From 7026ff8dbc62b9649e8be1793d3273d9cc162174 Mon Sep 17 00:00:00 2001
From: David Storch <david.storch@10gen.com>
Date: Fri, 23 Sep 2016 16:52:04 -0400
Subject: SERVER-26264 fail gracefully when attempting to join a cluster with
 featureCompatibilityVersion 3.4

---
 src/mongo/client/replica_set_monitor.cpp        | 8 ++++++++
 src/mongo/s/client/sharding_connection_hook.cpp | 9 +++++++++
 src/mongo/s/server.cpp                          | 6 ++++++
 src/mongo/s/sharding_initialization.cpp         | 5 +++++
 4 files changed, 28 insertions(+)

diff --git a/src/mongo/client/replica_set_monitor.cpp b/src/mongo/client/replica_set_monitor.cpp
index 0a471ffd088..8cf534b3fd5 100644
--- a/src/mongo/client/replica_set_monitor.cpp
+++ b/src/mongo/client/replica_set_monitor.cpp
@@ -835,6 +835,14 @@ HostAndPort Refresher::_refreshUntilMatches(const ReadPreferenceSetting* criteri
                     conn->isMaster(ignoredOutParam, &reply);
                     pingMicros = timer.micros();
                     conn.done();  // return to pool on success.
+                } catch (const DBException& ex) {
+                    // If this mongos is being used in an illegal configuration and must be
+                    // upgraded, bubble up the error.
+                    Status status = ex.toStatus();
+                    if (status == ErrorCodes::MustUpgrade) {
+                        throw;
+                    }
+                    reply = BSONObj();  // should be a no-op but want to be sure
                 } catch (...) {
                     reply = BSONObj();  // should be a no-op but want to be sure
                 }
diff --git a/src/mongo/s/client/sharding_connection_hook.cpp b/src/mongo/s/client/sharding_connection_hook.cpp
index bb368f443d1..f1db2092a6f 100644
--- a/src/mongo/s/client/sharding_connection_hook.cpp
+++ b/src/mongo/s/client/sharding_connection_hook.cpp
@@ -159,6 +159,15 @@ void ShardingConnectionHook::onCreate(DBClientBase* conn) {
             return;
         }
 
+        // Report a useful error message when we are illegally attempting to connect to 3.4 config
+        // servers in 3.4 feature compatibility mode.
+        uassert(ErrorCodes::MustUpgrade,
+                str::stream() << "Using a 3.2 mongos with a cluster that has "
+                                 "featureCompatibilityVersion 3.4 is not a supported "
+                                 "configuration. See "
+                                 "http://dochub.mongodb.org/core/3.4-feature-compatibility.",
+                configServerModeNumber < 2);
+
         uassert(28785,
                 str::stream() << "Unrecognized configsvr version number: " << configServerModeNumber
                               << ". Expected either 0 or 1",
diff --git a/src/mongo/s/server.cpp b/src/mongo/s/server.cpp
index e68fd12b66f..2200143c5bd 100644
--- a/src/mongo/s/server.cpp
+++ b/src/mongo/s/server.cpp
@@ -296,6 +296,12 @@ static ExitCode runMongosServer() {
                 return EXIT_CLEAN;
             }
             error() << "Error initializing sharding system: " << status;
+
+            // Exit gracefully if this mongos is being used incorrectly in a mixed 3.2/3.4 cluster.
+            if (status == ErrorCodes::MustUpgrade) {
+                dbexit(EXIT_SHARDING_ERROR);
+            }
+
             return EXIT_SHARDING_ERROR;
         }
 
diff --git a/src/mongo/s/sharding_initialization.cpp b/src/mongo/s/sharding_initialization.cpp
index 36a074742dd..99079c97c8c 100644
--- a/src/mongo/s/sharding_initialization.cpp
+++ b/src/mongo/s/sharding_initialization.cpp
@@ -199,6 +199,11 @@ Status initializeGlobalShardingState(OperationContext* txn,
                 // retry.
                 return status;
             }
+
+            if (status == ErrorCodes::MustUpgrade) {
+                return status;
+            }
+
             if (status == ErrorCodes::ReplicaSetNotFound) {
                 // ReplicaSetNotFound most likely means we've been waiting for the config replica
                 // set to come up for so long that the ReplicaSetMonitor stopped monitoring the set.
-- 
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