Modify Checkpoint state so it can only be closed

Modify Checkpoint of checkpoint.h to replace Checkpoint::setState() with
Checkpoint::close(), so that we can never update a checkpoint from the
closed state to the open state. Once a checkpoint has been marked closed
it should be immutable.

Change-Id: Ic0cae1e850205c73a768e3f8db87234e433706e0
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/172723
Tested-by: Build Bot <[email protected]>
Reviewed-by: Paolo Cocchi <[email protected]>

Author: rdemellow

engines/ep/src/checkpoint.h

@@ -335,11 +335,16 @@ class Checkpoint {
     }
 
     /**
-     * Set the current state of this checkpoint.
-     * @param state the checkpoint's new state
-     */
-    void setState(checkpoint_state state) {
-        *checkpointState.wlock() = state;
+     * Set the current state of this checkpoint to closed.
+     */
+    void close() {
+        auto& state = *checkpointState.wlock();
+        if (state == checkpoint_state::CHECKPOINT_CLOSED) {
+            throw std::logic_error(
+                    "Checkpoint::close() can't close a closed "
+                    "checkpoint!");
+        }
+        state = checkpoint_state::CHECKPOINT_CLOSED;
     }
 
     void incNumOfCursorsInCheckpoint() {

engines/ep/src/checkpoint_manager.cc

@@ -153,7 +153,7 @@ void CheckpointManager::addNewCheckpoint(
             oldOpenCkpt.getId(), vb.getId(), queue_op::checkpoint_end);
     oldOpenCkpt.queueDirty(qi);
     ++numItems;
-    oldOpenCkpt.setState(CHECKPOINT_CLOSED);
+    oldOpenCkpt.close();
 
     addOpenCheckpoint(snapStartSeqno,
                       snapEndSeqno,