On 09/10/2013 03:58 AM, Michal Privoznik wrote:
Some things to remember that affect VPATH builds, and where an in-tree build is blissfully unaware of the issues: if a VPATH build fails to find a file that was used as a prereq of any other target, then the rule for that file will expand $@ to prefer the current build dir (bad because a VPATH build on a fresh checkout will then stick $@ in the current directory instead of the desired srcdir); conversely, if a VPATH build finds the file in srcdir but decides it needs to be rebuilt, then the rule for that file will expand $@ to include the directory where it was found out-of-date (bad for an explicit listing of $(srcdir)/$@ because an incremental VPATH build will then expand srcdir twice). As we want these files to go into srcdir unconditionally, we have to massage or avoid $@ for any recipe that involves one of these files.
-$(srcdir)/remote_dispatch.h: $(srcdir)/../src/rpc/gendispatch.pl \ +remote_dispatch.h: $(srcdir)/../src/rpc/gendispatch.pl \ $(REMOTE_PROTOCOL) $(AM_V_GEN)$(PERL) -w $(srcdir)/../src/rpc/gendispatch.pl \ - --mode=server remote REMOTE $(REMOTE_PROTOCOL) > $@ + --mode=server remote REMOTE $(REMOTE_PROTOCOL) \ + > $(srcdir)/remote_dispatch.h
The other option is to have this line as:
$(srcdir)/$@
No, that explicitly does not work in VPATH setups, as mentioned in my commit message. On the first run (after a fresh checkout), $@ sees that remote_dispatch.h does not exist, so $@ expands to 'remote_dispatch.h', and the recipe creates $(srcdir)/remote_dispatch.h. But in an incremental rebuild, after changing a prerequisite file (such as the .x file embedded in $(REMOTE_PROTOCOL)), $@ discovers that the target file already exists in $(srcdir) via the VPATH lookup, so it expands $@ to '$(srcdir)/remote_dispatch.h', and the recipe attempts to build '$(srcdir)/$(srcdir)/remote_dispatch.h'. If $(srcdir) is .., then you have a mess on your hands (creating the wrong file, and the right file didn't get updated). Hence, the patch HAS to use absolute naming, rather than $@ magic, for anything that we _always_ want placed into srcdir. (For an in-tree build, $(srcdir) is '.', and since you can't tell between './remote_dispatch.h' vs. '././remote_dispatch.h', the problem only affects VPATH builds.)
But this version is okay as is. ACK.
Thanks for reviewing the series; I'll push shortly. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org