Deploydb should gracefully handle error when create artifact is called twice

for the same artifact
  - Fixes #153

features/api/artifact/creating.feature

@@ -199,3 +199,18 @@ Feature: Artifact CREATE APIs
       }
     """
     Then the response should be 404
+
+    @error
+    Scenario: Creating an existing artifact should fail
+
+     Given there is an artifact
+     When I POST to "/api/artifacts" with:
+    """
+      {
+        "group" : "com.example.cucumber",
+        "name" : "cucumber-artifact",
+        "version" : "1.0.1",
+        "sourceUrl" : "http://example.com/maven/com.example.cucumber/cucumber-artifact/1.0.1/cucumber-artifact-1.0.1.jar"
+      }
+    """
+    Then the response should be 409

src/main/groovy/deploydb/WorkFlow.groovy

@@ -357,6 +357,11 @@ class WorkFlow {
      */
     void triggerArtifactCreated(models.Artifact artifact) {
 
+        /* if artifacts already exists, send 409 back  */
+        if (artifactDAO.artifactExists(artifact.group, artifact.name, artifact.version)) {
+            throw new WebApplicationException(Response.Status.CONFLICT)
+        }
+
         /* Lookup Service(s) */
         List<models.Service> services = this.serviceRegistry.getAll().findAll() { models.Service service ->
             service.artifacts.find() { String artifactIdent ->

src/main/groovy/deploydb/dao/ArtifactDAO.groovy

@@ -69,4 +69,9 @@ class ArtifactDAO extends AbstractDAO<Artifact> {
         return artifacts
     }
 
+    boolean artifactExists(String group, String name, String version) {
+        return criteria().add(Restrictions.eq('group', group))
+                  .add(Restrictions.eq('name', name))
+                  .add(Restrictions.eq('version', version)).uniqueResult() == null ? false: true
+    }
 }