add c-api to get default cf handle

[979357361]

rocksdb_batched_multi_get_cf has performance improvement than normal multi_get, however it needs a cf_handle arg, so add a C-API to get and destroy the default cf_handle, as many user only use the default cf.

Fixes #12316

[facebook-github-bot]

@ajkr has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[ajkr]

It's kind of confusing because (1) it has "destroy" in the name but doesn't go through our regular destroy path (DestroyColumnFamilyHandle() or ~ColumnFamilyHandle()); and (2) it has "default" in the name but operates on any rocksdb_column_family_handle_t*.

What do you think about simply not offering such an API, and let the user call delete handle instead?

[979357361]

The reason I add this different api is that, I observed that "delete handle" api will destroy the cf together with the handle, for other cf this behavior is rational because except for the handle, user has no other way to get and work on it, free the handle should also free the cf, too.
But for the default cf, things have a little change, only the new batched multi_get api need the handle arg, what if I finish the call and just want to free the memory of default cf_handle? Maybe another api name is more suitable, what do you think?
Or we can modify the batched multi_get api, if the cf_handle arg is NULL, actions will go to the default cf by default? Maybe adding a new api "rocksdb_batched_multi_get" which use the default cf is more direct, as all other APIs have two version -- with or without "_cf" ending, except for batched multi_get.

[ajkr]

Oh ok I see the ambiguity of the term "destroy" in our C API is preexisting. How about this as the API change?

struct rocksdb_column_family_handle_t {
  ColumnFamilyHandle* rep;
  bool immortal;  // <-- New field
};

extern ROCKSDB_LIBRARY_API rocksdb_column_family_handle_t*
    rocksdb_get_default_column_family_handle(rocksdb_t* db);  // <-- New function

• rocksdb_get_default_column_family_handle() will set immortal == true
• Other creations of rocksdb_column_family_handle_t, where rep is not obtained by DefaultColumnFamilyHandle(), will set immortal == false
• rocksdb_column_family_handle_destroy() will delete rep only when !immortal

[979357361]

OK, that seems rational, thank you.
Can I expect to see this new API in the next release?

[ajkr]

Yes, we will be able to include it in the 9.2 release as long as we land it by Friday this week.