ChangesCollection: Return different text on multiple changes

Users are often confused when looking up a change just says "Not
found" when they are sure a change exists, but weren't aware that a
search returned multiple changes. Distinguish between these two cases
by changing the human-readable text in the response.

Change-Id: I87f6cd231be026e2bb48f26e18f11a5485aeb0e9

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java

@@ -39,6 +39,7 @@ import com.google.gerrit.extensions.api.changes.AddReviewerInput;
 import com.google.gerrit.extensions.api.changes.RebaseInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput;
 import com.google.gerrit.extensions.api.changes.RevisionApi;
+import com.google.gerrit.extensions.api.projects.BranchInput;
 import com.google.gerrit.extensions.client.ChangeStatus;
 import com.google.gerrit.extensions.client.ListChangesOption;
 import com.google.gerrit.extensions.common.AccountInfo;
@@ -50,6 +51,7 @@ import com.google.gerrit.extensions.common.LabelInfo;
 import com.google.gerrit.extensions.common.RevisionInfo;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
+import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.AccountGroup;
 import com.google.gerrit.reviewdb.client.Change;
@@ -94,6 +96,30 @@ public class ChangeIT extends AbstractDaemonTest {
     assertThat(c.owner.avatars).isNull();
   }
 
+  @Test
+  public void getAmbiguous() throws Exception {
+    PushOneCommit.Result r1 = createChange();
+    String changeId = r1.getChangeId();
+    gApi.changes().id(changeId).get();
+
+    BranchInput b = new BranchInput();
+    b.revision = repo().getRef("HEAD").getObjectId().name();
+    gApi.projects()
+        .name(project.get())
+        .branch("other")
+        .create(b);
+
+    PushOneCommit push2 = pushFactory.create(db, admin.getIdent(), testRepo,
+        PushOneCommit.SUBJECT, PushOneCommit.FILE_NAME,
+        PushOneCommit.FILE_CONTENT, changeId);
+    PushOneCommit.Result r2 = push2.to("refs/for/other");
+    assertThat(r2.getChangeId()).isEqualTo(changeId);
+
+    exception.expect(ResourceNotFoundException.class);
+    exception.expectMessage("Multiple changes found for " + changeId);
+    gApi.changes().id(changeId).get();
+  }
+
   @Test
   public void abandon() throws Exception {
     PushOneCommit.Result r = createChange();

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

@@ -92,9 +92,12 @@ public class ChangesCollection implements
         }
       }
     }
-    if (ctls.size() != 1) {
+    if (ctls.isEmpty()) {
       throw new ResourceNotFoundException(id);
     }
+    if (ctls.size() != 1) {
+      throw new ResourceNotFoundException("Multiple changes found for " + id);
+    }
 
     ChangeControl ctl = ctls.get(0);
     if (!ctl.isVisible(db.get())) {