ATN: Remove race condition in addState(ATNState*)

This is a proposed fix to bug #1826 which removes a race condition where
multiple threads could update ATNState::nextTokenWithinRule, leading to
corrupted std::vector instances in an InstanceSet.

runtime/Cpp/runtime/src/atn/ATN.cpp

@@ -88,10 +88,10 @@ misc::IntervalSet ATN::nextTokens(ATNState *s, RuleContext *ctx) const {
 }
 
 misc::IntervalSet& ATN::nextTokens(ATNState *s) const {
-  if (s->nextTokenWithinRule.isEmpty()) {
-    auto candidate = nextTokens(s, nullptr);
-    if (!candidate.isEmpty() || !s->nextTokenWithinRule.isReadOnly()) {
-      s->nextTokenWithinRule = candidate;
+  if (!s->nextTokenWithinRule.isReadOnly()) {
+    std::unique_lock<std::mutex> lock { _mutex };
+    if (!s->nextTokenWithinRule.isReadOnly()) {
+      s->nextTokenWithinRule = nextTokens(s, nullptr);
       s->nextTokenWithinRule.setReadOnly(true);
     }
   }

runtime/Cpp/runtime/src/atn/ATN.h

@@ -103,6 +103,9 @@ namespace atn {
     virtual misc::IntervalSet getExpectedTokens(size_t stateNumber, RuleContext *context) const;
 
     std::string toString() const;
+
+  private:
+    mutable std::mutex _mutex;
   };
 
 } // namespace atn

runtime/Cpp/runtime/src/misc/IntervalSet.h

@@ -6,6 +6,7 @@
 #pragma once
 
 #include "misc/Interval.h"
+#include <atomic>
 
 namespace antlr4 {
 namespace misc {
@@ -30,7 +31,7 @@ namespace misc {
   protected:
     /// The list of sorted, disjoint intervals.
     std::vector<Interval> _intervals;
-    bool _readonly;
+    std::atomic<bool> _readonly;
 
   public:
     IntervalSet();