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Re: On time64 and Large File Support

From: Russ Allbery
Subject: Re: On time64 and Large File Support
Date: Sat, 12 Nov 2022 12:54:42 -0800
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)

Wookey <> writes:

> Now, I'm not yet sure if just having autoconf 2.72 will actually break
> things. AIUI, these changes only apply where LFS
> (-D_FILE_OFFSET_BITS=64) is turned on, so in Debian at least, where that
> is not the default on 32bit arches, maybe this is OK. But probably quite
> a lot of packages already enable LFS so they are suddenly going to get a
> new ABI if they expose timet anywhere?
> shows
> 163 pages of hits, and a quick peruse suggsts that AC_SYS_LARGEFILE is
> used by a lot of packages (as you might expect - this transition has
> been going on for many years). And just having that macro in
> configure.(in|ac) will turn 64-bit timet on if you autoreconf with
> 2.72. Right?

If indeed pre-existing use of AC_SYS_LARGEFILES would suddenly enable
64-bit time_t on autoreconf, I can name two packages just off the top of
my head that this change to Autoconf will immediately break if their
Debian packages are rebuilt with a newer version of Autoconf, creating
severe bugs.

libremctl will have its ABI changed without any coordination or versioning
(which I will be doing, moving forward, but have not started tackling yet
in part because I was waiting to see what the plan would be and whether
there will be some coordinated change to SONAMEs, a new architecture, or
what).  And INN, which admittedly is a disaster about things like this for
lots of historical reasons, will have its *on-disk file format* changed
without notice in a way that will cause serious failure and possibly data
corruption on upgrades.

This is just wildly backward-incompatible and seems like an awful idea.
If we're going to throw a big switch and rebuild everything, it needs to
be done at a distro-wide level.  I believe the only safe thing for
Autoconf to do is to provide an opt-in facility, similar to what was done
for AC_SYS_LARGEFILE, and then leave deciding whether to opt in to
higher-level machinery.

> However my limited understanding as of right now says that autoconf 2.72
> tying 64bit time_t to use of AC_SYS_LARGEFILE means that 2.72 can't be
> used in debian yet. So I currently favour not tying them together in
> this release.

That's also my understanding from the thread so far, although I'm not sure
that I'm following all of the subtleties.

> People have been using AC_SYS_LARGEFILE without 64bit time_t for many
> years now so it's not yet clear to me why that cannot continue.

And these are conceptually not at all the same thing.  I saw Paul's
explanation for why he views them as fundamentally the same because of
their effect on system calls like stat, but I certainly don't think of
them that way and I am quite dubious many other people will either.  The
set of things that I have to check to ensure that time_t is handled
correctly is totally different than the set of things I thought about when
enabling AC_SYS_LARGEFILE many years in the past.

I recognize that there will be overlap once file timestamps are past 2038
and that will happen sooner than anyone plans for, but it's still true
that this has *not* happened right now and this therefore is not currently
creating many bugs, whereas this switch in this way will create many, very
serious bugs immediately.

Russ Allbery (             <>

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