Load an arbitrary version of change notes

The SHA1 of the change notes to load may be already
known to the caller of ChangeNotes.create() method hence
it can be passed as a version parameter to avoid an unneeded
ref lookup operation.

When the version passed isn't known, it can be given as
null parameter causing the lookup of the latest /meta ref
SHA1 from the repository's ref-database.

Allow skipping potentially hundreds of thousands of refs lookups
for large mono-repos when filtering refs from the repository based
on the change status.

Also document the different variants of ChangeNotes.create()
methods, highlighting their specific purpose and use-case.

This is a partial cherry-pick of Ia54acb37e1187.

Original-Author: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I359578a05ecad595fb64909398ea3fd069ff1a5f

4 files changed, 39 insertions, 14 deletions

diff --git a/java/com/google/gerrit/server/notedb/AbstractChangeNotes.java b/java/com/google/gerrit/server/notedb/AbstractChangeNotes.java
index a5acbe3ee3..e81160abaa 100644
--- a/java/com/google/gerrit/server/notedb/AbstractChangeNotes.java
+++ b/java/com/google/gerrit/server/notedb/AbstractChangeNotes.java
@@ -118,9 +118,10 @@ public abstract class AbstractChangeNotes<T> {
   private ObjectId revision;
   private boolean loaded;
 
-  protected AbstractChangeNotes(Args args, Change.Id changeId) {
+  protected AbstractChangeNotes(Args args, Change.Id changeId, @Nullable ObjectId metaSha1) {
     this.args = requireNonNull(args);
     this.changeId = requireNonNull(changeId);
+    this.revision = metaSha1;
   }
 
   public Change.Id getChangeId() {
@@ -152,7 +153,7 @@ public abstract class AbstractChangeNotes<T> {
     try (Timer0.Context timer = args.metrics.readLatency.start();
         // Call openHandle even if reading is disabled, to trigger
         // auto-rebuilding before this object may get passed to a ChangeUpdate.
-        LoadHandle handle = openHandle(repo)) {
+        LoadHandle handle = openHandle(repo, revision)) {
       revision = handle.id();
       onLoad(handle);
       loaded = true;
@@ -174,15 +175,16 @@ public abstract class AbstractChangeNotes<T> {
    * <p>Implementations may override this method to provide auto-rebuilding behavior.
    *
    * @param repo open repository.
+   * @param id version SHA1 of the change notes to load
    * @return handle for reading the entity.
    * @throws NoSuchChangeException change does not exist.
    * @throws IOException a repo-level error occurred.
    */
-  protected LoadHandle openHandle(Repository repo) throws NoSuchChangeException, IOException {
-    return openHandle(repo, readRef(repo));
-  }
-
-  protected LoadHandle openHandle(Repository repo, ObjectId id) {
+  protected LoadHandle openHandle(Repository repo, @Nullable ObjectId id)
+      throws NoSuchChangeException, IOException {
+    if (id == null) {
+      id = readRef(repo);
+    }
     return new LoadHandle(repo, id);
   }
 
diff --git a/java/com/google/gerrit/server/notedb/ChangeNotes.java b/java/com/google/gerrit/server/notedb/ChangeNotes.java
index 95095686e3..8eb5c5cdaa 100644
--- a/java/com/google/gerrit/server/notedb/ChangeNotes.java
+++ b/java/com/google/gerrit/server/notedb/ChangeNotes.java
@@ -110,9 +110,23 @@ public class ChangeNotes extends AbstractChangeNotes<ChangeNotes> {
       return createChecked(c.getProject(), c.getId());
     }
 
-    public ChangeNotes createChecked(Project.NameKey project, Change.Id changeId) {
+    public ChangeNotes createChecked(
+        Repository repo,
+        Project.NameKey project,
+        Change.Id changeId,
+        @Nullable ObjectId metaRevId) {
       Change change = newChange(project, changeId);
-      return new ChangeNotes(args, change, true, null).load();
+      return new ChangeNotes(args, change, true, null, metaRevId).load(repo);
+    }
+
+    public ChangeNotes createChecked(
+        Project.NameKey project, Change.Id changeId, @Nullable ObjectId metaRevId) {
+      Change change = newChange(project, changeId);
+      return new ChangeNotes(args, change, true, null, metaRevId).load();
+    }
+
+    public ChangeNotes createChecked(Project.NameKey project, Change.Id changeId) {
+      return createChecked(project, changeId, null);
     }
 
     public static Change newChange(Project.NameKey project, Change.Id changeId) {
@@ -344,14 +358,23 @@ public class ChangeNotes extends AbstractChangeNotes<ChangeNotes> {
   private ImmutableListMultimap<PatchSet.Id, PatchSetApproval> approvals;
   private ImmutableSet<Comment.Key> commentKeys;
 
-  @VisibleForTesting
-  public ChangeNotes(Args args, Change change, boolean shouldExist, @Nullable RefCache refs) {
-    super(args, change.getId());
+  public ChangeNotes(
+      Args args,
+      Change change,
+      boolean shouldExist,
+      @Nullable RefCache refs,
+      @Nullable ObjectId metaSha1) {
+    super(args, change.getId(), metaSha1);
     this.change = new Change(change);
     this.shouldExist = shouldExist;
     this.refs = refs;
   }
 
+  @VisibleForTesting
+  public ChangeNotes(Args args, Change change, boolean shouldExist, @Nullable RefCache refs) {
+    this(args, change, shouldExist, refs, null);
+  }
+
   public Change getChange() {
     return change;
   }
diff --git a/java/com/google/gerrit/server/notedb/DraftCommentNotes.java b/java/com/google/gerrit/server/notedb/DraftCommentNotes.java
index 9b403e8ce4..49884068a1 100644
--- a/java/com/google/gerrit/server/notedb/DraftCommentNotes.java
+++ b/java/com/google/gerrit/server/notedb/DraftCommentNotes.java
@@ -59,7 +59,7 @@ public class DraftCommentNotes extends AbstractChangeNotes<DraftCommentNotes> {
   }
 
   DraftCommentNotes(Args args, Change.Id changeId, Account.Id author, @Nullable Ref ref) {
-    super(args, changeId);
+    super(args, changeId, null);
     this.author = requireNonNull(author);
     this.ref = ref;
     if (ref != null) {
diff --git a/java/com/google/gerrit/server/notedb/RobotCommentNotes.java b/java/com/google/gerrit/server/notedb/RobotCommentNotes.java
index fe0564356e..d53b2caaf6 100644
--- a/java/com/google/gerrit/server/notedb/RobotCommentNotes.java
+++ b/java/com/google/gerrit/server/notedb/RobotCommentNotes.java
@@ -47,7 +47,7 @@ public class RobotCommentNotes extends AbstractChangeNotes<RobotCommentNotes> {
 
   @Inject
   RobotCommentNotes(Args args, @Assisted Change change) {
-    super(args, change.getId());
+    super(args, change.getId(), null);
     this.change = change;
   }