MB-56094: Add 'cursor_op' to CM cursors stats

We already dump the seqno, the 'operation' is another useful
information at diagnostic. In particular in the case where cursor points
to a meta-item, as consecutive meta-items share the same seqno.

Output example:

  vb_0:persistence:cursor_op:             set_vbucket_state
  vb_0:persistence:cursor_seqno:          1

Note: In the example above the checkpoint is like [e:1 cs:1 vbs:1), so
the seqno isn't enough for knowing the precise cursor position.

Change-Id: I280eb9789eb45e2ecdfb51df0fbabe75801655aa
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/189981
Tested-by: Paolo Cocchi <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: paolococchi

engines/ep/docs/stats.org

@@ -993,6 +993,8 @@ each stat name.
 |                                     | name 'cursor_name' is pointing now        |
 | cursor_name:cursor_seqno            | The seqno at which the cursor             |
 |                                     | 'cursor_name' is pointing now             |
+| cursor_name:cursor_op               | The type of operation of the item pointed |
+|                                     | by cursor                                 |
 | cursor_name:num_visits              | Number of times a batch of items have been|
 |                                     | drained from a checkpoint of 'cursor_name'|
 | cursor_name:num_items_for_cursor    | Number of items remaining for the cursor  |

engines/ep/src/checkpoint_manager.cc

@@ -1572,39 +1572,49 @@ void CheckpointManager::addStats(const AddStatFn& add_stat,
         add_casted_stat(buf.data(), queuedItemsMemUsage, add_stat, cookie);
 
         for (const auto& cursor : cursors) {
+            const auto& name = cursor.second->getName();
+
             checked_snprintf(buf.data(),
                              buf.size(),
                              "vb_%d:%s:cursor_checkpoint_id",
                              vbucketId.get(),
-                             cursor.second->getName().c_str());
+                             name.c_str());
             add_casted_stat(buf.data(),
                             (*(cursor.second->getCheckpoint()))->getId(),
                             add_stat,
                             cookie);
 
+            const auto pos = cursor.second->getPos();
             checked_snprintf(buf.data(),
                              buf.size(),
-                             "vb_%d:%s:cursor_seqno",
+                             "vb_%d:%s:cursor_op",
                              vbucketId.get(),
-                             cursor.second->getName().c_str());
+                             name.c_str());
             add_casted_stat(buf.data(),
-                            (*(cursor.second->getPos()))->getBySeqno(),
+                            to_string((*pos)->getOperation()),
                             add_stat,
                             cookie);
 
+            checked_snprintf(buf.data(),
+                             buf.size(),
+                             "vb_%d:%s:cursor_seqno",
+                             vbucketId.get(),
+                             name.c_str());
+            add_casted_stat(buf.data(), (*pos)->getBySeqno(), add_stat, cookie);
+
             checked_snprintf(buf.data(),
                              buf.size(),
                              "vb_%d:%s:num_visits",
                              vbucketId.get(),
-                             cursor.second->getName().c_str());
+                             name.c_str());
             add_casted_stat(
                     buf.data(), cursor.second->getNumVisit(), add_stat, cookie);
 
             checked_snprintf(buf.data(),
                              buf.size(),
                              "vb_%d:%s:num_items_for_cursor",
                              vbucketId.get(),
-                             cursor.second->getName().c_str());
+                             name.c_str());
             add_casted_stat(buf.data(),
                             getNumItemsForCursor(lh, *cursor.second),
                             add_stat,

engines/ep/tests/ep_testsuite.cc

@@ -7246,6 +7246,7 @@ static enum test_result test_mb19687_fixed(EngineIface* h) {
         // Add stats which are only available for persistent buckets:
         std::initializer_list<std::string_view> persistence_stats = {
                 "vb_0:persistence:cursor_checkpoint_id",
+                "vb_0:persistence:cursor_op",
                 "vb_0:persistence:cursor_seqno",
                 "vb_0:persistence:num_visits",
                 "vb_0:persistence:num_items_for_cursor"};