Prevent posting reviews on change edits

Posting a review triggers a stream event, but this fails with a
NullPointerException when the creation date is set for the event since
the change edit does not have a creation date.

Posting reviews on a change edit and sending a stream event does not
make sense since the change edit is not visible to anyone except the
author.

Change-Id: I8d2de5c0368d2f4211bc756ff9e989350e18df0d
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/rest-api-changes.txt

@@ -2067,6 +2067,9 @@ describes the applied labels.
   }
 ----
 
+A review cannot be set on a change edit. Trying to post a review for a
+change edit fails with `409 Conflict`.
+
 [[rebase-revision]]
 === Rebase Revision
 --

gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java

@@ -37,6 +37,7 @@ import com.google.gerrit.extensions.api.changes.ReviewInput.NotifyHandling;
 import com.google.gerrit.extensions.common.Comment.Side;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.BadRequestException;
+import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.extensions.restapi.RestModifyView;
 import com.google.gerrit.extensions.restapi.UnprocessableEntityException;
 import com.google.gerrit.extensions.restapi.Url;
@@ -130,8 +131,11 @@ public class PostReview implements RestModifyView<RevisionResource, ReviewInput>
 
   @Override
   public Output apply(RevisionResource revision, ReviewInput input)
-      throws AuthException, BadRequestException, UnprocessableEntityException,
-      OrmException, IOException {
+      throws AuthException, BadRequestException, ResourceConflictException,
+      UnprocessableEntityException, OrmException, IOException {
+    if (revision.getEdit().isPresent()) {
+      throw new ResourceConflictException("cannot post review on edit");
+    }
     if (input.onBehalfOf != null) {
       revision = onBehalfOf(revision, input);
     }