[PATCH 11/13] gc: separate checkpoint references from wal consumers

[Konstantin Osipov]

* Vladimir Davydov <vdavydov.dev at gmail.com> [18/10/05 00:11]:

Pinning and referencing are different concepts :)
I agree with your reasoning and the patch is OK to push.

> Initially, gc_consumer object was used for pinning both checkpoint and
> WAL files, but commit 9c5d851d7830 ("replication: remove old snapshot
> files not needed by replicas") changed that. Now whether a consumer pins
> WALs or checkpoints or both depends on gc_consumer_type. This was done
> so that replicas wouldn't prevent garbage collection of checkpoint
> files, which they don't need after initial join is complete.
> 
> The way the feature was implemented is rather questionable though:
>  - Since consumers of both types are stored in the same binary search
>    tree, we have to iterate through the tree to find the leftmost
>    checkpoint consumer, see gc_tree_first_checkpoint. This looks
>    inefficient and ugly.
>  - The notion of advancing a checkpoint consumer (gc_consumer_advance)
>    is dubious: there's no point to move on to the next checkpoint after
>    reading one - instead the consumer needs incremental changes, i.e.
>    WALs.
> 
> To eliminate those questionable aspects and make the code easier for
> understanding, let's separate WAL and checkpoint consumers. We do this
> by removing gc_consumer_type and making gc_consumer track WALs only.
> For pinning the files corresponding to a checkpoint a new object class
> is introduced, gc_checkpoint_ref. To pin a checkpoint, gc_ref_checkpoint
> needs to be called. It is passed the gc_checkpoint object to pin, the
> consumer name, and the gc_checkpoint_ref to store the ref in. To unpin a
> previously pinned checkpoint, gc_checkpoint_unref should be called.
> 
> References are listed by box.info.gc() for each checkpoint under
> 'references' key.

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