MergeOp: Avoid (one instance of) scanning all refs

We were scanning all refs just to check that each patch set SHA-1 was
not completely orphaned. Instead, tighten this check slightly and
enforce that a patch set is present at its expected ref name. This
allows us to use a limited exactRef(String...) call rather than
scanning all potentially many thousands of refs.

Change-Id: I5308682b111bd69ade0345c51c63ee341226c1e2

gerrit-server/src/main/java/com/google/gerrit/server/git/MergeOp.java

@@ -16,12 +16,12 @@ package com.google.gerrit.server.git;
 
 import static com.google.common.base.Preconditions.checkState;
 import static com.google.gerrit.server.notedb.ReviewerStateInternal.REVIEWER;
-import static org.eclipse.jgit.lib.RefDatabase.ALL;
 
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Optional;
 import com.google.common.base.Predicate;
 import com.google.common.collect.HashBasedTable;
+import com.google.common.collect.HashMultimap;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Maps;
@@ -581,18 +581,7 @@ public class MergeOp {
       Collection<ChangeData> submitted) throws IntegrationException {
     logDebug("Validating {} changes", submitted.size());
     List<ChangeData> toSubmit = new ArrayList<>(submitted.size());
-
+    Multimap<ObjectId, PatchSet.Id> revisions = getRevisions(submitted);
-    Map<String, Ref> allRefs;
-    try {
-      allRefs = repo.getRefDatabase().getRefs(ALL);
-    } catch (IOException e) {
-      throw new IntegrationException(e.getMessage(), e);
-    }
-
-    Set<ObjectId> tips = new HashSet<>();
-    for (Ref r : allRefs.values()) {
-      tips.add(r.getObjectId());
-    }
 
     SubmitType submitType = null;
     ChangeData choseSubmitTypeFrom = null;
@@ -601,8 +590,7 @@ public class MergeOp {
       Change chg;
       try {
         ctl = cd.changeControl();
-        // Reload change in case index was stale.
+        chg = cd.change();
-        chg = cd.reloadChange();
       } catch (OrmException e) {
         throw new IntegrationException("Failed to validate changes", e);
       }
@@ -641,18 +629,13 @@ public class MergeOp {
         continue;
       }
 
-      if (!tips.contains(id)) {
+      if (!revisions.containsEntry(id, ps.getId())) {
-        // TODO Technically the proper way to do this test is to use a
-        // RevWalk on "$id --not --all" and test for an empty set. But
-        // that is way slower than looking for a ref directly pointing
-        // at the desired tip. We should always have a ref available.
-        //
         // TODO this is actually an error, the branch is gone but we
         // want to merge the issue. We can't safely do that if the
         // tip is not reachable.
         //
         logError("Revision " + idstr + " of patch set " + ps.getId()
-            + " is not contained in any ref");
+            + " does not match " + ps.getId().toRefName());
         commits.put(changeId, CodeReviewCommit.revisionGone(ctl));
         continue;
       }
@@ -705,6 +688,31 @@ public class MergeOp {
     return new AutoValue_MergeOp_BranchBatch(submitType, toSubmit);
   }
 
+  private Multimap<ObjectId, PatchSet.Id> getRevisions(
+      Collection<ChangeData> cds) throws IntegrationException {
+    try {
+      List<String> refNames = new ArrayList<>(cds.size());
+      for (ChangeData cd : cds) {
+        // Reload change in case index was stale.
+        cd.reloadChange();
+        Change c = cd.change();
+        if (c != null) {
+          refNames.add(c.currentPatchSetId().toRefName());
+        }
+      }
+      Multimap<ObjectId, PatchSet.Id> revisions =
+          HashMultimap.create(cds.size(), 1);
+      for (Map.Entry<String, Ref> e : repo.getRefDatabase().exactRef(
+          refNames.toArray(new String[refNames.size()])).entrySet()) {
+        revisions.put(
+            e.getValue().getObjectId(), PatchSet.Id.fromRef(e.getKey()));
+      }
+      return revisions;
+    } catch (IOException | OrmException e) {
+      throw new IntegrationException("Failed to validate changes", e);
+    }
+  }
+
   private SubmitType getSubmitType(ChangeControl ctl, PatchSet ps) {
     try {
       ChangeData cd = changeDataFactory.create(db, ctl);