From: Patrick Steinhardt <ps@pks.im>
To: Taylor Blau <me@ttaylorr.com>
Cc: git@vger.kernel.org, hanwenn@gmail.com
Subject: Re: [PATCH 3/4] refs: complete list of special refs
Date: Thu, 30 Nov 2023 08:44:05 +0100 [thread overview]
Message-ID: <ZWg9RW4L8nBhYmaB@tanuki> (raw)
In-Reply-To: <ZWe0RzOoHI9QZMox@nand.local>
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On Wed, Nov 29, 2023 at 04:59:35PM -0500, Taylor Blau wrote:
> On Wed, Nov 29, 2023 at 09:14:20AM +0100, Patrick Steinhardt wrote:
> > We have some references that are more special than others. The reason
> > for them being special is that they either do not follow the usual
> > format of references, or that they are written to the filesystem
> > directly by the respective owning subsystem and thus circumvent the
> > reference backend.
> >
> > This works perfectly fine right now because the reffiles backend will
> > know how to read those refs just fine. But with the prospect of gaining
> > a new reference backend implementation we need to be a lot more careful
> > here:
> >
> > - We need to make sure that we are consistent about how those refs are
> > written. They must either always be written via the filesystem, or
> > they must always be written via the reference backend. Any mixture
> > will lead to inconsistent state.
> >
> > - We need to make sure that such special refs are always handled
> > specially when reading them.
> >
> > We're already mostly good with regard to the first item, except for
> > `BISECT_EXPECTED_REV` which will be addressed in a subsequent commit.
> > But the current list of special refs is missing a lot of refs that
> > really should be treated specially. Right now, we only treat
> > `FETCH_HEAD` and `MERGE_HEAD` specially here.
> >
> > Introduce a new function `is_special_ref()` that contains all current
> > instances of special refs to fix the reading path.
> >
> > Based-on-patch-by: Han-Wen Nienhuys <hanwenn@gmail.com>
> > Signed-off-by: Patrick Steinhardt <ps@pks.im>
> > ---
> > refs.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 56 insertions(+), 2 deletions(-)
> >
> > diff --git a/refs.c b/refs.c
> > index 7d4a057f36..2d39d3fe80 100644
> > --- a/refs.c
> > +++ b/refs.c
> > @@ -1822,15 +1822,69 @@ static int refs_read_special_head(struct ref_store *ref_store,
> > return result;
> > }
> >
> > +static int is_special_ref(const char *refname)
> > +{
> > + /*
> > + * Special references get written and read directly via the filesystem
> > + * by the subsystems that create them. Thus, they must not go through
> > + * the reference backend but must instead be read directly. It is
> > + * arguable whether this behaviour is sensible, or whether it's simply
> > + * a leaky abstraction enabled by us only having a single reference
> > + * backend implementation. But at least for a subset of references it
> > + * indeed does make sense to treat them specially:
> > + *
> > + * - FETCH_HEAD may contain multiple object IDs, and each one of them
> > + * carries additional metadata like where it came from.
> > + *
> > + * - MERGE_HEAD may contain multiple object IDs when merging multiple
> > + * heads.
> > + *
> > + * - "rebase-apply/" and "rebase-merge/" contain all of the state for
> > + * rebases, where keeping it closely together feels sensible.
> > + *
> > + * There are some exceptions that you might expect to see on this list
> > + * but which are handled exclusively via the reference backend:
> > + *
> > + * - CHERRY_PICK_HEAD
> > + * - HEAD
> > + * - ORIG_HEAD
> > + *
> > + * Writing or deleting references must consistently go either through
> > + * the filesystem (special refs) or through the reference backend
> > + * (normal ones).
> > + */
> > + const char * const special_refs[] = {
> > + "AUTO_MERGE",
> > + "BISECT_EXPECTED_REV",
> > + "FETCH_HEAD",
> > + "MERGE_AUTOSTASH",
> > + "MERGE_HEAD",
> > + };
>
> Is there a reason that we don't want to declare this statically? If we
> did, I think we could drop one const, since the strings would instead
> reside in the .rodata section.
Not really, no.
> > + int i;
>
> Not that it matters for this case, but it may be worth declaring i to be
> an unsigned type, since it's used as an index into an array. size_t
> seems like an appropriate choice there.
Hm. We do use `int` almost everywhere when iterating through an array
via `ARRAY_SIZE`, but ultimately I don't mind whether it's `int`,
`unsigned` or `size_t`.
> > + for (i = 0; i < ARRAY_SIZE(special_refs); i++)
> > + if (!strcmp(refname, special_refs[i]))
> > + return 1;
> > +
> > + /*
> > + * git-rebase(1) stores its state in `rebase-apply/` or
> > + * `rebase-merge/`, including various reference-like bits.
