OK I fixed the configure.ac checking program.  Hopefully it works properly now.

There is nothing to be sorry about, Martin. Thank you for your work, keep contributing.

Patch 3 tested on glibc-2.17, glibc-2.32 and sun os.

Sorry Dmitry I'm still really confused.  Maybe it would help if you made more clear what you discovered.

GNU make always sets POSIX_SPAWN_USEVFORK if it exists, and it does exist in all versions of glibc we are discussing.

You have discovered that in glibc 2.17, posix_spawn() has a bug in its behavior, with POSIX_SPAWN_USEVFORK set (since we've determined it is set everywhere).

In glibc >=2.24, posix_spawn() does not have this bug with POSIX_SPAWN_USEVFORK set (and, presumably, without it set either although I didn't check this).

The question is, does posix_spawn() in glibc <2.24 (e.g., 2.17) and WITHOUT POSIX_SPAWN_USEVFORK set also show this bug, or does it work properly without vfork?

If older glibc works if you don't set POSIX_SPAWN_USEVFORK, then I suggest that the thing to do is to detect whether this old version of glibc is being used and if it is, then we don't use POSIX_SPAWN_USEVFORK regardless of whether it exists or not, and just use normal posix_spawn().

If older glibc doesn't work even if you do use POSIX_SPAWN_USEVFORK, then I'm not sure why we're even testing for this since it doesn't seem to make any difference either way.

Am I missing something?

Let me describe differently.

posix_spawn calls either vfork or clone, depending on arguments being passed. See the man page referenced above.
I observed that when posix_spawn calls vfork, that's is when POSIX_SPAWN_USEVFORK is set in flags, errno is set to ENOEXEC. This allows the patch to work.
When posix_spawn calls clone, that is POSIX_SPAWN_USEVFORK is not set in flags, errno it not set. In this situation the patch does not help.

In other words, make wants to set POSIX_SPAWN_USEVFORK in order to let the patch work.

The change in configure.ac in case there is such a even older version of glibc where posix_spawn returns 0 and there is no POSIX_SPAWN_USEVFORK (i am not sure there is such a version, but if there is, it's definitely not 2.17). In that case the configure check disables posix_spawn.

posix_spawn calls either vfork of spawn.
On glibc-2.17 i observed that when posix_spawn calls vfork the return code is zero, but errno is set to ENOEXEC.
When posix_spawn calls spawn, both return code and errno are zero.

This man page
https://man7.org/linux/man-pages/man3/posix_spawn.3.html
describes when vfork is called.
This is a quote

       Before glibc 2.24, the child process is created using vfork(2)
       instead of fork(2) when either of the following is true:

       *  the spawn-flags element of the attributes object pointed to by
          attrp contains the GNU-specific flag POSIX_SPAWN_USEVFORK; or

       *  file_actions is NULL and the spawn-flags element of the
          attributes object pointed to by attrp does not contain
          POSIX_SPAWN_SETSIGMASK, POSIX_SPAWN_SETSIGDEF,
          POSIX_SPAWN_SETSCHEDPARAM, POSIX_SPAWN_SETSCHEDULER,
          POSIX_SPAWN_SETPGROUP, or POSIX_SPAWN_RESETIDS.

Make specifies POSIX_SPAWN_USEVFORK, when avaiable, and this allows the patch to help.
The patch cannot not help some version of glibc which lacks POSIX_SPAWN_USEVFORK and returns 0 and keeps errno intact.
So, i updated the patch to check for __GLIBC__ at configure time and disable posix_spawn, unless POSIX_SPAWN_USEVFORK is available. This check should not affect modern versions of glibc, they have POSIX_SPAWN_USEVFORK.
Tested on glibc-2.17, glibc-2.32 and sun os.

This is strange.  I looked in the glibc bug tracker and couldn't find anything about this issue.  I also looked at the actual implementation of glibc 2.17 and it doesn't seem to me like this should happen (although I'm not a glibc developer so I might have missed something).  I was looking at the version for x86_64 Linux kernel though: Dmitry can you give more info about what system you were testing on that had this issue?

The patch does clear errno before invoking posix_spawn() so it's LIKELY that if errno is ENOEXEC after return then we had this issue, but it is true that the value of errno is not determinate if the function returns success.

Unfortunately due to cross-compilation etc. it's not feasible to test the runtime behavior of posix_spawn() in configure and even if it were, that would only be relevant for the version of glibc used at compile time, which could well not be the same version used at runtime.  So a configure check is problematic at best.

I looked through the history of glibc and saw that posix_spawn() was completely rewritten in glibc 2.24.  But, the issue might not be with posix_spawn() at all, it could be in fork() or vfork().

Dmitry can you modify the code in src/job.c to remove the POSIX_SPAWN_USEVFORK flag and see if you still see the problem with glibc 2.17?  Maybe that will help narrow down where the problem actually is.

posix_spawn fails to run a child process.

Some versions of glibc (i observed this with glibc-2.17) fail to spawn a shell
program missing a shbang and return 0 from posix_spawn.

$ ls
hello.sh  makefile
$ cat hello.sh
printf "hello, world\n"
$ cat makefile
all:; ./hello.sh
$ # this is make built from master
$ ~/src/make/m64/make
./hello.sh
make: *** [makefile:1: all] Error 127
$ # this is make with the patch applied
$ ~/src/make/l64/make
./hello.sh
hello, world

Even though posix_spawn returns 0 in this situation, it still sets errno to
ENOEXEC.
This patch causes child_execute_job to compare errno against ENOEXEC after
posix_spawn returned 0.