fix: fix flaky tests

Author: zepfred

core/src/test/java/ai/timefold/solver/core/testutil/AbstractMeterTest.java

@@ -8,6 +8,8 @@
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.parallel.Execution;
+import org.junit.jupiter.api.parallel.ExecutionMode;
 import org.junit.jupiter.api.parallel.ResourceLock;
 
 import io.micrometer.core.instrument.Metrics;
@@ -16,7 +18,10 @@
 import io.micrometer.core.instrument.simple.SimpleConfig;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
 
-@ResourceLock(value = "metrics") // Mutual exclusion to prevent flaky tests.
+// Mutual exclusion to prevent flaky tests from different test suites
+@ResourceLock(value = "metrics")
+// We also want to ensure that tests of the same suite do not update the registry concurrently
+@Execution(ExecutionMode.SAME_THREAD)
 public abstract class AbstractMeterTest {
 
     @BeforeEach // To guard against nasty tests which do not do this.

core/src/test/java/ai/timefold/solver/core/testutil/PlannerTestUtils.java

@@ -70,11 +70,12 @@ public static <Solution_> SolverConfig buildSolverConfig(Class<Solution_> soluti
                                 new TerminationConfig().withStepCountLimit(TERMINATION_STEP_COUNT_LIMIT)));
     }
 
-    public static <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem) {
+    public static synchronized <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem) {
         return solve(solverConfig, problem, true);
     }
 
-    public static <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem, boolean bestSolutionEventExists) {
+    public static synchronized <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem,
+            boolean bestSolutionEventExists) {
         SolverFactory<Solution_> solverFactory = SolverFactory.create(solverConfig);
         var solver = solverFactory.buildSolver();
         var eventBestSolutionRef = new AtomicReference<Solution_>();
@@ -100,10 +101,10 @@ public static TestdataSolution generateTestdataSolution(String code, int entityA
         var solution = new TestdataSolution(code);
         solution.setValueList(IntStream.range(1, entityAndValueCount + 1)
                 .mapToObj(i -> new TestdataValue("v" + i))
-                .collect(Collectors.toList()));
+                .toList());
         solution.setEntityList(IntStream.range(1, entityAndValueCount + 1)
                 .mapToObj(i -> new TestdataEntity("e" + i))
-                .collect(Collectors.toList()));
+                .toList());
         return solution;
     }