fix: Catch timer exceptions (Eppo-exp#85)

* fix: catch timer exception

* test timer exception

Author: typotter

src/main/java/cloud/eppo/FetchConfigurationTask.java

@@ -32,7 +32,12 @@ public void scheduleNext() {
     long delay = this.intervalInMillis - jitter;
     FetchConfigurationTask nextTask =
         new FetchConfigurationTask(runnable, timer, intervalInMillis, jitterInMillis);
-    timer.schedule(nextTask, delay);
+
+    try {
+      timer.schedule(nextTask, delay);
+    } catch (IllegalStateException e) {
+      log.error("[Eppo SDK] Error scheduling next fetch task", e);
+    }
   }
 
   @Override

src/test/java/cloud/eppo/BaseEppoClientTest.java

@@ -22,6 +22,7 @@
 import java.util.*;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionException;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.stream.Stream;
 import okhttp3.mockwebserver.MockResponse;
 import okhttp3.mockwebserver.MockWebServer;
@@ -570,6 +571,33 @@ public void testAssignmentLogErrorNonFatal() {
     verify(mockAssignmentLogger, times(1)).logAssignment(assignmentLogCaptor.capture());
   }
 
+  @Test
+  public void testGracefulPolling() {
+    Timer pollTimer = new Timer();
+    final AtomicInteger callCount = new AtomicInteger(0);
+    Runnable runnerTask =
+        new Runnable() {
+          @Override
+          public void run() {
+            callCount.incrementAndGet();
+          }
+        };
+
+    FetchConfigurationTask task = new FetchConfigurationTask(runnerTask, pollTimer, 50, 5);
+
+    // Trigger an unexpected state; the timer is cancelled but the FetchConfigurationTask attempts
+    // to schedule a runnable.
+    pollTimer.cancel();
+    task.scheduleNext();
+
+    sleepUninterruptedly(50);
+
+    // If the Timer has been cancelled, FetchConfigurationTask doesn't  attempt to reschedule.
+    assertEquals(0, callCount.get());
+
+    // No exception to be thrown if illegal timer state is properly caught.
+  }
+
   @Test
   public void testPolling() {
     EppoHttpClient httpClient = mockHttpResponse(BOOL_FLAG_CONFIG);