Avoid looking up refs for all filtered changes

When filtering a huge list of refs all the closed
changes need to be identified and hidden; however, the mere
lookup of the changes' SHA1s from the repository's ref-database
can cause a considerable system load because of the high
number of change refs.

The initial list of refs to be filtered contains already
all the '/meta' refs and their associated SHA1s hence it can
be reused for looking up the change-id to their latest version
instead of re-reading the same information from the repository
on disk.

Depends-On: https://gerrit-review.googlesource.com/c/gerrit/+/319756
Change-Id: I164addd6abc7c1dff1cfeedcec27415e6b88f3b3

src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ForProjectWrapper.java

diff --git a/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ForProjectWrapper.java b/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ForProjectWrapper.java
index 0809840..3080694 100644
--- a/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ForProjectWrapper.java
+++ b/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ForProjectWrapper.java
@@ -30,8 +30,10 @@
 import com.google.inject.Inject;
 import com.google.inject.assistedinject.Assisted;
 import java.util.Collection;
+import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
+import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
 
@@ -79,6 +81,10 @@
   @Override
   public Collection<Ref> filter(Collection<Ref> refs, Repository repo, RefFilterOptions opts)
       throws PermissionBackendException {
+    Map<Change.Id, ObjectId> changeRevisions =
+        refs.stream()
+            .filter(ref -> ref.getName().endsWith("/meta"))
+            .collect(Collectors.toMap(ForProjectWrapper::changeIdFromRef, Ref::getObjectId));
     return defaultForProject
         .filter(refs, repo, opts)
         .parallelStream()
@@ -87,13 +93,20 @@
         .filter(config::isRefToShow)
         .filter(
             (ref) -> {
+              Change.Id changeId = changeIdFromRef(ref);
               String refName = ref.getName();
               return (!isChangeRef(refName)
-                  || (!isChangeMetaRef(refName) && isOpen(repo, refName)));
+                  || (!isChangeMetaRef(refName)
+                      && changeId != null
+                      && isOpen(repo, changeId, changeRevisions.get(changeId))));
             })
         .collect(Collectors.toList());
   }
 
+  private static Change.Id changeIdFromRef(Ref ref) {
+    return Change.Id.fromRef(ref.getName());
+  }
+
   private boolean isChangeRef(String changeKey) {
     return changeKey.startsWith("refs/changes");
   }
@@ -102,14 +115,14 @@
     return isChangeRef(changeKey) && changeKey.endsWith("/meta");
   }
 
-  private boolean isOpen(Repository repo, String refName) {
+  private boolean isOpen(Repository repo, Change.Id changeId, ObjectId changeRevision) {
     try {
-      Change.Id changeId = Change.Id.fromRef(refName);
-      ChangeNotes changeNotes = changeNotesFactory.create(repo, project, changeId);
+      ChangeNotes changeNotes =
+          changeNotesFactory.createChecked(repo, project, changeId, changeRevision);
       return changeNotes.getChange().getStatus().isOpen();
     } catch (NoSuchChangeException e) {
       logger.atWarning().withCause(e).log(
-          "Ref %s refers to a non-existent change: hiding from the advertised refs", refName);
+          "Change %d does not exist: hiding from the advertised refs", changeId);
       return false;
     }
   }