fWatermark.compare_exchange_strong(startValue,temp);
+		     return true;
+		   }
+		 }
+		 //Didn't find it so now go back around
+		 temp = fBuffers+fNumBuffers;
+		 loopEndCondition = startValue;
+	      }
+	      return false;
+	   }
+
+	   char* pop() {
+	      if(fNumAvailableBuffers <= 0) {
+	         return nullptr;
+	      }
+	      std::atomic<Buffer_t>* temp = fWatermark.load();
+	      std::atomic<Buffer_t>* startValue = temp;
+	      std::atomic<Buffer_t>* loopEndCondition = fBuffers+fNumBuffers;
+	      for(int mode = 0; mode !=2; ++mode) {
+		 while(temp < loopEndCondition) {
+		    Buffer_t retValue = temp->exchange(nullptr);
+		    if(nullptr != retValue) {
+		       --fNumAvailableBuffers;
+		       fWatermark.compare_exchange_strong(startValue,temp+1);
+		       return retValue;
+		    }
+		    ++temp;
+		 }
+		 //Didn't find it so now go back around
+		 temp = fBuffers;
+		 loopEndCondition = startValue;
+	      }
+	      return 0;
+	   }
+	 private:
+	   typedef char* Buffer_t;
+
+	   std::atomic<Buffer_t>* fBuffers; // array of buffers,
+	   std::atomic<std::atomic<Buffer_t>*> fWatermark; // most recently filled slot
+	   size_t fNumBuffers; // size of fBuffers
+	   std::atomic<long> fNumAvailableBuffers; //number of non null buffers
+	 };
+
+#else
          class Bucket {
          public:
             Bucket():
@@ -92,7 +186,7 @@ namespace Cint {
             Buffer_t* fWatermark; // most recently filled slot
             size_t fNumBuffers; // size of fBuffers
          };
-
+#endif
       private:
          G__BufferReservoir() {
             static size_t numBuffers[fgNumBuckets] = {256, 64, 16, 8, 4, 4, 2};