Fix NPE in SnapshottingInstanceRepository

fixes #1125

Author: joshiste

spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/domain/entities/SnapshottingInstanceRepository.java

@@ -83,14 +83,14 @@ public void stop() {
     }
 
     protected Mono<Instance> rehydrateSnapshot(InstanceId id) {
-        Instance outdatedSnapshot = this.snapshots.get(id);
-        return super.find(id).map(instance -> {
-            if (this.snapshots.replace(id, outdatedSnapshot, instance)) {
+        return super.find(id).map(instance -> this.snapshots.compute(id, (key, snapshot) -> {
+            //check if the loaded version hasn't been already outdated by a snapshot
+            if (snapshot == null || instance.getVersion() > snapshot.getVersion()) {
                 return instance;
             } else {
-                return this.snapshots.get(id);
+                return snapshot;
             }
-        });
+        }));
     }
 
     protected void updateSnapshot(InstanceEvent event) {

spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/domain/entities/SnapshottingInstanceRepositoryTest.java

@@ -21,6 +21,7 @@
 import de.codecentric.boot.admin.server.domain.values.Registration;
 import de.codecentric.boot.admin.server.domain.values.StatusInfo;
 import de.codecentric.boot.admin.server.eventstore.InMemoryEventStore;
+import de.codecentric.boot.admin.server.eventstore.OptimisticLockingException;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
@@ -100,6 +101,16 @@ public void should_update_cache_after_error() {
                     .verifyComplete();
     }
 
+    @Test
+    public void should_return_outdated_instance_not_present_in_cache() {
+        this.repository.stop();
+        //given
+        StepVerifier.create(this.repository.save(this.instance)).expectNext(this.instance).verifyComplete();
+        StepVerifier.create(this.repository.save(this.instance)).verifyError(OptimisticLockingException.class);
+        //when
+        StepVerifier.create(this.repository.find(this.instance.getId())).expectNext(this.instance).verifyComplete();
+    }
+
     @Test
     public void should_refresh_snapshots_eagerly_on_optimistick_lock_exception() {
         //given