SERVER-24817 Make ShardLocal wait for writes it performed when doing majority reads

This gives config servers read-your-own-write semantics, even when the writes are performed as w:1
and the reads have readConcern:majority

1 files changed, 19 insertions, 0 deletions

diff --git a/src/mongo/s/client/shard_local.h b/src/mongo/s/client/shard_local.h
index b39aba621d0..1c65015b78e 100644
--- a/src/mongo/s/client/shard_local.h
+++ b/src/mongo/s/client/shard_local.h
@@ -30,7 +30,9 @@
 
 #include "mongo/base/disallow_copying.h"
 #include "mongo/client/dbclientinterface.h"
+#include "mongo/db/repl/optime.h"
 #include "mongo/s/client/shard.h"
+#include "mongo/stdx/mutex.h"
 
 namespace mongo {
 
@@ -75,6 +77,23 @@ private:
         const BSONObj& query,
         const BSONObj& sort,
         boost::optional<long long> limit) final;
+
+    /**
+     * Checks if an OpTime was set on the current Client (ie if the current operation performed a
+     * write) and if so updates _lastOpTime to the OpTime from the write that was just performed.
+     */
+    void _updateLastOpTimeFromClient(OperationContext* txn);
+
+    repl::OpTime _getLastOpTime();
+
+    // Guards _lastOpTime below.
+    stdx::mutex _mutex;
+
+    // Stores the optime that was generated by the last operation to perform a write that was run
+    // through _runCommand.  Used in _exhaustiveFindOnConfig for waiting for that optime to be
+    // committed so that readConcern majority reads will read the writes that were performed without
+    // a w:majority write concern.
+    repl::OpTime _lastOpTime{};
 };
 
 }  // namespace mongo