Re: [PATCH] gdb: Fix instability in thread groups test

[Pedro Alves <palves@redhat.com>]

On 08/10/2018 10:26 PM, Simon Marchi wrote:
> On 2018-08-10 05:57, Andrew Burgess wrote:
>> In the test script gdb.mi/list-thread-groups-available.exp we ask GDB
>> to list all thread groups, and match the output against a
>> regexp. Occasionally, I would see this test fail.
>>
>> The expected output is a list of entries, each entry looking roughly
>> like this:
>>
>>   {id="<DECIMAL>",type="process",description="<STRING>",
>>    user="<STRING>",cores=["<DECIMAL>","<DECIMAL>",...]}
>>
>> All the fields after 'id' and 'type' are optional, and the 'cores'
>> list can contain 1 or more "<DECIMAL>" entries.
>>
>> On my machine (Running Fedora 27, kernel 4.17.3-100.fc27.x86_64)
>> usually the 'description' is a non-empty string, and the 'cores' list
>> has at least one entry in it.  But sometimes, very rarely, I'll see an
>> entry in the process group list where the 'description' is an empty
>> string, the 'user' is the string "?", and the 'cores' list is empty.
>> Such an entry looks like this:
>>
>>    {id="19863",type="process",description="",user="?",cores=[]}
>>
>> I believe that this is caused by the process exiting while GDB is
>> scanning /proc for process information.  The current code in
>> gdb/nat/linux-osdata.c is not (I think) resilient against exiting
>> processes.
>>
>> This commit adjusts the regex that matches the 'cores' list so that an
>> empty list is acceptable, with this patch in place the test script
>> gdb.mi/list-thread-groups-available.exp never fails for me now.
>>
>> gdb/testsuite/ChangeLog:
>>
>>     * gdb.mi/list-thread-groups-available.exp: Update test regexp.
>> ---
>>  gdb/testsuite/ChangeLog                               | 4 ++++
>>  gdb/testsuite/gdb.mi/list-thread-groups-available.exp | 2 +-
>>  2 files changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
>> b/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
>> index c4dab2a2c34..88f9ee9b63d 100644
>> --- a/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
>> +++ b/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
>> @@ -45,7 +45,7 @@ set id_re "id=\"$decimal\""
>>  set type_re "type=\"process\""
>>  set description_re "description=\"$string_re\""
>>  set user_re "user=\"$string_re\""
>> -set cores_re "cores=\\\[\"$decimal\"(,\"$decimal\")*\\\]"
>> +set cores_re "cores=\\\[(\"$decimal\"(,\"$decimal\")*)?\\\]"
>>
>>  # List all available processes.
>>  set process_entry_re
>> "{${id_re},${type_re}(,$description_re)?(,$user_re)?(,$cores_re)?}"
> 
> Hi Andrew,
> 
> The patch LGTM.  I manually reproduced this case by spawning a process (tail -f /dev/null) and noting its pid.  In linux_xfer_osdata_processes, I added:
> 
>   if (pid == <pid>)
>     sleep (5);
> 
> and killing the process during that sleep.

But shouldn't we make GDB handle this better?  Make the output
more "atomic" in the sense that we either show a valid complete
entry, or no entry?  There's an inherent race
here, since we use multiple /proc accesses to fill up a process
entry.  If we start fetching process info for a process, and the process
disappears midway, I'd think it better to discard that process's entry,
as-if we had not even seen it, i.e., as if we had listed the set of
processes a tiny moment later.

Thanks,
Pedro Alves