bug#36684: "View build log" - but it's empty (at first)!

[Chris Marusich <cmmarusich@gmail.com>]

Hi,

Sometimes, I see an error like the following (it is not specific to
ant-bootstrap):

--8<---------------cut here---------------start------------->8---
$ guix package -m ~/profile-packages.scm 
...
building /gnu/store/76apf0hpcdabpjy0839nhkwgfrz3m8z5-ant-bootstrap-1.8.4.drv...
| 'build' phasebuilder for `/gnu/store/76apf0hpcdabpjy0839nhkwgfrz3m8z5-ant-bootstrap-1.8.4.drv' failed with exit code 1
build of /gnu/store/76apf0hpcdabpjy0839nhkwgfrz3m8z5-ant-bootstrap-1.8.4.drv failed
View build log at '/var/log/guix/drvs/76/apf0hpcdabpjy0839nhkwgfrz3m8z5-ant-bootstrap-1.8.4.drv.bz2'.
cannot build derivation `/gnu/store/3l11fmnyc4k1dcwlijk4vdsq70jc4lqs-icedtea-3.7.0.drv': 1 dependencies couldn't be built
killing process 20600
cannot build derivation `/gnu/store/b071r8bm2n01mp50g6r5ghxl0m09akdx-profile.drv': 1 dependencies couldn't be built
guix package: error: build of `/gnu/store/b071r8bm2n01mp50g6r5ghxl0m09akdx-profile.drv' failed
--8<---------------cut here---------------end--------------->8---

But when I try to view the log file very quickly after getting these
messages (within about 1 minute or so), I get:

--8<---------------cut here---------------start------------->8---
$ file  /var/log/guix/drvs/7f/n896phqyv9r636yhjipa9k4qm7g9ni-evolution-data-server-3.30.2.drv.bz2
/var/log/guix/drvs/7f/n896phqyv9r636yhjipa9k4qm7g9ni-evolution-data-server-3.30.2.drv.bz2: empty
--8<---------------cut here---------------end--------------->8---

However, when I come back a few minutes later, mysteriously the file has
been populated:

--8<---------------cut here---------------start------------->8---
$ file  /var/log/guix/drvs/7f/n896phqyv9r636yhjipa9k4qm7g9ni-evolution-data-server-3.30.2.drv.bz2 
/var/log/guix/drvs/7f/n896phqyv9r636yhjipa9k4qm7g9ni-evolution-data-server-3.30.2.drv.bz2: bzip2 compressed data, block size = 900k
--8<---------------cut here---------------end--------------->8---

Indeed, I was originally going to report that the file was empty, but
while I was writing this email, it filled up!  Is this behavior
expected?  I anticipated that the file would be finalized (empty or not)
before the helpful message "View built log at ..." was printed, but it
seems like that message is proleptic, which makes it a little confusing.

-- 
Chris