[Bug driver/105568] New: Superfluous --jobserver-auth= check taints further diagnostics

[pexu@gcc-bugzilla.mail.kapsi.fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105568

            Bug ID: 105568
           Summary: Superfluous --jobserver-auth= check taints further
                    diagnostics
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pexu@gcc-bugzilla.mail.kapsi.fi
  Target Milestone: ---

Hi.

When linking with -flto=jobserver both gcc/gcc.cc and gcc/lto-wrapper.cc check
if MAKEFLAGS contains "--jobserver-auth=" and if so attempt to further
scrutinize its contents.

Unfortunately, detect_jobserver() (at gcc/gcc.cc), which is executed always
when linking, silently removes the entire "--jobserver-auth=" (up until next
space) shall this scrutinization attempt fail.

This makes jobserver_active_p() (at gcc/lto-wrapper.cc) always complain about a
missing "--jobserver-auth=".  Indeed, this might be now technically true, but
it is very confusing and misleading if this was done because detect_jobserver()
was not satisfied with the given "--jobserver-auth=".

There a many reasons why the given "--jobserver-auth=" might be treated as
non-functioning.  Either GNU make is using an another jobserver type (named
semaphore), the given file descriptors are inaccessible because GNU make and
GCC are using different emulation layers (e.g. Mingw and Cygwin) or those
simply are indeed not inherited, i.e. user forgot to add `+' to invoking GNU
rule.

Therefore it might be still necessary to remove the "--jobserver-auth=" but I
presume this should done after jobserver_active_p().  Actually, this is already
being done, as the entire MAKEFLAGS is emptied unless jobserver logic is
enabled.