[Bug libc/18433] posix_spawn does not return correctly upon failure to execute

["nachms+sourceware at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18433

--- Comment #3 from Nach M. S. <nachms+sourceware at gmail dot com> ---
Note the return value (emphasis included):
Upon successful completion, posix_spawn() and posix_spawnp() shall return the
process ID of the child process to the parent process, in the variable pointed
to by a non-NULL pid argument, and shall return zero as the function return
value. Otherwise, no child process shall be created, the value stored into the
variable pointed to by a non-NULL pid is unspecified, ***and an error number
shall be returned as the function return value to indicate the error***. If the
pid argument is a null pointer, the process ID of the child is not returned to
the caller.

You are correct that the standard allows to exit with 127, however it
discourages that behavior and says only if the library is unable to detect all
issues is it okay to do so. I can't speak for other platforms, but it is
perfectly possible to implement this function in a library on Linux in a
feature-complete fashion.

FreeBSD, NetBSD, and musl-libc (Linux) at the library level are all capable of
detecting the errors and properly returning them without any special kernel
supports. I've also implemented this function on Linux correctly and
feature-complete with proper error handling, and it's not really a big deal to
do so.

Using any simple form of IPC, the error can be returned from the child to the
parent. The BSDs use vfork() with a variable that the child modifies and the
parent looks at. musl-libc uses a pipe. Personally, I recommend the latter
approach.

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