ChangeBundle: Handle leading whitespace in ReviewDb topics

JGit's footer parser trims leading whitespace, so a topic with leading
whitespace in ReviewDb is trimmed in ReviewDb.

Change-Id: I9566c67227b1b46a202bd33b0cc924d3c0a798ca

gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeBundle.java

@@ -489,7 +489,9 @@ public class ChangeBundle {
       aSubj = s.trimLeadingFrom(aSubj);
       excludeSubject = bSubj.startsWith(aSubj);
       excludeOrigSubj = true;
-      excludeTopic = "".equals(a.getTopic()) && b.getTopic() == null;
+      String aTopic = trimLeadingOrNull(a.getTopic());
+      excludeTopic = Objects.equals(aTopic, b.getTopic())
+          || "".equals(aTopic) && b.getTopic() == null;
       aUpdated = bundleA.getLatestTimestamp();
     } else if (bundleA.source == NOTE_DB && bundleB.source == REVIEW_DB) {
       excludeCreatedOn = !timestampsDiffer(
@@ -497,7 +499,9 @@ public class ChangeBundle {
       bSubj = s.trimLeadingFrom(bSubj);
       excludeSubject = aSubj.startsWith(bSubj);
       excludeOrigSubj = true;
-      excludeTopic = a.getTopic() == null && "".equals(b.getTopic());
+      String bTopic = trimLeadingOrNull(b.getTopic());
+      excludeTopic = Objects.equals(bTopic, a.getTopic())
+          || a.getTopic() == null && "".equals(bTopic);
       bUpdated = bundleB.getLatestTimestamp();
     }
 
@@ -530,6 +534,10 @@ public class ChangeBundle {
     }
   }
 
+  private static String trimLeadingOrNull(String s) {
+    return s != null ? CharMatcher.whitespace().trimLeadingFrom(s) : null;
+  }
+
   /**
    * Set of fields that must always exactly match between ReviewDb and NoteDb.
    * <p>

gerrit-server/src/test/java/com/google/gerrit/server/notedb/ChangeBundleTest.java

@@ -271,6 +271,54 @@ public class ChangeBundleTest {
     assertDiffs(b1, b2,
         "topic differs for Change.Id " + c1.getId() + ":"
             + " {topic} != {null}");
+
+    // Null is equal to a string that is all whitespace.
+    Change c4 = clone(c1);
+    c4.setTopic("  ");
+    b1 = new ChangeBundle(c4, messages(), patchSets(), approvals(), comments(),
+        reviewers(), REVIEW_DB);
+    b2 = new ChangeBundle(c2, messages(), patchSets(), approvals(), comments(),
+        reviewers(), NOTE_DB);
+    assertNoDiffs(b1, b2);
+    assertNoDiffs(b2, b1);
+  }
+
+  @Test
+  public void diffChangesIgnoresLeadingWhitespaceInReviewDbTopics()
+      throws Exception {
+    Change c1 = TestChanges.newChange(
+        new Project.NameKey("project"), new Account.Id(100));
+    c1.setTopic(" abc");
+    Change c2 = clone(c1);
+    c2.setTopic("abc");
+
+    // Both ReviewDb, exact match required.
+    ChangeBundle b1 = new ChangeBundle(c1, messages(), patchSets(), approvals(),
+        comments(), reviewers(), REVIEW_DB);
+    ChangeBundle b2 = new ChangeBundle(c2, messages(), patchSets(), approvals(),
+        comments(), reviewers(), REVIEW_DB);
+    assertDiffs(b1, b2,
+        "topic differs for Change.Id " + c1.getId() + ":"
+            + " { abc} != {abc}");
+
+    // Leading whitespace in ReviewDb topic is ignored.
+    b1 = new ChangeBundle(c1, messages(), patchSets(), approvals(), comments(),
+        reviewers(), REVIEW_DB);
+    b2 = new ChangeBundle(c2, messages(), patchSets(), approvals(), comments(),
+        reviewers(), NOTE_DB);
+    assertNoDiffs(b1, b2);
+    assertNoDiffs(b2, b1);
+
+    // Must match except for the leading whitespace.
+    Change c3 = clone(c1);
+    c3.setTopic("cba");
+    b1 = new ChangeBundle(c1, messages(), patchSets(), approvals(), comments(),
+        reviewers(), REVIEW_DB);
+    b2 = new ChangeBundle(c3, messages(), patchSets(), approvals(), comments(),
+        reviewers(), NOTE_DB);
+    assertDiffs(b1, b2,
+        "topic differs for Change.Id " + c1.getId() + ":"
+            + " { abc} != {cba}");
   }
 
   @Test