add test for cached unificator thread safety

unify/src/jvmTest/kotlin/it/unibo/tuprolog/unify/TestThreadSafety.kt

@@ -0,0 +1,31 @@
+package it.unibo.tuprolog.unify
+
+import it.unibo.tuprolog.core.Substitution
+import it.unibo.tuprolog.unify.testutils.UnificatorUtils.successfulUnifications
+import java.util.*
+import java.util.concurrent.Executors
+import java.util.concurrent.Future
+import java.util.concurrent.TimeUnit
+import kotlin.test.Test
+import kotlin.test.assertTrue
+
+class TestThreadSafety {
+    @Test
+    fun testCachedUnificatorThreadSafety() {
+        val executors = Executors.newFixedThreadPool(8)
+        val results: MutableList<Future<Substitution>> = Collections.synchronizedList(LinkedList())
+        val unificator = Unificator.cached(Unificator.strict())
+        for (i in 1 .. 100) {
+            for (equation in successfulUnifications.keys) {
+                results.add(executors.submit<Substitution> {
+                    unificator.mgu(equation.lhs, equation.rhs)
+                })
+            }
+        }
+        executors.shutdown()
+        executors.awaitTermination(1, TimeUnit.MINUTES)
+        for (res in results) {
+            assertTrue { res.get() is Substitution.Unifier }
+        }
+    }
+}