Re: [PATCH 07/10] t1404: demonstrate two problems with reference transactions

> On Tue, Aug 29, 2017 at 1:20 AM, Michael Haggerty <mhagger@alum.mit.edu> wrote:
>> [...]
>> +test_expect_failure 'no bogus intermediate values during delete' '
>> +       prefix=refs/slow-transaction &&
>> +       # Set up a reference with differing loose and packed versions:
>> +       git update-ref $prefix/foo $C &&
>> +       git pack-refs --all &&
>> +       git update-ref $prefix/foo $D &&
>> +       git for-each-ref $prefix >unchanged &&
>> +       # Now try to update the reference, but hold the `packed-refs` lock
>> +       # for a while to see what happens while the process is blocked:
>> +       : >.git/packed-refs.lock &&
>> +       test_when_finished "rm -f .git/packed-refs.lock" &&
>> +       {
>> +               sleep 1 &&
>> +               rm -f .git/packed-refs.lock &
>> +       } &&
>> +       pid1=$! &&
>> +       {
>> +               # Note: the following command is intentionally run in the
>> +               # background. We extend the timeout so that `update-ref`
>> +               # tries to acquire the `packed-refs` lock longer than it
>> +               # takes the background process above to delete it:
>> +               git -c core.packedrefstimeout=2000 update-ref -d $prefix/foo &
>> +       } &&
>> +       pid2=$! &&
>> +       ok=true &&
>> +       while kill -0 $pid2 2>/dev/null
> 
>     If sig is 0, then no signal is sent, but error checking is still
>     performed; this can be used to check for the existence of a
>     process ID or process group ID.
> 
> So the kill -0 is the idiomatic form of "while $pid2 is still alive"?
> ignoring errors due to the dev/null redirection?
> 
> And due to the nature of this test we have to have a busy
> loop, we cannot rate limit the cpu usage inside the loop
> via some shorter sleeps, as ideally we want to observe
> the ref at any time.

>> +       do
>> +               sha1=$(git rev-parse --verify --quiet $prefix/foo || echo undefined) &&
>> +               case "$sha1" in
>> +               $D)
>> +                       # This is OK; it just means that nothing has happened yet.
>> +                       : ;;
>> +               undefined)
>> +                       # This is OK; it means the deletion was successful.
>> +                       : ;;
>> +               $C)
>> +                       # This value should never be seen. Probably the loose
>> +                       # reference has been deleted but the packed reference
>> +                       # is still there:
>> +                       echo "$prefix/foo incorrectly observed to be C" &&
>> +                       break
>> +                       ;;
>> +               *)
>> +                       # WTF?
>> +                       echo "$prefix/foo unexpected value observed: $sha1" &&
>> +                       break
>> +                       ;;
>> +               esac
>> +       done >out &&
>> +       wait $pid1 &&
>> +       wait $pid2 &&
> 
> oh, you use explicit pids here to check each exit code.
> 
>> If anybody has suggestions for better ways to test these things,
>> please speak up :-)
> 
> I don't think I'd have a satisfactory answer to that, as the timing is inherent
> to the things we test. In other software projects that are less low level, I
> would have suggested to use a time/clock mock, which can be stopped
> and then inspection can be performed at defined states.