Re: New feature: Post-requisites

[Edward Welbourne]

Alejandro Colomar (Friday, July 22, 2022 14:05)

> I'd like to suggest a new feature: post-requisites.

>

> As the word suggests, as pre-requisites are satisfied before a given

> target, post-requisites would be satisfied _after_ a given target.

and just as a prerequisite can be built at any time before, but must be

built before, the post-requisite can be run at any time after, but must

be built after.  So if several targets that are being built require some

given post-requisite, it waits until all of them have been built.

> Why would someone ever need that in a Makefile:

>

> In an install target for a library, you may want to run ldconfig(8)

> after any install targets have executed.

>

> In an install target for the linux man-pages, you may want to run

> mandb(8)  after any pages have been installed in the system.

I think the traditional solution to this would be to make the install

depend on rules that get individual files installed and have its command

run the post-install steps, or have the install rule depend on a set of

post-install rules (each of which does one post-install task), that in

turn depend on all the files whose updates would trigger them.  However,

the problem with that is that if I want to just update the one man page,

the mandb(8) run that's needed following it won't be run unless I build

the mandb-run rule, that depends on *all* my man pages.  So I can't do

fine-grained targeted "build this one thing" parts of my build process.

Post-requisites would take care of that; if I build and install an

update to any man page, then mandb gets run, regardless of what else I

am also building, aside from mandb's run happening after any of those

that need it to be done later.

> The proposed syntax would be similar to that of order only

> prerequisites, but using '>' instead of '|':

>

> [

> [...]

>

> $(DESTDIR)$(man3dir)/foo.3: man3/foo.3 | $(DESTDIR)$(man3dir) > update-mandb

>         $(info INSTALL  $@)

>         @$(INSTALL_DATA) -T $< $@

>

> $(DESTDIR)$(man3dir)/bar.3: man3/bar.3 | $(DESTDIR)$(man3dir) > update-mandb

>         $(info INSTALL  $@)

>         @$(INSTALL_DATA) -T $< $@

>

> .PHONY: update-mandb

> update-mandb:

>         $(info MANDB)

>         @mandb

>

> .PHONY: install

> install: $(DESTDIR)$(man3dir)/foo.3 $(DESTDIR)$(man3dir)/bar.3

> ]

>

> So that `make install` would install foo.3 and bar.3, and after all of

> the pages have been installed, mandb(8) would be run.

> And `make /usr/local/share/man/man3/foo.3` would install foo.3, and

> after it, it would run mandb(8).

>

> Does that make sense to you?

and if my package (e.g. perl) has a whole library of man pages, I'd be

able to do updates to just one (or a few) of them at a time and get

mandb run after that update, regardless of which subset of pages I

update, rather than only being able to get that run when I install the

whole library.

I guess it would also be useful for tidying away scratch directories and

the like, used by rules for their transient or intermediate files.

It sounds like a neat idea.  I wonder how practical it would be to

implement ...

      Eddy.