> > + */
> > + if (starts_with(refname, "rebase-apply/") ||
> > + starts_with(refname, "rebase-merge/"))
>
> Do we care about case sensitivity here? Definitely not on case-sensitive
> filesystems, but I'm not sure about case-insensitive ones. For instance,
> on macOS, I can do:
>
> $ git rev-parse hEAd
>
> and get the same value as "git rev-parse HEAD" (on my Linux workstation,
> this fails as expected).
>
> I doubt that there are many users in the wild asking to resolve
> reBASe-APPLY/xyz, but I think that after this patch that would no longer
> work as-is, so we may want to replace this with istarts_with() instead.
In practice I'd argue that nobody is ever going to ask for something in
`rebase-apply/` outside of Git internals or scripts, and I'd expect
these to always use proper casing. So I rather lean towards a "no, we
don't care about case sensitivity".
Patrick
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next prev parent reply other threads:[~2023-11-30 7:44 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-29 8:14 [PATCH 0/4] refs: improve handling of special refs Patrick Steinhardt
2023-11-29 8:14 ` [PATCH 1/4] wt-status: read HEAD and ORIG_HEAD via the refdb Patrick Steinhardt
2023-11-29 21:45 ` Taylor Blau
2023-11-30 7:42 ` Patrick Steinhardt
2023-11-30 17:36 ` Taylor Blau
2023-11-29 8:14 ` [PATCH 2/4] refs: propagate errno when reading special refs fails Patrick Steinhardt
2023-11-29 21:51 ` Taylor Blau
2023-11-30 7:43 ` Patrick Steinhardt
2023-11-29 8:14 ` [PATCH 3/4] refs: complete list of special refs Patrick Steinhardt
2023-11-29 21:59 ` Taylor Blau
2023-11-30 7:44 ` Patrick Steinhardt [this message]
2023-11-30 15:42 ` Phillip Wood
2023-12-01 6:43 ` Patrick Steinhardt
2023-12-04 14:18 ` Phillip Wood
2023-11-29 8:14 ` [PATCH 4/4] bisect: consistently write BISECT_EXPECTED_REV via the refdb Patrick Steinhardt
2023-11-29 22:13 ` Taylor Blau
2023-11-29 22:14 ` [PATCH 0/4] refs: improve handling of special refs Taylor Blau
2023-11-30 7:46 ` Patrick Steinhardt
2023-11-30 17:35 ` Taylor Blau
2023-12-12 7:18 ` [PATCH v2 " Patrick Steinhardt
2023-12-12 7:18 ` [PATCH v2 1/4] wt-status: read HEAD and ORIG_HEAD via the refdb Patrick Steinhardt
2023-12-12 20:24 ` Junio C Hamano
2023-12-12 23:32 ` Ramsay Jones
2023-12-13 0:36 ` Junio C Hamano
2023-12-13 7:38 ` Patrick Steinhardt
2023-12-13 15:15 ` Junio C Hamano
2023-12-14 9:04 ` Patrick Steinhardt
2023-12-14 16:41 ` Junio C Hamano
2023-12-14 13:21 ` Patrick Steinhardt
2023-12-12 7:18 ` [PATCH v2 2/4] refs: propagate errno when reading special refs fails Patrick Steinhardt
2023-12-12 20:28 ` Junio C Hamano
2023-12-13 7:28 ` Patrick Steinhardt
2023-12-12 7:18 ` [PATCH v2 3/4] refs: complete list of special refs Patrick Steinhardt
2023-12-12 7:19 ` [PATCH v2 4/4] bisect: consistently write BISECT_EXPECTED_REV via the refdb Patrick Steinhardt
2023-12-14 13:36 ` [PATCH v3 0/4] refs: improve handling of special refs Patrick Steinhardt
2023-12-14 13:36 ` [PATCH v3 1/4] wt-status: read HEAD and ORIG_HEAD via the refdb Patrick Steinhardt
2023-12-14 13:37 ` [PATCH v3 2/4] refs: propagate errno when reading special refs fails Patrick Steinhardt
2023-12-14 13:37 ` [PATCH v3 3/4] refs: complete list of special refs Patrick Steinhardt
2023-12-14 13:37 ` [PATCH v3 4/4] bisect: consistently write BISECT_EXPECTED_REV via the refdb Patrick Steinhardt
2023-12-20 19:28 ` [PATCH v3 0/4] refs: improve handling of special refs Junio C Hamano
2023-12-21 10:08 ` Patrick Steinhardt
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