Re: [PATCH 06/25] Generate c for feature instead of tdesc

[Pedro Alves <palves@redhat.com>]

On 06/22/2017 03:49 PM, Yao Qi wrote:
> Pedro Alves <palves@redhat.com> writes:
> 
>> > On 06/12/2017 09:41 AM, Yao Qi wrote:
>>> >> +$(FEATURE_CFILES): %.c: %.xml.tmp
>>> >> +	$(GDB) -nx -q -batch \
>>> >> +	  -ex 'maint print c-tdesc $<' > $@.tmp
>>> >> +	sh ../../move-if-change $@.tmp $@
>>> >> +	rm $<
>>> >> +
>>> >> +%.xml.tmp: %.xml
>>> >> +	echo "<?xml version=\"1.0\"?>" > $@
>>> >> +	echo "<!DOCTYPE target SYSTEM \"gdb-target.dtd\">" >> $@
>>> >> +	echo "<target>" >> $@
>>> >> +	echo "  <architecture>" >> $@
>>> >> +	if test $(findstring i386/32bit-,$@); then echo "i386" >> $@ ; fi;
>>> >> +	echo "  </architecture>" >> $@
>>> >> +	echo "  <xi:include href=\"$(notdir $<)\"/>" >> $@
>>> >> +	echo "</target>" >> $@
>>> >> +
>> >
>> > Don't we need move-if-change here?
>> >
> move-if-change from what to what?  *.xml.tmp is removed after *.c is
> generated.

"*.xml.tmp.tmp" to "*.xml.tmp".  I.e., from $@.tmp to $@, just
like the rule further above.

I may be missing something, but AFAICS, the "%.xml.tmp: %.xml"
rule is writing to the target directly.  If the build
is interrupted in the middle of that rule, we end up with an
incomplete "%.xml.tmp" file left over in the file system.
Then the next time you invoke make, make will think that
the "%.xml.tmp" file is already up to date and won't rebuild it.
As a result, you'll pass a broken xml.tmp file to
"maint print c-tdesc".

Avoiding these issues is the whole point of move-if-change,
by atomically creating the target file.

Thanks,
Pedro Alves