Ævar Arnfjörð Bjarmason [off-list ref] writes:
Per the log of changes to the Makfile and Junio's recent comment about
[1] why that pattern got introduced it was for a different reason
entirely, i.e. ("[]" edits are mine, for brevity):
[T]hat age old convention [...] is spelled [as]:
thing:
rm -f thing thing+
prepare contents for thing >thing+
Did I say that? I recall I specifically avoided the "redirection"
because this is *NOT* shell-script only principle. If a command is
so broken that "cmd -o thing" that fails to work correctly leaves a
broken output in thing, then the pattern should be applied and made
to "cmd -o thing+ && mv thing+ thing".
On the other hand, if "cmd" refrains from leaving a half-baked
result in "-o thing" (and I believe $(CC) is well-behaved even on
AIX), I do not think it is a good idea to use that pattern. One
version of AIX may refuse to overwrite a file in use because
creat("thing") that is necessary for "-o thing" may fail while
"thing" is in use), but another system may refuse to rename over a
file in use (i.e. "-o thing+" into a brand new "thing+" may be OK
but the final "mv thing+ thing" may fail). So pretending that it is
a cure for broken use case is cluttering Makefile for no real
benefit and leading readers into confused thinking.
mv thing+ thing
It protects us from a failure mode where "prepare contents for
thing" step is broken and leaves a "thing" that does not work, but
confuses make that make does not need to rebuild it, if you wrote it
as such:
thing:
prepare contents for thing >thing
[It might leave behind a corrupt 'thing'.] In any case, it is not
"we are trying to make thing available while it is being
rewritten" at all.
That makes perfect sense for shellscripts, but as this change shows
there's other good reasons to use this age old convention that weren't
considered at the time.
So, no, the age old convention may have considered and does not
apply to such a use case.
quoted hunk
git$X: git.o GIT-LDFLAGS $(BUILTIN_OBJS) $(GITLIBS)
- $(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
- $(filter %.o,$^) $(LIBS)
+ $(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@+ $(ALL_LDFLAGS) \
+ $(filter %.o,$^) $(LIBS) && \
+ mv $@+ $@
Really, does anybody else use "$(CC) -o $@" in such a way in their
Makefile? Having to do this smells simply crazy (I am not saying
you are crazy---the platform that forces you to write such a thing
is crazy).
So, while I do not think the end result would break the build (other
than it probably would leave crufts "make clean" would not notice
behind when interrupted in the middle), I am moderately negative on
this change.
Thanks.
On Mon, Mar 29 2021, Junio C Hamano wrote:
Ævar Arnfjörð Bjarmason [off-list ref] writes:
quoted
Per the log of changes to the Makfile and Junio's recent comment about
[1] why that pattern got introduced it was for a different reason
entirely, i.e. ("[]" edits are mine, for brevity):
[T]hat age old convention [...] is spelled [as]:
thing:
rm -f thing thing+
prepare contents for thing >thing+
Did I say that? I recall I specifically avoided the "redirection"
because this is *NOT* shell-script only principle. If a command is
so broken that "cmd -o thing" that fails to work correctly leaves a
broken output in thing, then the pattern should be applied and made
to "cmd -o thing+ && mv thing+ thing".
[I see this was already noted downthread...]
On the other hand, if "cmd" refrains from leaving a half-baked
result in "-o thing" (and I believe $(CC) is well-behaved even on
AIX), I do not think it is a good idea to use that pattern. One
version of AIX may refuse to overwrite a file in use because
creat("thing") that is necessary for "-o thing" may fail while
"thing" is in use), but another system may refuse to rename over a
file in use (i.e. "-o thing+" into a brand new "thing+" may be OK
but the final "mv thing+ thing" may fail). So pretending that it is
a cure for broken use case is cluttering Makefile for no real
benefit and leading readers into confused thinking.
It does fix this issue entirely on AIX. So it's a cure for the broken
case.
I can assure you that not having to regularly log in to the GCC farm AIX
box and remember how to deal with IBM's ps/kill etc. just to do another
build/test is a real benefit :)
Whereas maybe there's another system we're not fixing with this patch,
but does it matter? I don't see how it would make things worse for that
OS, if it exists. But it sounds like it's just a hypothetical.
quoted
mv thing+ thing
It protects us from a failure mode where "prepare contents for
thing" step is broken and leaves a "thing" that does not work, but
confuses make that make does not need to rebuild it, if you wrote it
as such:
thing:
prepare contents for thing >thing
[It might leave behind a corrupt 'thing'.] In any case, it is not
"we are trying to make thing available while it is being
rewritten" at all.
That makes perfect sense for shellscripts, but as this change shows
there's other good reasons to use this age old convention that weren't
considered at the time.
So, no, the age old convention may have considered and does not
apply to such a use case.
The reason I mentioned this was to specifically address the implied "why
would we need this?" part of your E-Mail that you're quoting.
I think that started out as us talking past one another, so let me try
to summarize.
I was basically saying "here's a well-known command idiom" that can be
used for XYZ.
I think you're basically saying "in git.git, I introduced this idiom to
deal with problem ABC".
ABC and XYZ aren't incompatible. I'm just saying this can and does solve
both problems[continued below].
quoted
git$X: git.o GIT-LDFLAGS $(BUILTIN_OBJS) $(GITLIBS)
- $(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
- $(filter %.o,$^) $(LIBS)
+ $(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@+ $(ALL_LDFLAGS) \
+ $(filter %.o,$^) $(LIBS) && \
+ mv $@+ $@
Really, does anybody else use "$(CC) -o $@" in such a way in their
Makefile? Having to do this smells simply crazy (I am not saying
you are crazy---the platform that forces you to write such a thing
is crazy).
Yes, if you do say a Google search for "Cannot open or remove a file
containing a running program" you'll find that there's 15k results of
people basically (re)discovering this problem in porting their software
to AIX, and the solutions being some variant of "yes AIX sucks, just use
this 'cmd >x+ && mv x+ x' trick".
You can also invoke a "slibclean" program to reticulate AIX's splines,
but I thought this sucked less.