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[bug#42023] [PATCH] Retry deduplication on ENOENT
From: |
Ludovic Courtès |
Subject: |
[bug#42023] [PATCH] Retry deduplication on ENOENT |
Date: |
Wed, 16 Sep 2020 22:37:05 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
Hi!
Caleb Ristvedt <caleb.ristvedt@cune.org> skribis:
[...]
>> 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.
Ah ha!
> We should probably still add a test in 'deduplicate' for whether
> symlinks can be hardlinked to.
If GNU/Linux and GNU/Hurd support it, it’s unnecessary.
> 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 see, good catch!
> From 12f5848e79b0ede95babebea240264b32e39812c 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] 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.
>
> * 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. Also modified to support
> canonicalizing symbolic links, though it is the caller's responsibility to
> ensure that the system supports hardlinking to a symbolic link (on Linux it
> does).
> (deduplicate): modified to use canonicalize-with-link.
[...]
> + (lambda args
> + (let ((errno (system-error-errno args)))
> + (cond
> + ((= errno ENOENT)
> + ;; either SWAP-DIRECTORY has missing directory
> + ;; components or TARGET was deleted - this is a
> + ;; fundamental ambiguity to the errno produced by
> + ;; link()
> + (if (file-exists? swap-directory)
> + ;; we must assume link failed because target doesn't
> + ;; exist, so create it.
Nitpick: Please capitalize sentences, add a period at the end, and write
“'link'” instead of “link()” or “link” for clarity.
Otherwise LGTM.
I think we’ll have to stress-test it through offloading to catch any
remaining issues.
Thank you!
Ludo’.