[bug#42023] [PATCH] Retry deduplication on ENOENT

[Caleb Ristvedt]

Continued where left off from 42023:

Ludovic Courtès <ludo@gnu.org> writes:

>> From b992a3aaac7e3b30222e0bf1df09093f18e25e6a Mon Sep 17 00:00:00 2001
>> From: Caleb Ristvedt <caleb.ristvedt@cune.org>
>> Date: Sat, 8 Aug 2020 11:25:57 -0500
>> Subject: [PATCH 3/6] deduplication: retry on ENOENT.
>>
>> It's possible for the garbage collector to remove the "canonical" link after
>> it's been detected as existing by 'deduplicate'.  This would cause an ENOENT
>> error when replace-with-link attempts to create the temporary link.  This
>> changes it so that it will properly handle that by retrying.
>
> Would that ENOENT cause an error, or just a missed deduplication
> opportunity?

Depends on how we handle it. Previously the error would be uncaught
entirely; simply skipping deduplication of that file would work, though
it would be suboptimal.

>
>> * guix/store/deduplication.scm (replace-with-link): renamed to
>>   canonicalize-with-link, now also handles the case where the target link
>>   doesn't exist yet, and retries on ENOENT.
>>   (deduplicate): modified to use canonicalize-with-link.
>
> There’s an issue with this patch.  I gave it a spin (offloading a few
> builds) and it got stuck in a infinite loop:
>
> stat("/gnu/store/.links/0k63r0n3681r2gqd00blq4z5xd7cw1knv0x049p99f0pq31brhrk",
>  0x7ffe43898cd0) = -1 ENOENT (Dosiero aŭ dosierujo ne ekzistas)
> link("/gnu/store/83jy739bn644w3pnwgb5kwjig0kzs92f-libwps-0.4.12/lib/libwps-0.4.so",
>  "/gnu/store/.links/0k63r0n3681r2gqd00blq4z5xd7cw1knv0x049p99f0pq31brhrk") = 
> -1 EEXIST (Dosiero jam ekzistas)
> stat("/gnu/store/.links/0k63r0n3681r2gqd00blq4z5xd7cw1knv0x049p99f0pq31brhrk",
>  0x7ffe43898cd0) = -1 ENOENT (Dosiero aŭ dosierujo ne ekzistas)
> link("/gnu/store/83jy739bn644w3pnwgb5kwjig0kzs92f-libwps-0.4.12/lib/libwps-0.4.so",
>  "/gnu/store/.links/0k63r0n3681r2gqd00blq4z5xd7cw1knv0x049p99f0pq31brhrk") = 
> -1 EEXIST (Dosiero jam ekzistas)
> stat("/gnu/store/.links/0k63r0n3681r2gqd00blq4z5xd7cw1knv0x049p99f0pq31brhrk",
>  0x7ffe43898cd0) = -1 ENOENT (Dosiero aŭ dosierujo ne ekzistas)
> link("/gnu/store/83jy739bn644w3pnwgb5kwjig0kzs92f-libwps-0.4.12/lib/libwps-0.4.so",
>  "/gnu/store/.links/0k63r0n3681r2gqd00blq4z5xd7cw1knv0x049p99f0pq31brhrk") = 
> -1 EEXIST (Dosiero jam ekzistas)
>

I believe I can explain this. In 'deduplicate' we currently treat
anything that isn't a directory as a hardlinkable thing. This includes
symlinks (although it's implementation-defined whether symlinks can be
hardlinked to - we use CAN_LINK_SYMLINK to test this in
nix/libstore/optimise-store.cc). This means that at present we
unconditionally attempt to deduplicate symlinks (which happens to work
on linux). However, 'file-exists?' uses stat, not lstat, to check for
file existence. Thus, if there is a dangling symlink, 'file-exists?'
will return #f when passed it, but of course attempting to call link()
to create it will fail with EEXIST. Attached is a modified patch that
tests for file existence with lstat instead. I expect that will fix the
problem.

We should probably still add a test in 'deduplicate' for whether
symlinks can be hardlinked to.

Tangent: I was curious why libwps-0.4.so would be a dangling symlink,
and it turns out that it's actually a relative symlink, so when
accessing it via /gnu/store/...-libwps-0.4.12/lib/libwps-0.4.so it isn't
dangling, but when accessing it via /gnu/store/.links/0k63r... it is.

> I think we should work on reducing the complexity of that code (e.g.,
> there are several layers of system-error handling).

I've since flattened it down into just one layer of catch'es. It adds a
bit of redundancy, but might make it clearer.

- reepca

0001-deduplication-retry-on-ENOENT.patch
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