git: fc13cbaf6942 - stable/13 - sigwait: add comment explaining EINTR/ERESTART details

[Konstantin Belousov]

The branch stable/13 has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=fc13cbaf6942af73d50659e4728b6b1299a0d028

commit fc13cbaf6942af73d50659e4728b6b1299a0d028
Author:     Konstantin Belousov <kib at FreeBSD.org>
AuthorDate: 2021-06-07 14:59:41 +0000
Commit:     Konstantin Belousov <kib at FreeBSD.org>
CommitDate: 2021-06-22 01:45:31 +0000

    sigwait: add comment explaining EINTR/ERESTART details
    
    (cherry picked from commit acced8b043c5df0ebd51934bca6dcae3322cf890)
---
 sys/kern/kern_sig.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index a2709f38c5cb..1cab25aa5a40 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -1167,6 +1167,13 @@ sys_sigwait(struct thread *td, struct sigwait_args *uap)
 
 	error = kern_sigtimedwait(td, set, &ksi, NULL);
 	if (error) {
+		/*
+		 * sigwait() function shall not return EINTR, but
+		 * the syscall does.  Non-ancient libc provides the
+		 * wrapper which hides EINTR.  Otherwise, EINTR return
+		 * is used by libthr to handle required cancellation
+		 * point in the sigwait().
+		 */
 		if (error == EINTR && td->td_proc->p_osrel < P_OSREL_SIGWAIT)
 			return (ERESTART);
 		td->td_retval[0] = error;