SERVER-41925 enumerate the FCV 4.4 dependencies in the sharding code

Author: Misha Tyulenev

jstests/sharding/stats.js

@@ -15,21 +15,8 @@ function numKeys(o) {
 }
 
 db.foo.drop();
-//	SERVER-29678 changed collStats so versions > 4.0 now return 0s on NS not found
-if (MongoRunner.getBinVersionFor(jsTest.options().mongosBinVersion) === '4.0') {
-    // TODO: This should be fixed in 4.4
-    let res = db.foo.stats();
-    if (res.ok === 1) {
-        // Possible to hit a shard that is actually version >= 4.2 => result should be zeros
-        assert(res.size === 0 && res.count === 0 && res.storageSize === 0 && res.nindexes === 0);
-    } else {
-        assert.commandFailed(db.foo.stats(),
-                             'db.collection.stats() should fail non-existent in versions <= 4.0');
-    }
-} else {
-    assert.commandWorked(db.foo.stats(),
-                         'db.collection.stats() should return 0s on non-existent collection');
-}
+assert.commandWorked(db.foo.stats(),
+                     'db.collection.stats() should return 0s on non-existent collection');
 
 // ---------- load some data -----
 

src/mongo/db/s/config/configsvr_shard_collection_command.cpp

@@ -545,6 +545,7 @@ class ConfigSvrShardCollectionCommand : public BasicCommand {
                 return true;
             }
 
+            // TODO: SERVER-42594
             // This check for empty collection is racy, because it is not guaranteed that documents
             // will not show up in the collection right after the count below has executed. It is
             // left here for backwards compatiblity with pre-4.0.4 clusters, which do not support

src/mongo/db/s/config/sharding_catalog_manager_collection_operations.cpp

@@ -370,6 +370,7 @@ void ShardingCatalogManager::shardCollection(OperationContext* opCtx,
     // Fail if there are partially written chunks from a previous failed shardCollection.
     checkForExistingChunks(opCtx, nss);
 
+    // TODO: SERVER-42593
     // Prior to 4.0.5, zones cannot be taken into account at collection sharding time, so ignore
     // them and let the balancer apply them later
     const std::vector<TagsType> treatAsNoZonesDefined;

src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp

@@ -337,6 +337,7 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
                                                 << "field when attempting to add "
                                                 << connectionString.toString() << " as a shard");
     }
+    // TODO: SERVER-42592
     if (serverGlobalParams.featureCompatibility.getVersion() >
         ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo40) {
         // If the cluster's FCV is 4.2, or upgrading to / downgrading from, the node being added
@@ -646,6 +647,7 @@ StatusWith<std::string> ShardingCatalogManager::addShard(
 
     AddShard addShardCmd = add_shard_util::createAddShardCmd(opCtx, shardType.getName());
 
+    // TODO: SERVER-42592
     auto addShardCmdBSON = [&]() {
         // In 4.2, use the _addShard command to add the shard, which in turn inserts a
         // shardIdentity document into the shard and triggers sharding state initialization.
@@ -680,6 +682,7 @@ StatusWith<std::string> ShardingCatalogManager::addShard(
         invariant(!opCtx->lockState()->isLocked());
         Lock::SharedLock lk(opCtx->lockState(), FeatureCompatibilityVersion::fcvLock);
 
+        // TODO: SERVER-42592
         BSONObj setFCVCmd;
         switch (serverGlobalParams.featureCompatibility.getVersion()) {
             case ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo42:

src/mongo/db/s/txn_two_phase_commit_cmds.cpp

@@ -221,12 +221,6 @@ class CoordinateCommitTransactionCmd : public TypedCommand<CoordinateCommitTrans
                 uassertStatusOK(ShardingState::get(opCtx)->canAcceptShardedCommands());
             }
 
-            uassert(ErrorCodes::CommandNotSupported,
-                    "'coordinateCommitTransaction' is only supported in feature compatibility "
-                    "version 4.2",
-                    (serverGlobalParams.featureCompatibility.getVersion() ==
-                     ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo42));
-
             const auto& cmd = request();
             const auto tcs = TransactionCoordinatorService::get(opCtx);
 

src/mongo/s/sharding_mongod_test_fixture.cpp

@@ -144,6 +144,7 @@ void ShardingMongodTestFixture::setUp() {
     repl::setOplogCollectionName(service);
     repl::createOplog(_opCtx.get());
 
+    // TODO: SERVER-42592
     // Set the highest FCV because otherwise it defaults to the lower FCV. This way we default to
     // testing this release's code, not backwards compatibility code.
     serverGlobalParams.featureCompatibility.setVersion(