bug#71477: 30.0.50; Lock files are not deleted on Windows 98

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 12 Jun 2024 09:07:52 -0700
> Cc: 71477@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> 
> 
> On 2024-06-12 01:25, Eli Zaretskii wrote:
> 
> > -  if (! c_isdigit (dot[1]))
> > +  if (! (c_isdigit (dot[1])
> > +	 /* Windows 9X report negative PID values.  */
> > +	 || (dot[1] == '-' && c_isdigit (dot[2]))))
> 
> Faster is "if (! c_isdigit[(dot[1] == '-') + 1])", as it avoids a 
> conditional branch on most platforms.

OK.

> > -      else if (0 < pid && pid <= TYPE_MAXIMUM (pid_t)
> > +      else if (pid != -1 && pid <= TYPE_MAXIMUM (pid_t)
> >                  && (kill (pid, 0) >= 0 || errno == EPERM)
> 
> This looks dubious for most systems, where 'kill' has special behavior 
> when pid < -1 or pid == 0; it tests a process group. That's not the test 
> we want here, since we want to check whether Emacs can be sent a signal, 
> not whether any process in its process group can be sent a signal (this 
> can be valid even after Emacs has exited). The code should use calls 
> like kill (-2, 0) and kill (0, 0) only on platforms where we know the 
> calls do not test a process group.

But on all platforms except Windows 9X we shouldn't see a negative PID
here, so what you say is purely theoretical, no?

> Even on MS Windows 98 we should check that TYPE_MINIMUM (pid_t) <= pid. 

Since pid_t is typedefed as 'int', that's always true, no?

> Also, is there a special meaning for kill (0, 0) on MS Windows 98?

No.  And our emulation of 'kill' fails with EPERM when called witgh
both arguments zero.

> If so, we should also check that pid != 0.

There are no processes on Windows whose PID is zero, so getting zero
here is impossible.

> Do any MS-Windows platforms support process groups, i.e., kill (-2, 0) 
> operates on process group 2 rather than on an individual process with 
> process ID -2? If so, these platforms should be careful too, and should 
> not use kill (-2, 0) or kill (0, 0).

Windows does support process groups, but our emulation of 'kill'
pretends that each process is its own group.

> How about the attached patch instead? You can adjust the 
> Microsoft-specific .h files to define VALID_PROCESS_ID appropriately for 
> MS Windows 98, and for any other MS platform where kill (-2, 0) is known 
> to check for just the individual process -2.

Fine with me, please install and I will followup.

Thanks.