Log external ID differential cache loader failure

We've investigated this issue and tried different ways of getting this
to fail, but haven't been successful.

We've investigated the algorithm, but haven't found any faults. We also
considered concurrency, but as we lock the cache before calling the
differential loader, we're not sure how that could be the issue.

We've managed to recreate the issue by fudging the input to the private
method, where we pass a key to `additions` that is already present in
the cache, but we don't see how that would be possible from the public
`load` entrypoint.

In order to continue the investigation we require more information, we
have opted to add some further logs when the differential cache loader
errors.

We now log the tip of the external ID cache as well as the external ID
ref on disk. We also log the set of additions, to confirm a key clash.

Bug: Issue 16384
Release-Notes: skip
Change-Id: I3836f28285b4fdcfd2e26720bdbf82cf046960c7

1 files changed, 13 insertions, 2 deletions

diff --git a/java/com/google/gerrit/server/account/externalids/ExternalIdCacheLoader.java b/java/com/google/gerrit/server/account/externalids/ExternalIdCacheLoader.java
index 27672bdcf6..43e316a978 100644
--- a/java/com/google/gerrit/server/account/externalids/ExternalIdCacheLoader.java
+++ b/java/com/google/gerrit/server/account/externalids/ExternalIdCacheLoader.java
@@ -43,7 +43,9 @@ import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.TimeUnit;
+import java.util.stream.Collectors;
 import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectReader;
@@ -184,8 +186,17 @@ public class ExternalIdCacheLoader {
         }
       }
 
-      AllExternalIds allExternalIds =
-          buildAllExternalIds(repo, oldExternalIds, additions, removals);
+      AllExternalIds allExternalIds;
+      try {
+        allExternalIds = buildAllExternalIds(repo, oldExternalIds, additions, removals);
+      } catch (IllegalArgumentException e) {
+        Set<String> additionKeys =
+            additions.keySet().stream().map(AnyObjectId::getName).collect(Collectors.toSet());
+        logger.atSevere().withCause(e).log(
+            "Failed to load external ID cache. Repository ref is %s, cache ref is %s, additions are %s",
+            extIdRef.getObjectId().getName(), parentWithCacheValue.getId().getName(), additionKeys);
+        throw e;
+      }
       reloadCounter.increment(true);
       reloadDifferential.record(System.nanoTime() - start, TimeUnit.NANOSECONDS);
       return allExternalIds;