Re: [PATCH 07] Use append mode to capture parallel test output.

[Ralf Wildenhues]

* Akim Demaille wrote on Sat, Mar 14, 2009 at 01:24:28PM CET:
> Le 14 mars 09 à 11:56, Ralf Wildenhues a écrit :
>
>> D'oh.  Wonder how often we will keep encountering this.

> I have not understood what's going on here.  Are these files running  
> concurrently in the same pwd?

No.  The issue is different.  Whenever you have
  $program > file

and $program itself spawns other processes which inherit the file
descriptor to 'file', and write to it concurrently, then POSIX does
not guarantee that those writes are atomic.  More specifically, while
it guarantees that all writes happen, there may be a time at which one
of the processes has already written, but not updated the file offset.
When another process then starts a write, it will happen at the old
offset, and overwrite the output from the first process.

Using O_APPEND avoids this race: POSIX guarantees atomicity in this
case, for writes below (depending on the file type, there may be a limit
on the maximum number of bytes guaranteed to be written atomically).

An alternative to O_APPEND is to use a pipe:
  make -j | cat > file

works just as well.  Or a named pipe, I guess.

This issue really shows up in practice, but is probably very hard to
reproduce on systems with only one processor (I've never seen that).

Hope that helps.

Cheers,
Ralf

>>    Use append mode to capture parallel test output.
>>
>>    * tests/lisp8.test: Use append mode for output from `make -j',
>>    to avoid dropped lines.
>>    * tests/parallel-tests3.test: Likewise.
>>
>> diff --git a/tests/lisp8.test b/tests/lisp8.test
>> index 578cd9c..3e4981c 100755
>> --- a/tests/lisp8.test
>> +++ b/tests/lisp8.test
>> @@ -39,7 +39,8 @@ $AUTOCONF
>> $AUTOMAKE --add-missing
>> ./configure
>>
>> -$MAKE -j >stdout || { cat stdout; Exit 1; }
>> +: >stdout
>> +$MAKE -j >>stdout || { cat stdout; Exit 1; }
>>
>> cat stdout
>> test 1 -eq `grep 'Warnings can be ignored' stdout | wc -l`
>> @@ -51,7 +52,8 @@ test -f elc-stamp
>>
>> rm -f am-*.elc
>>
>> -$MAKE -j >stdout
>> +: >stdout
>> +$MAKE -j >>stdout