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Re: [PATCH] Fix "database is locked" error when committing during rep-cache.db verification

From: Daniel Shahaf <d.s_at_daniel.shahaf.name>
Date: Thu, 21 May 2020 22:16:46 +0000

Johan Corveleyn wrote on Thu, 21 May 2020 17:53 +0200:
> Trying to summarize this thread a bit. I apologize in advance if I
> forgot something, or have misrepresented any of the points that were
> raised (feel free to correct / add).
>

I have additions, below.

> Denis summed up the following problems that might happen while
> 'verify' locks the repcache.db:
>
> > 1. a post-commit error "database is locked"
> > 2. new representations will not be added in the rep-cache.db
> > 3. deduplication does not work for new data committed at this time
> > 4. commits work with delays.
>
> We have also established that the new tool build-repcache is not
> suitable for post-factum fixing of 3). It does not reprocess already
> committed revisions.

Note, however, that (3) is simply an observation that Denis made about
the semantics of the incumbent code. We do not know of a user who needs
rep-cache.db entries to be added so soon after the commit that running
«build-repcache» after «verify» finishes would be too late (or impractical).

> We are currently considering two approaches to address these issues:
>

Hold on. Those issues are what happens *whenever* the post-commmit FS
processing INSERTs are starved. They happen when a «verify» starves
INSERTs, but they are known to have at least one other case: when two
commits happen in quick succession, the INSERTs that happen during
post-commit FS processing of the first commit can starve the second
commit's INSERTs. (Don't we have a jira issue for this variant? I
thought we did, but my searches came up dry.)

Which is to say, the presentation of the matter as "four issues to be
fixed" isn't accurate. There are _eight_ issues to consider: (1) to (4)
when they are caused by a «verify» starving INSERTs, and (1) to (4) when
they are caused by an «INSERT» starving INSERTs.

Thus, your analysis of the pros and cons is incomplete: it glosses over
the case of an «INSERT» starving other INSERTs.

Furthermore, over the course of the thread several other ideas have been
floated, intended to address various subsets of the eight issues. The
"Perform INSERTs asynchronously" idea, for example (raised by me, but
inspired by a remark of Brane's), can basically fix all eight issues by
itself. There was also the idea to stop marshalling post-commit FS
processing errors to the client, which would fix both variants of (1)
but none of the other six. I think there were other ideas as well, but
I've run out of time for this thread for tonight, sorry.

> I'll add one more concern of my own here, regarding the 'sharding' approach:
> I'd like to warn for the NIHS (Not Invented Here Syndrome) that comes
> peeking around the corner if we say "SQLite might have subtle bugs
> that might hurt us if we do X, but rolling our own solution might be
> better". Why would "rolling our own solution" like sharding
> repcache.db be less susceptible to such subtle bugs than SQLite? Okay,
> on the one hand SQLite is more complex, because it's generic database
> software. But on the other hand it presumably has a lot more users /
> audience than just Subversion. I have no clear answer here.

Thanks, Johan.

Daniel
Received on 2020-05-22 00:17:01 CEST

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