SERVER-47694: fix multikey. again

Split the single _isMultikey variable on an IndexCatalogEntry(Impl) into two
separate variables: _isMultikeyForReader and _isMultikeyForWriter.

_isMultikeyForReader is flipped as early as possible. Readers concurrent
with multikey flipping may forgo a possible optimization when their snapshot
sees no multikey data.

_isMultikeyForWriter is flipped after the storage engine commits a multikey
change to the on-disk catalog. At this point, writers may, under some
circumstances, optimize away some catalog writes.

Move logic for optimizing readers (multikey paths, clearing query cache) outside
of the onCommit.

Adds a failpoint widenWUOWChangesWindow which sleeps transaction commit and
onCommit/onRollback handlers.

Have validate assert multikey paths are set correctly for the documents observed
during its collection scan.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/mongo/db/multi_key_path_tracker.h b/src/mongo/db/multi_key_path_tracker.h
index b155bc1fe58..d6efe1e71db 100644
--- a/src/mongo/db/multi_key_path_tracker.h
+++ b/src/mongo/db/multi_key_path_tracker.h
@@ -69,6 +69,11 @@ public:
      */
     static bool isMultikeyPathsTrivial(const MultikeyPaths& paths);
 
+    /**
+     * Return true iff the child's paths are a subset of the parent.
+     */
+    static bool covers(const MultikeyPaths& parent, const MultikeyPaths& child);
+
     // Decoration requires a default constructor.
     MultikeyPathTracker() = default;