Re: [PATCH 4/4] terminal: restore settings on SIGTSTP

[Phillip Wood <phillip.wood123@gmail.com>]

On 07/03/2022 14:45, Ævar Arnfjörð Bjarmason wrote:
>[...] 
> On Mon, Mar 07 2022, Phillip Wood wrote:
> 
>> On 07/03/2022 11:49, Ævar Arnfjörð Bjarmason wrote:
>>> On Mon, Mar 07 2022, Phillip Wood wrote:
>>>
>>>> Hi Ævar
>>>>
>>>> On 05/03/2022 13:59, Ævar Arnfjörð Bjarmason wrote:
>>>>> [...]
>>>>>>     int save_term(unsigned flags)
>>>>>>     {
>>>>>> +	struct sigaction sa;
>>>>>> +
>>>>>>     	if (term_fd < 0)
>>>>>>     		term_fd = (flags & SAVE_TERM_STDIN) ? 0
>>>>>>     						    : open("/dev/tty", O_RDWR);
>>>>>> @@ -44,6 +136,26 @@ int save_term(unsigned flags)
>>>>>>     	if (tcgetattr(term_fd, &old_term) < 0)
>>>>>>     		return -1;
>>>>>>     	sigchain_push_common(restore_term_on_signal);
>>>>>> +	/*
>>>>>> +	 * If job control is disabled then the shell will have set the
>>>>>> +	 * disposition of SIGTSTP to SIG_IGN.
>>>>>> +	 */
>>>>>> +	sigaction(SIGTSTP, NULL, &sa);
>>>>>> +	if (sa.sa_handler == SIG_IGN)
>>>>>> +		return 0;
>>>>>> +
>>>>>> +	/* avoid calling gettext() from signal handler */
>>>>>> +	background_resume_msg = xstrdup(_("error: cannot resume in the background"));
>>>>>> +	restore_error_msg = xstrdup(_("error: cannot restore terminal settings"));
>>>>> You don't need to xstrdup() the return values of gettext() (here
>>>>> _()),
>>>>> you'll get a pointer to static storage that's safe to hold on to for the
>>>>> duration of the program.
>>>>
>>>> I had a look at the documentation and could not see anything about the
>>>> lifetime of the returned string, all it says is "don't alter it"
>>> I think this is overed in "11.2.7 Optimization of the *gettext
>>> functions", a pedantic reading might suggest not, but what's meant with
>>> the combination of that API documentation & the description of how MO
>>> files work is that you're just getting pointers into the already-loaded
>>> translation catalog, so it's safe to hold onto the pointer and re-use it
>>> later.
>>
>> Strictly that section only shows it is safe if there are no other
>> calls to gettext() before the returned string is used. I agree the
>> implementation is likely to be just returning static strings but I
>> can't find anywhere that says another implementation (e.g. on
>> macos/*bsd) has to do that.
> 
> I agree. I'm 99.99% sure this is safe & portable use of the API, but I'm
> having some trouble finding documentation for that...
> 
>>> In any case, if we're going to be paranoid about gettext() it would make
>>> sense to propose that as some general change to how we use it, we rely
>>> on this assumption holding in a lot of our use of the API:
>>>       git grep '= _\('
>>> Rather than sneak that partcular new assumption in here in this
>>> already
>>> tricky code...
>>
>> The ones I looked at are mostly not calling gettext() again before
>> using the translated string (there is one exception in
>> builtin/remote.c).
> 
> Doesn't validate_encoding() in convert.c, process_entry() in
> merge-ort.c, setup_unpack_trees_porcelain() in unpack-trees.c cmd_mv()
> in builtin/mv.c etc. qualify?

I only checked a few, cmd_mv() always assigns to the same variable so 
the previous value is overwritten anyway, some of the others such as 
unpack_trees are assuming the return value is valid after a subsequent 
call to gettext(). I found[1] which states

     The string returned must not be modified by the program and can
     be invalidated by a subsequent call to bind_textdomain_codeset()
     or setlocale(3C).

so I think we can drop the copying.

> I.e. for a hypothetical gettext() that always returned the same pointer
> and just overwrote it with the latest message those would all emit bad
> output, wouldn't they?
> 
>> In restore_term() I'm checking if the messages are NULL to see if job
>> control is enabled, I could use a flag but I'm inclined to just keep
>> coping the strings.
> 
> Checking if they're NULL is orthagonal to whether we xstrdup()
> them. I.e. you'd just skip the xstrdup() and replace the FREE_AND_NULL
> with a "= NULL" assignment, no?

Yes, I'm not sure what I was thinking when I wrote that.

Best Wishes

Phillip

[1] 
https://docs.oracle.com/cd/E88353_01/html/E37843/gettext-3c.html#REFMAN3Agettext-3c