Change kind cache: short-circuit on root commits

See our stack trace at

https://phabricator.wikimedia.org/P5612

It's leading us to this
https://github.com/GerritCodeReview/gerrit/blob/c7746ffec770953c22d9b88d658e6f526db09a59/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java#L261
Line

Bug: Issue 8558
Change-Id: I4861b98e309a32c3c11a40dd0e4675c5ecb846a8

1 files changed, 9 insertions, 2 deletions

diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java
index 030ddd2a15..c75a41342d 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java
@@ -224,8 +224,15 @@ public class ChangeKindCacheImpl implements ChangeKindCache {
           return ChangeKind.NO_CHANGE;
         }
 
-        if ((prior.getParentCount() != 1 || next.getParentCount() != 1)
-            && (!onlyFirstParentChanged(prior, next) || prior.getParentCount() == 0)) {
+        if (prior.getParentCount() == 0 || next.getParentCount() == 0) {
+          // At this point we have considered all the kinds that could be applicable to root
+          // commits; the remainder of the checks in this method all assume that both commits have
+          // at least one parent.
+          return ChangeKind.REWORK;
+        }
+
+        if ((prior.getParentCount() > 1 || next.getParentCount() > 1)
+            && !onlyFirstParentChanged(prior, next)) {
           // Trivial rebases done by machine only work well on 1 parent.
           return ChangeKind.REWORK;
         }