Re: [PATCH] Makefile: add and use the ".DELETE_ON_ERROR" flag

Ævar Arnfjörð Bjarmason wrote:

On Wed, Jun 23 2021, Felipe Contreras wrote:

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Ævar Arnfjörð Bjarmason wrote:

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On Wed, Jun 23 2021, Felipe Contreras wrote:

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Ævar Arnfjörð Bjarmason wrote:

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As in db10fc6c09f this allows us to remove patterns of removing
leftover $@ files at the start of rules, since previous failing runs
of the Makefile won't have left those littered around anymore.

I'm not as confident that we should be replacing the "mv $@+ $@"
pattern entirely, since that means that external programs or one of
our other Makefiles might race and get partial content.

The reason I did it in db10fc6c09 is because both asciidoctor and
asciidoc should deal with temporary files by themselves (like gcc). If
you interrupt the build nothing gets generated.

If you interrupt the build default make behavior without
.DELETE_ON_ERROR kicks in.

Generally yes, but it's possible the program traps the interrupt signal,
in which case make never receives it.

Okey, so by "should deal with [it]" you meant that would be ideal, not
that it's something they're doing now. I misunderstood you there.

It is tricky.

For example asciidoctor does trap interrupt signals, but deals with them
correctly. On the other hand asciidoc does not, but they clearly did
intent to, just did it wrong. I sent a pull request for asciidoc to fix
that [1], but so far no response.

So it's a mixture of both; ideally they should do it, and they kind of
do, but not all of them. Certainly git scripts do not. But they could.

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My gcc 8.3.0 just does an unlink()/openat(..., O_RDWR|O_CREAT|O_TRUNC)
dance followed by chmod() when I do e.g.:

    gcc -o main main.c

So no in-place atomic renaming, does yours do something different?

It doesn't rename the file, but if interrupted the file is unlinked.

Right, and with .DELETE_ON_ERROR that "interrupted" is extended to
"interrupted, or errors", but bringing this discussion around that's why
I was confident in replacing the "rm" pattern at the start (which really
is 100% replaced by .DELETE_ON_ERROR), but not the "mv" at the end
(which isn't, and is an orthagonal feature).

Depnds on what "the feature" is.

If the feature is not having lingering partial files on error, then gcc already
deals with that.

If the feature is never having partial files at all, then yeah, you need
the "mv" at the end, but as Jeff and Junio already pointed out: that
feature is of doubtful value.

I see value on .DELTE_ON_ERROR, not so much on never having partial
files. I have tried to imagine why anybody would want this, and I just
can't picture it, though that could be a failure of my imagination.

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However, other scripts like build-docdep.perl would indeed generate
partial output.

In my opinion it's the scripts themselves that should be fixed, and not
the Makefile, *if* we care about this at all.

I don't think default tool/make/*nix semantics are broken, I just think
it's neat to do that rename dance yourself, it's a cheap way to
guarantee that we always have working tools for use by other concurrent
scripts.

It is cheap in the sense that it doesn't cost the computer much, but it
makes the code less maintenable and harder to read.

To me it's a layering violation. If the tool is already dealing with
interrupted builds, and on top of that make is doing the same, not only
for interrupted builds but also failures, then it makes little sense to
add even more safeties on top of that in the Makefile.

I agree for interrupted builds, but we're talking about
in-place-renaming, which is orthogonal.

In-place-renaming is the means, the end-goal (I presume) is to never
have partial files.

Yes, it's orthogonal, but also I don't see the point.

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If this was really an important feature, it should be part of make
itself, or ninja, or whatever.

IMO the whole point of DELETE_ON_ERROR is to avoid everyone doing the
exact same dance in their Makefiles.

I agree it would be an interesting make feature, but something pretty
far from what it's doing now.

In general "make" has been intentionally sloppy about this sort of
thing. When you make a file "foo" it doesn't enforce that you fsync it
either, or that if it's being created the directory it's inserted into
is fsync'd.

In a POSIXly-strict sense it can't assume that it can operate properly
without those things happening, but in practice modern OS's deal with it
just fine, so "make" leaves that to the rule itself.

It would be nice to have a make feature to e.g. have individual rules
say "I emit on stdout, put it into $@ for me", then it could in-place
rename, fsync, display progress through "pv(1)" or whatever.

Perhaps. I still don't see why this is something important.

Either way a pattern I've seen lately in a lot of software is a
reluctance to modernize itself, and that results in other software
starting from scracth (GCC vs. LLVM, vim vs. neovim, and make vs. ninja).

If we are reaching the limit to what make can offer us--and plenty of
other projects are already using more modern alternatives--does it
really make much sense to focus on a small thing make can't offer us
natively and work around that?

Maybe it would make more sense to stop relying on make so much and
attempt to make other tools support this feature natively.

Cheers.

[1] https://github.com/asciidoc-py/asciidoc-py/pull/195

-- 
Felipe Contreras