SERVER-22498 fix migration session id multiversion failure

Author: Dianna Hohensee

src/mongo/db/s/migration_session_id.cpp

@@ -92,4 +92,8 @@ std::string MigrationSessionId::toString() const {
     return (_sessionId ? *_sessionId : "");
 }
 
+bool MigrationSessionId::isEmpty() const {
+    return !_sessionId;
+}
+
 }  // namespace mongo

src/mongo/db/s/migration_session_id.h

@@ -73,6 +73,8 @@ class MigrationSessionId {
 
     std::string toString() const;
 
+    bool isEmpty() const;
+
 private:
     MigrationSessionId();
     explicit MigrationSessionId(std::string sessionId);

src/mongo/db/s/migration_source_manager.cpp

@@ -326,7 +326,10 @@ bool MigrationSourceManager::transferMods(OperationContext* txn,
             return false;
         }
 
-        if (!_sessionId->matches(sessionId)) {
+        // TODO after 3.4 release, !sessionId.isEmpty() can be removed: versions >= 3.2 will
+        // all have sessionId implemented. (two more instances below).
+        // A mongod version < v3.2 will not have sessionId, in which case it is empty and ignored.
+        if (!sessionId.isEmpty() && !_sessionId->matches(sessionId)) {
             errmsg = str::stream() << "requested migration session id " << sessionId.toString()
                                    << " does not match active session id "
                                    << _sessionId->toString();
@@ -482,7 +485,8 @@ bool MigrationSourceManager::clone(OperationContext* txn,
             return false;
         }
 
-        if (!_sessionId->matches(sessionId)) {
+        // A mongod version < v3.2 will not have sessionId, in which case it is empty and ignored.
+        if (!sessionId.isEmpty() && !_sessionId->matches(sessionId)) {
             errmsg = str::stream() << "requested migration session id " << sessionId.toString()
                                    << " does not match active session id "
                                    << _sessionId->toString();
@@ -512,7 +516,8 @@ bool MigrationSourceManager::clone(OperationContext* txn,
             return false;
         }
 
-        if (!_sessionId->matches(sessionId)) {
+        // A mongod version < v3.2 will not have sessionId, in which case it is empty and ignored.
+        if (!sessionId.isEmpty() && !_sessionId->matches(sessionId)) {
             errmsg = str::stream() << "migration session id changed from " << sessionId.toString()
                                    << " to " << _sessionId->toString()
                                    << " while initial clone was active";