MB-41407: [BP] Do not use old StoredValue after soft delete

Backport of http://review.couchbase.org/c/kv_engine/+/139274 .

Fixes tsan-identified race between EphemeralVBucket::pageOut and
EphemeralVBucket::StaleItemDeleter immediately deleting the newly stale
item.

softDeleteStoredValue may lead to the existing item being marked stale.
At this point, is is no longer safe to use the old stored value, as the
StaleItemDeleter may delete it at any time.

Change-Id: I89101bb26dec6ae99b58f7b4df3e1203cdb7b8d9
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/141012
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>
Well-Formed: Build Bot <[email protected]>

Author: jameseh96

engines/ep/src/ephemeral_vb.cc

@@ -107,20 +107,24 @@ bool EphemeralVBucket::pageOut(
     if (!eligibleToPageOut(lh, *v)) {
         return false;
     }
+    auto cid = v->getKey().getCollectionID();
     VBQueueItemCtx queueCtx;
     v->setRevSeqno(v->getRevSeqno() + 1);
-    StoredValue* newSv;
     DeletionStatus status;
     VBNotifyCtx notifyCtx;
-    std::tie(newSv, status, notifyCtx) = softDeleteStoredValue(
+    // After soft delete it is no longer safe to manipulate v the original
+    // StoredValue v.
+    // If v was made stale (because a range lock covers it) it may be
+    // deleted at any time. Only the new SV ptr which has been returned should
+    // be handled now.
+    std::tie(v, status, notifyCtx) = softDeleteStoredValue(
             lh, *v, /*onlyMarkDeleted*/ false, queueCtx, 0);
 
     switch (status) {
     case DeletionStatus::Success:
-        ht.updateMaxDeletedRevSeqno(newSv->getRevSeqno());
+        ht.updateMaxDeletedRevSeqno(v->getRevSeqno());
         notifyNewSeqno(notifyCtx);
-        doCollectionsStats(readHandle, v->getKey().getCollectionID(),
-                notifyCtx);
+        doCollectionsStats(readHandle, cid, notifyCtx);
         autoDeleteCount++;
         return true;