SERVER-25725 Kill 3.2 secondaries when featureCompatibilityVersion=3.4.

Creates a v=2 index on the "admin.system.version" collection when the
featureCompatibilityVersion is set to 3.4. The index version of this
index is returned as a decimal value in the "listIndexes" command
response in order to prevent 3.2 secondaries from performing initial
sync from a 3.4 mongod with featureCompatibilityVersion=3.4.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/mongo/db/commands/list_indexes.cpp b/src/mongo/db/commands/list_indexes.cpp
index a943061a510..53e563ab7bd 100644
--- a/src/mongo/db/commands/list_indexes.cpp
+++ b/src/mongo/db/commands/list_indexes.cpp
@@ -35,11 +35,13 @@
 #include "mongo/db/catalog/database.h"
 #include "mongo/db/clientcursor.h"
 #include "mongo/db/commands.h"
+#include "mongo/db/commands/feature_compatibility_version.h"
 #include "mongo/db/concurrency/write_conflict_exception.h"
 #include "mongo/db/curop.h"
 #include "mongo/db/db_raii.h"
 #include "mongo/db/exec/queued_data_stage.h"
 #include "mongo/db/exec/working_set.h"
+#include "mongo/db/index/index_descriptor.h"
 #include "mongo/db/query/cursor_request.h"
 #include "mongo/db/query/cursor_response.h"
 #include "mongo/db/query/find_common.h"
@@ -162,6 +164,27 @@ public:
             }
             MONGO_WRITE_CONFLICT_RETRY_LOOP_END(txn, "listIndexes", ns.ns());
 
+            if (ns.ns() == FeatureCompatibilityVersion::kCollection &&
+                indexNames[i] == FeatureCompatibilityVersion::k32IncompatibleIndexName) {
+                BSONObjBuilder bob;
+
+                for (auto&& indexSpecElem : indexSpec) {
+                    auto indexSpecElemFieldName = indexSpecElem.fieldNameStringData();
+                    if (indexSpecElemFieldName == IndexDescriptor::kIndexVersionFieldName) {
+                        // Include the index version in the command response as a decimal type
+                        // instead of as a 32-bit integer. This is a new BSON type that isn't
+                        // supported by versions of MongoDB earlier than 3.4 that will cause 3.2
+                        // secondaries to crash when performing initial sync.
+                        bob.append(IndexDescriptor::kIndexVersionFieldName,
+                                   indexSpecElem.numberDecimal());
+                    } else {
+                        bob.append(indexSpecElem);
+                    }
+                }
+
+                indexSpec = bob.obj();
+            }
+
             WorkingSetID id = ws->allocate();
             WorkingSetMember* member = ws->get(id);
             member->keyData.clear();