Re: mtime of installed files

[Ralf Wildenhues]

Hi Ludovic,

* Ludovic Courtès wrote on Tue, Jul 20, 2010 at 06:28:46PM CEST:
> Background: Guile 1.9/2.0 has an auto-compilation feature whereby if a
> source file has no corresponding object file in the search path, or if
> the object file is older than the source file, then the source file is
> automatically recompiled and stored in ~/.cache/guile/ccache.

I assume this part has nothing to do with Automake per se, right?

> Packages that use Guile can also choose to pre-compile all their source
> files and install both the source and object files.  This saves the need
> for users to auto-compile the source.  (This is what Guile does with its
> own source files.)

I assume this, too, is currently done without help of Automake, or make,
for that matter.

> For this to work, we need “make install” to guarantee the relation
> mtime(installed-object) >= mtime(installed-source), assuming we have
> mtime(builddir-object) >= mtime(srcdir-source), which will always be the
> case unless the computer’s clock is skewed.
> 
> Do Automake-generated makefiles provide such a guarantee?  Regardless of
> the ‘make’ implementation, OS, etc.?

This is actually a tough question; it has little to do with Automake,
much more with any or all of make, tar, cp, touch, install, libc, file
system, and the kernel.

`info Autoconf --index timestamp' leads to two nodes describing some of
the problems with time stamps.  The gist is that in the worst case, any
of the above tools independently may have full or only limited timestamp
resolution (nowadays that would mostly be either nanosecond or 1 second).
Even well-run software distributions without user-compiled tools
typically can't afford to (or don't) update all of these tools at once.

Tools with limited resolution are expected to cut off fractional bits,
but there has been at least one report on autotools lists (ping me if
you need a pointer) of tools that seem to round, which violates Posix
and leads to disaster.  I do not recollect how wide-spread they are.

Anyway, most of the time, these issues can be worked around, for example
by sleeping for a second before building builddir objects, or between
installing sources and installing object files, depending on whether
install -C is used or not.

Hope that helps.  It might be a bit tricky or ugly to actually put this
sleep command in a normal automake Makefile.am, so if you know what
exactly you need, we can create an example.

Cheers,
Ralf