Revert "SERVER-25623 addShard should fail if shard's binary version < the cluster's featureCompatibilityVersion"

This reverts commit 80b4173ab3ad46eed8f4f482e5fe34bf6546d2e6.

2 files changed, 33 insertions, 23 deletions

diff --git a/jstests/sharding/add_invalid_shard.js b/jstests/sharding/add_invalid_shard.js
index 5f2484bae73..655b57cd605 100644
--- a/jstests/sharding/add_invalid_shard.js
+++ b/jstests/sharding/add_invalid_shard.js
@@ -11,14 +11,7 @@
     var shardDoc = configDB.shards.findOne();
 
     // Can't add mongos as shard.
-    assert.commandFailedWithCode(st.admin.runCommand({addshard: st.s.host}),
-                                 ErrorCodes.IllegalOperation);
-
-    // Can't add a mongod with a lower binary version than our featureCompatibilityVersion.
-    var lastStableMongod = MongoRunner.runMongod({binVersion: "last-stable"});
-    assert.commandFailedWithCode(st.admin.runCommand({addshard: lastStableMongod.host}),
-                                 ErrorCodes.IncompatibleServerVersion);
-    MongoRunner.stopMongod(lastStableMongod);
+    assert.commandFailed(st.admin.runCommand({addshard: st.s.host}));
 
     // Can't add config servers as shard.
     assert.commandFailed(st.admin.runCommand({addshard: st._configDB}));
diff --git a/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp b/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp
index b8ce6b79995..5e73c2577af 100644
--- a/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp
+++ b/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp
@@ -303,10 +303,26 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
     std::shared_ptr<RemoteCommandTargeter> targeter,
     const std::string* shardProposedName,
     const ConnectionString& connectionString) {
+
+    // Check if the node being added is a mongos or a version of mongod too old to speak the current
+    // communication protocol.
     auto swCommandResponse =
         _runCommandForAddShard(opCtx, targeter.get(), "admin", BSON("isMaster" << 1));
     if (!swCommandResponse.isOK()) {
-        return swCommandResponse.getStatus();
+        if (swCommandResponse.getStatus() == ErrorCodes::RPCProtocolNegotiationFailed) {
+            // Mongos to mongos commands are no longer supported in the wire protocol
+            // (because mongos does not support OP_COMMAND), similarly for a new mongos
+            // and an old mongod. So the call will fail in such cases.
+            // TODO: If/When mongos ever supports opCommands, this logic will break because
+            // cmdStatus will be OK.
+            return {ErrorCodes::RPCProtocolNegotiationFailed,
+                    str::stream() << targeter->connectionString().toString()
+                                  << " does not recognize the RPC protocol being used. This is"
+                                  << " likely because it contains a node that is a mongos or an old"
+                                  << " version of mongod."};
+        } else {
+            return swCommandResponse.getStatus();
+        }
     }
 
     // Check for a command response error
@@ -321,14 +337,15 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
 
     auto resIsMaster = std::move(swCommandResponse.getValue().response);
 
-    // Fail if the node being added is a mongos.
-    const std::string msg = resIsMaster.getStringField("msg");
-    if (msg == "isdbgrid") {
-        return {ErrorCodes::IllegalOperation, "cannot add a mongos as a shard"};
-    }
+    // Check that the node being added is a new enough version.
+    // If we're running this code, that means the mongos that the addShard request originated from
+    // must be at least version 3.4 (since 3.2 mongoses don't know about the _configsvrAddShard
+    // command).  Since it is illegal to have v3.4 mongoses with v3.2 shards, we should reject
+    // adding any shards that are not v3.4.  We can determine this by checking that the
+    // maxWireVersion reported in isMaster is at least COMMANDS_ACCEPT_WRITE_CONCERN.
+    // TODO(SERVER-25623): This approach won't work to prevent v3.6 mongoses from adding v3.4
+    // shards, so we'll have to rethink this during the 3.5 development cycle.
 
-    // Fail if the node being added's binary version is lower than the cluster's
-    // featureCompatibilityVersion.
     long long maxWireVersion;
     Status status = bsonExtractIntegerField(resIsMaster, "maxWireVersion", &maxWireVersion);
     if (!status.isOK()) {
@@ -339,15 +356,15 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
                                     << " as a shard: "
                                     << status.reason());
     }
-    if (serverGlobalParams.featureCompatibility.version.load() !=
-            ServerGlobalParams::FeatureCompatibility::Version::k34 &&
-        maxWireVersion < WireVersion::LATEST_WIRE_VERSION) {
-        return {ErrorCodes::IncompatibleServerVersion,
-                str::stream() << "Cannot add " << connectionString.toString()
-                              << " as a shard because its binary version is not compatible with "
-                                 "the cluster's featureCompatibilityVersion."};
+    if (maxWireVersion < WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN) {
+        return Status(ErrorCodes::IncompatibleServerVersion,
+                      str::stream() << "Cannot add " << connectionString.toString()
+                                    << " as a shard because we detected a mongod with server "
+                                       "version older than 3.4.0.  It is invalid to add v3.2 and "
+                                       "older shards through a v3.4 mongos.");
     }
 
+
     // Check whether there is a master. If there isn't, the replica set may not have been
     // initiated. If the connection is a standalone, it will return true for isMaster.
     bool isMaster;