NoteDb: Avoid writing partial change graphs

If we naively turn on notedb.writeChanges without previously running
RebuildNoteDb to backfill existing changes, a single ChangeUpdate may
end up writing a partial note graph, for example containing a single
commit with only a "Topic" field and no other information. Such a
partial graph will almost certainly fail to parse; better to avoid
writing them out in the first place.

Teach NoteDbUpdateManager to skip writing updates unless the first
update in the list has a special allowWriteToNewRef. This means
writing a change will automatically "turn on" on a per-change basis as
soon as a backfill is completed, whether in a running server or in
between restarts.

There are three cases where we want to allow writing updates to new
refs:
 - Any ChangeDraftUpdate, since having a "partial" draft notes ref
   doesn't cause parsing to fail, and executing the same update
   multiple times is idempotent with respect to the note data.
 - A ChangeUpdate that creates a change for the first time, which is
   by definition not a partial graph. This is handled in BatchUpdate,
   which already knows which ChangeUpdates correspond to new changes.
 - In ChangeRebuilder, which again is guaranteed to write a complete
   graph.

Change-Id: Ic8c6c28e68aef22be7569d2b0cb409eae22e3034

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

@@ -204,7 +204,9 @@ public abstract class AbstractChangeNotesTest extends GerritBaseTests {
 
   protected ChangeUpdate newUpdate(Change c, CurrentUser user)
       throws Exception {
-    return TestChanges.newUpdate(injector, c, user);
+    ChangeUpdate update = TestChanges.newUpdate(injector, c, user);
+    update.setAllowWriteToNewRef(true);
+    return update;
   }
 
   protected ChangeNotes newNotes(Change c) throws OrmException {

gerrit-server/src/test/java/com/google/gerrit/testutil/NoteDbChecker.java

@@ -14,13 +14,18 @@
 
 package com.google.gerrit.testutil;
 
+import static com.google.common.truth.Truth.assertThat;
+
 import com.google.common.base.Joiner;
 import com.google.common.collect.Iterables;
 import com.google.gerrit.reviewdb.client.Change;
+import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.reviewdb.server.ReviewDbUtil;
 import com.google.gerrit.server.PatchLineCommentsUtil;
+import com.google.gerrit.server.git.GitRepositoryManager;
 import com.google.gerrit.server.notedb.ChangeBundle;
+import com.google.gerrit.server.notedb.ChangeNoteUtil;
 import com.google.gerrit.server.notedb.ChangeNotes;
 import com.google.gerrit.server.notedb.ChangeRebuilder;
 import com.google.gerrit.server.schema.DisabledChangesReviewDbWrapper;
@@ -29,6 +34,7 @@ import com.google.inject.Provider;
 import com.google.inject.Singleton;
 
 import org.eclipse.jgit.errors.RepositoryNotFoundException;
+import org.eclipse.jgit.lib.Repository;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -41,6 +47,7 @@ public class NoteDbChecker {
   static final Logger log = LoggerFactory.getLogger(NoteDbChecker.class);
 
   private final Provider<ReviewDb> dbProvider;
+  private final GitRepositoryManager repoManager;
   private final TestNotesMigration notesMigration;
   private final ChangeNotes.Factory notesFactory;
   private final ChangeRebuilder changeRebuilder;
@@ -48,11 +55,13 @@ public class NoteDbChecker {
 
   @Inject
   NoteDbChecker(Provider<ReviewDb> dbProvider,
+      GitRepositoryManager repoManager,
       TestNotesMigration notesMigration,
       ChangeNotes.Factory notesFactory,
       ChangeRebuilder changeRebuilder,
       PatchLineCommentsUtil plcUtil) {
     this.dbProvider = dbProvider;
+    this.repoManager = repoManager;
     this.notesMigration = notesMigration;
     this.notesFactory = notesFactory;
     this.changeRebuilder = changeRebuilder;
@@ -106,6 +115,14 @@ public class NoteDbChecker {
     checkActual(readExpected(changeIds), new ArrayList<String>());
   }
 
+  public void assertNoChangeRef(Project.NameKey project, Change.Id changeId)
+      throws Exception {
+    try (Repository repo = repoManager.openMetadataRepository(project)) {
+      assertThat(repo.exactRef(ChangeNoteUtil.changeRefName(changeId)))
+          .isNull();
+    }
+  }
+
   private List<ChangeBundle> readExpected(Iterable<Change.Id> changeIds)
       throws Exception {
     ReviewDb db = unwrapDb();