module/Makefile.in: don't run xargs if empty

[gmelikov]

Motivation and Context

If stdin is empty - don't run xargs command,
otherwise we can get cp: missing file operand
error (reproducer triggers on spl dir).

How Has This Been Tested?

Reproducer - 'make -j 2 pkg-utils deb-kmod'

list='avl/ icp/ lua/ nvpair/ spl/ os/linux/spl/ unicode/ zcommon/ zfs/ os/linux/zfs/'; for objdir in $list; do \
	(cd ../module && find $objdir \
	-name '*.c' -o -name '*.h' -o -name '*.S' | \
	xargs cp --parents -t /home/travis/build/gmelikov/zfs/module/$distdir); \
done

cp: missing file operand

Don't reproduce after this patch.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[gmelikov]

Hmm, looks like now we have some problems with make -j X with X >1. I've got another error with it.

[codecov]

Codecov Report

Merging #9418 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9418      +/-   ##
==========================================
+ Coverage   78.95%   79.02%   +0.07%     
==========================================
  Files         410      410              
  Lines      122499   122499              
==========================================
+ Hits        96723    96809      +86     
+ Misses      25776    25690      -86

Flag	Coverage Δ
#kernel	79.74% <ø> (-0.01%)	⬇️
#user	66.8% <ø> (+0.58%)	⬆️

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[ghost]

This will be fine for FreeBSD.

[behlendorf]

Looks good, just one portability question.

[behlendorf]

looks like now we have some problems with make -j X with X >1. I've got another error with it.

I don't think I've seen this. What's the error?

[gmelikov]

@behlendorf

# make -j 2 pkg-utils deb-kmod > out.log 2> out2.log
# echo $?
2
# cat out2.log
cp: cannot create regular file 'zfs-0.8.0/config/ax_python_devel.m4': File exists
make[3]: *** [Makefile:965: distdir-am] Error 1
make[2]: *** [Makefile:960: distdir] Error 2
make[1]: *** [Makefile:1065: dist] Error 2
make: *** [Makefile:1302: srpm-utils] Error 2
make: *** Waiting for unfinished jobs....

Looks like now each thread tries to do the same thing with ./zfs-0.8.0. Maybe there is a better fix, but the fast one is to ignore such cases.

[behlendorf]

@gmelikov the -j option is not needed when building packages. The spec files internally use %{?_smp_mflags} will set -j appropriately for the system. Though, I agree it shouldn't result in an error.