Re: [PATCH] Fix testsuite hangs when gdb_test_multiple body errors out (Re: GDB 8.2.90 available for testing)

[Pedro Alves <palves at redhat dot com>]

On 03/22/2019 08:44 PM, Pedro Franco de Carvalho wrote:
> Pedro Alves <palves@redhat.com> writes:
> 
> Hello,
> 
>> I spent a while trying to fix this, and I came up with the patch below.
>>
>> WDYT?
> 
> Makes sense to me.
> 
>> So this commit solves this by appending an "eof" with an empty
>> spawn_id list, so that it won't ever match.
> 
> This is a clever solution, but just to be sure, can we actually rely on
> this behavior when the list is empty?  This did work when I tested it,
> but could some Expect version conceivably do something else like
> returning an error when parsing a body with a "-i" that uses an empty
> list?

I'd think not, but I can't predict the future so I can't be sure,
of course.

I looked at expect's source code a little and (in my untrained eyes)
it didn't seem like there would be a reason for it not to work,
since -i's argument is just read as a list.  I think a reasonable use
case would be to allow an empty variable as spawn id, so that
some patterns would be disabled depending on the variable being
empty or not (e.g., "-i $my_spawn_id_if_any").

When I thought of this trick, I was going to use an indirect
spawn id:

 The -i flag may also name a global variable in which case the variable
 is read for a list of spawn ids. The variable is reread whenever it changes.
 This provides a way of changing the I/O source while the command is in
 execution. Spawn ids provided this way are called "indirect" spawn ids. 

and having that global variable be an empty list.  That came to mind
since I had experience with using indirect spawn ids in another case
when I needed an empty spawn id list:

    # Use an indirect spawn id list, and remove the inferior spawn id
    # from the expected output as soon as it matches, in case
    # $inferior_pattern happens to be a prefix of the resulting full
    # gdb pattern below (e.g., "\r\n").
    global gdb_test_stdio_spawn_id_list
    set gdb_test_stdio_spawn_id_list "$inferior_spawn_id"

And then I realized the to-the-point '-i ""' would work just as well.

I think that if this stops working, likely either empty variable or
indirect spawn id pointing at an empty list would still keep working,
so we would likely be able to switch to one of those.

Maybe meanwhile, we could ask on the dejagnu list what's the purpose
of this picking one of eof/timeout/default as an error block, see if
that could be removed.  I'd guess that dejagnu would change faster
than expect here, or maybe I should say, less slowly.  :-)

Thanks,
Pedro Alves