build: don't link to optional_mutex if it doesn't exist

[gperciva]

No description provided.

[gperciva]

This has no effect on any project which has liball_optional_mutex_normal (such as libcperciva, spiped, and kivaloo). However, if you updated the build system for imds-filterd (for some crazy reason), you would be unable to compile it, because the build would try to link to liball_optional_mutex_normal.a.

This PR is probably pointless for any practical projects, because once we have noeintr_close, we'll need optional_mutex for everything. But it's rather untidy to have a build system which just plain breaks if we don't have certain files.

[gperciva]

I have no firm feelings about what we should have here. We could spell out the full path to liball_optional_mutex_normal.a (or even that and liball_optional_mutex_pthread.a!).

I initially had the full path, but it looked messy, and I figured the directory would be sufficient to be confident that the project either had optional_mutex or not.

[cperciva]

I'm not sure how this doesn't result in an infinite loop. Running make -V SUBDIR_DEPTH would process the Makefile and come across this line again, right?

[gperciva]

Short answer: because make -v SUBDIR_DEPTH gets it from Makefile (where it's defined in full), not Makefile.BSD.

Medium answer: I'll update the comment.

Better answer: hmm, that's weird, make -f Makefile.BSD -v SUBDIR_DEPTH and -V FINALIZED_SUBDIR_DEPTH both work, instead of going into an infinite loop. I need to investigate more.

[gperciva]

Addendum to that "better answer": even if we run make -f Makefile.BSD -v SUBDIR_DEPTH on the command-line, the top-level Makefile.inc runs make -v SUBDIR_DEPTH (which has the fully defined value) so there's no loop.

[cperciva]

Hmm, so what happens if we run make Makefiles and there isn't a Makefile already? We get a broken one and then it gets fixed the next time we run make Makefiles?

[gperciva]

Not broken. (Although admittedly, that's a happy accident rather than by design.)

metabuild.sh line 25:

SUBDIR_DEPTH=$(${MAKEBSD} -v SUBDIR_DEPTH)

That works fine regardless of whether there's a Makefile or not.

metabuild.sh line 172 puts SUBDIR_DEPTH into the Makefile. Until that point,

.info ${FINALIZED_SUBDIR_DEPTH}

in Makefile.inc shows that it's empty. But after line 172 of metabuild.sh, it's fine.

Line 179 deals with liball via copyvar_LIBALL_optional_mutex.

I've added a comment to metabuild.sh to note this dependency.

[cperciva]

Can we write this instead as as LIBALL=.../liball.a followed by an optional LIBALL+=.../liball_optional_mutex_normal.a ?

[gperciva]

Updated. The comment for FINALIZED_SUBDIR_DEPTH states why it avoids an infinite loop, and I added a LIBALL += for the optional_mutex stuff.

[cperciva]

I wish there was a way to do this without needing the FINALIZED_SUBDIR_DEPTH hack, but I don't see any better alternative.