SERVER-63763 Track initial number of orphans in range deletion task documents (donor-side)

3 files changed, 46 insertions, 0 deletions

diff --git a/src/mongo/db/s/migration_coordinator.cpp b/src/mongo/db/s/migration_coordinator.cpp
index dfab697554e..5096442f56e 100644
--- a/src/mongo/db/s/migration_coordinator.cpp
+++ b/src/mongo/db/s/migration_coordinator.cpp
@@ -231,6 +231,18 @@ SemiFuture<void> MigrationCoordinator::_commitMigrationOnDonorAndRecipient(
 
     hangBeforeSendingCommitDecision.pauseWhileSet();
 
+    LOGV2_DEBUG(6376300,
+                2,
+                "Retrieving number of orphan documents from recipient",
+                "migrationId"_attr = _migrationInfo.getId());
+
+    auto numOrphans = migrationutil::retrieveNumOrphansFromRecipient(opCtx, _migrationInfo);
+
+    if (numOrphans > 0) {
+        migrationutil::persistUpdatedNumOrphans(
+            opCtx, BSON("_id" << _migrationInfo.getId()), numOrphans);
+    }
+
     LOGV2_DEBUG(23896,
                 2,
                 "Deleting range deletion task on recipient",
diff --git a/src/mongo/db/s/migration_util.cpp b/src/mongo/db/s/migration_util.cpp
index dffa086da60..364f5813f0a 100644
--- a/src/mongo/db/s/migration_util.cpp
+++ b/src/mongo/db/s/migration_util.cpp
@@ -678,6 +678,34 @@ void persistUpdatedNumOrphans(OperationContext* opCtx,
         WriteConcernOptions());
 }
 
+int retrieveNumOrphansFromRecipient(OperationContext* opCtx,
+                                    const MigrationCoordinatorDocument& migrationInfo) {
+    const auto recipientShard = uassertStatusOK(
+        Grid::get(opCtx)->shardRegistry()->getShard(opCtx, migrationInfo.getRecipientShardId()));
+    FindCommandRequest findCommand(NamespaceString::kRangeDeletionNamespace);
+    findCommand.setFilter(BSON("_id" << migrationInfo.getId()));
+    findCommand.setReadConcern(BSONObj());
+    Shard::QueryResponse rangeDeletionResponse =
+        uassertStatusOK(recipientShard->runExhaustiveCursorCommand(
+            opCtx,
+            ReadPreferenceSetting(ReadPreference::PrimaryOnly),
+            NamespaceString::kRangeDeletionNamespace.db().toString(),
+            findCommand.toBSON(BSONObj()),
+            Milliseconds(-1)));
+    if (rangeDeletionResponse.docs.empty()) {
+        // In case of shutdown/stepdown, the recipient may have already deleted its range deletion
+        // document. A previous call to this function will have already returned the correct number
+        // of orphans, so we can simply return 0.
+        LOGV2_DEBUG(6376301,
+                    2,
+                    "No matching document found for migration",
+                    "recipientId"_attr = migrationInfo.getRecipientShardId(),
+                    "migrationId"_attr = migrationInfo.getId());
+        return 0;
+    }
+    return rangeDeletionResponse.docs[0].getIntField("numOrphanDocs");
+}
+
 void persistCommitDecision(OperationContext* opCtx,
                            const MigrationCoordinatorDocument& migrationDoc) {
     invariant(migrationDoc.getDecision() &&
diff --git a/src/mongo/db/s/migration_util.h b/src/mongo/db/s/migration_util.h
index 43d2c71cff4..73012403600 100644
--- a/src/mongo/db/s/migration_util.h
+++ b/src/mongo/db/s/migration_util.h
@@ -152,6 +152,12 @@ void persistUpdatedNumOrphans(OperationContext* opCtx,
                               const int& changeInOrphans);
 
 /**
+ * Retrieves the value of 'numOrphanedDocs' from the recipient shard's range deletion task document.
+ */
+int retrieveNumOrphansFromRecipient(OperationContext* opCtx,
+                                    const MigrationCoordinatorDocument& migrationInfo);
+
+/**
  * Updates the migration coordinator document to set the decision field to "committed" and waits for
  * majority writeConcern.
  */