SERVER-44165 Remove the config.rangeDeletions collection on downgrade

jstests/multiVersion/delete_pending_range_deletions_on_downgrade.js

@@ -0,0 +1,51 @@
+(function() {
+"use strict";
+
+load("jstests/libs/uuid_util.js");
+
+const dbName = "test";
+const collName = "foo";
+const ns = dbName + "." + collName;
+const rangeDeletionNs = "config.rangeDeletions";
+
+// Create 2 shards with 3 replicas each.
+let st = new ShardingTest({shards: {rs0: {nodes: 3}, rs1: {nodes: 3}}});
+
+// Create a sharded collection with two chunks: [-inf, 50), [50, inf)
+assert.commandWorked(st.s.adminCommand({enableSharding: dbName}));
+assert.commandWorked(st.s.adminCommand({movePrimary: dbName, to: st.shard0.shardName}));
+assert.commandWorked(st.s.adminCommand({shardCollection: ns, key: {x: 1}}));
+assert.commandWorked(st.s.adminCommand({split: ns, middle: {x: 50}}));
+
+// Pause range deletion.
+let originalShard0Primary = st.rs0.getPrimary();
+originalShard0Primary.adminCommand({configureFailPoint: 'suspendRangeDeletion', mode: 'alwaysOn'});
+
+// Write range to deletion collection
+let deletionTask = {
+    _id: UUID(),
+    nss: ns,
+    collectionUuid: UUID(),
+    donorShardId: "unused",
+    range: {min: {x: 50}, max: {x: MaxKey}},
+    whenToClean: "now"
+};
+
+let deletionsColl = st.shard0.getCollection(rangeDeletionNs);
+
+// Write range to deletion collection
+deletionsColl.insert(deletionTask);
+
+// Verify deletion count.
+assert.eq(deletionsColl.find().itcount(), 1);
+
+print("setting fcv: " + lastStableFCV);
+assert.commandWorked(
+    st.s.getDB("admin").runCommand({setFeatureCompatibilityVersion: lastStableFCV}));
+checkFCV(st.shard0.getDB("admin"), lastStableFCV);
+
+// Verify deletion count.
+assert.eq(deletionsColl.find().itcount(), 0);
+
+st.stop();
+})();

src/mongo/db/commands/set_feature_compatibility_version_command.cpp

@@ -45,6 +45,7 @@
 #include "mongo/db/repl/repl_client_info.h"
 #include "mongo/db/s/active_shard_collection_registry.h"
 #include "mongo/db/s/config/sharding_catalog_manager.h"
+#include "mongo/db/s/migration_util.h"
 #include "mongo/db/server_options.h"
 #include "mongo/rpc/get_status_from_command_result.h"
 #include "mongo/s/catalog/type_collection.h"
@@ -234,6 +235,9 @@ public:
             }
 
             if (serverGlobalParams.clusterRole == ClusterRole::ShardServer) {
+                LOG(0) << "Downgrade: dropping config.rangeDeletions collection";
+                migrationutil::dropRangeDeletionsCollection(opCtx);
+
                 // The primary shard sharding a collection will write the initial chunks for a
                 // collection directly to the config server, so wait for all shard collections to
                 // complete to guarantee no chunks are missed by the update on the config server.

src/mongo/db/s/migration_util.cpp

@@ -198,6 +198,12 @@ void resubmitRangeDeletionsOnStepUp(ServiceContext* serviceContext) {
     });
 }
 
+void dropRangeDeletionsCollection(OperationContext* opCtx) {
+    DBDirectClient client(opCtx);
+    client.dropCollection(NamespaceString::kRangeDeletionNamespace.toString(),
+                          WriteConcerns::kMajorityWriteConcern);
+}
+
 void persistMigrationCoordinatorLocally(OperationContext* opCtx,
                                         const MigrationCoordinatorDocument& migrationDoc) {
     PersistentTaskStore<MigrationCoordinatorDocument> store(

src/mongo/db/s/migration_util.h

@@ -84,6 +84,8 @@ void submitPendingDeletions(OperationContext* opCtx);
 // Asynchronously calls submitPendingDeletions using the fixed executor pool.
 void resubmitRangeDeletionsOnStepUp(ServiceContext* serviceContext);
 
+void dropRangeDeletionsCollection(OperationContext* opCtx);
+
 /**
  * Writes the migration coordinator document to config.migrationCoordinators and waits for majority
  * write concern.