[libvirt] [PATCH] docs: ensure outputfile is deleted if rst2html/rst2man fail

[Eric Blake]

On 12/11/19 12:59 PM, Daniel P. Berrangé wrote:
> On Wed, Dec 11, 2019 at 12:52:20PM -0600, Eric Blake wrote:
>> On 12/11/19 12:45 PM, Daniel P. Berrangé wrote:
>>> This avoids leaving a zero length or partially generated output
>>> file on errors.
>>>
>>> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
>>> ---
>>>    docs/Makefile.am | 10 +++++-----
>>>    1 file changed, 5 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/docs/Makefile.am b/docs/Makefile.am
>>> index eb8de80b9c..9a1f7a6117 100644
>>> --- a/docs/Makefile.am
>>> +++ b/docs/Makefile.am
>>> @@ -259,21 +259,21 @@ man8_MANS = $(manpages8_rst:%.rst=%.8)
>>>    	   grep -v '^\.\. contents::' < $< | \
>>>    	   sed -e 's|SYSCONFDIR|$(sysconfdir)|g' \
>>>    	       -e 's|RUNSTATEDIR|$(runstatedir)|g' | \
>>> -	   $(RST2MAN) > $@
>>> +	   $(RST2MAN) > $@ || { rm $@ && exit 1; }
>>
>> But still allows a truncated view of the file if another process accesses
>> the file while RST2MAN is still running.  Even better is to generate output
>> to a temp file then atomically mv it into place, so that no concurrent
>> process can ever see an incomplete file.
> 
> Is that a problem that actually impacts negatively in real world for builds?
> In various places in the make rules, we've either done the "|| rm" approach,
> or the temp file and rename approach. Personally I find the recipes using
> the "|| rm" approach are more maintainable/readable.

If you're trying to browse the docs and build at the same time, such as 
checking if your tweak to a doc renders well, you could hit it; but 
you're also right that it's not a show-stopper problem.  Go with 
whatever is easier to maintain if you don't care about the issue of 
atomicity in the files (the rm to avoid a long-term corruption is more 
important than the atomic mv to avoid a short-term corruption).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org