Add posix_spawnattr_{get,set}procdescp_np(3)

[kib]

Summary

The function set the address where to store resulting procdesc, which also explcitly request posix_spawn() to use pdfork() (really pdrfork()) instead of fork, and create procdesc for the child.

Since it is needed, also add pdrfork_thread(3) on x86.

The documentation will come later after we agree on the API. I choose this since it allows to unify use of pdrfork, so consumers still use posix_spawn instead of some other function.

An illustration for use of the all new proposed API is there https://reviews.freebsd.org/P686. I used this program to debug the code.

[kib]

Rename to spawnattr_setprocdescp_np. IMO pdrfork is the implementation detail there, new name is less cryptic and easier to remember.
Export pdrfork_thread(3) on x86.

[kib]

Export pdrfork_thread on i386 for real

[asomers]

I didn't read the assembly code (I haven't had to write x86 assembly since University), but otherwise it looks good to me. And thanks for adding the P686 example, too.

lib/libc/gen/posix_spawn.c

56–57 ↗

(On Diff #170418)

If I'm counting right, then you should reverse the order of these two fields to eliminate alignment padding.

[kib]

lib/libc/gen/posix_spawn.c

56–57 ↗

(On Diff #170418)

I do not think it matters. This is an internal structure used without exposing in ABI, and we do not care about it layout. Eliminating padding is not needed.