Take care of the case where another thread inserts the same key between unlock()/lock()

parallel_hashmap/phmap.h

@@ -3805,9 +3805,18 @@ protected:
         mutexlock    = std::move(typename Lockable::ReadWriteLock(inner));
         size_t offset = set._find_key(key, hashval);
         if (offset == (size_t)-1) {
-            mutexlock.switch_to_unique();
-            offset = set.prepare_insert(hashval);
-            return std::make_tuple(&inner, offset, true);
+            if (mutexlock.switch_to_unique()) {
+                // we did an unlock/lock, and another thread could have inserted the same key, so we need to
+                // do a find() again.
+                offset = set._find_key(key, hashval);
+                if (offset == (size_t)-1) {
+                    offset = set.prepare_insert(hashval);
+                    return std::make_tuple(&inner, offset, true);
+                }
+            } else {
+                offset = set.prepare_insert(hashval);
+                return std::make_tuple(&inner, offset, true);
+            }
         }
         return std::make_tuple(&inner, offset, false);
     }

parallel_hashmap/phmap_base.h

@@ -4635,7 +4635,7 @@ public:
         void unlock() {}
         void lock_shared() {}
         void unlock_shared() {}
-        void switch_to_unique() {}
+        bool switch_to_unique() { return false; }
     };
 
     // ----------------------------------------------------
@@ -4711,7 +4711,7 @@ public:
 
         mutex_type *mutex() const noexcept { return m_; }
         
-        void switch_to_unique() {}
+        bool switch_to_unique() { return false; }
 
     private:
         mutex_type *m_;
@@ -4791,7 +4791,7 @@ public:
 
         mutex_type *mutex() const noexcept { return m_; }
 
-        void switch_to_unique() {}
+        bool switch_to_unique() { return false; }
 
     private:
         mutex_type *m_;
@@ -4873,10 +4873,11 @@ public:
 
         mutex_type *mutex() const noexcept { return m_; }
 
-        void switch_to_unique() {
+        bool switch_to_unique() {
             assert(locked_shared_);
             unlock_shared();
             lock();
+            return true;
         }