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kqueue(2): add a note about EV_RECEIPT

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In the below-referenced PR, a case is attached of a simple reproducer that
exhibits suboptimal behavior: EVFILT_READ and EVFILT_WRITE being set in the
same kevent(2) call will only honor the first one. This is, in-fact, how
it's supposed to work.

A read of the manpage leads me to believe we could be more clear about this;
right now there's a logical leap to make in the relevant statement: "When
passed as input, it forces EV_ERROR to always be returned." -- the logical
leap being that this indicates the caller should have allocated space for
the change to be returned with EV_ERROR indicated in the events, or
subsequent filters will get dropped on the floor.

Another possible workaround that accomplishes similar effect without needing
space for all events is just setting EV_RECEIPT on the final change being
passed in; if any errored before it, the kqueue would not be drained. If we
made it to the final change with EV_RECEIPT set, then we would return that
one with EV_ERROR and still not drain the kqueue. This would seem to not be
all that advisable.

PR:		229741
MFC after:	1 week
This commit is contained in:
Kyle Evans 2020-04-22 03:45:52 +00:00
parent 2cd0c52978
commit 00b0f94c58
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=360182
1 changed files with 7 additions and 3 deletions
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10
lib/libc/sys/kqueue.2
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@ -24,7 +24,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd July 27, 2018
.Dd April 21, 2020
.Dt KQUEUE 2
.Os
.Sh NAME
@ -63,8 +63,7 @@ the event no longer holds, the kevent is removed from the kqueue and
is not returned.
.Pp
Multiple events which trigger the filter do not result in multiple
kevents being placed on the kqueue; instead, the filter will aggregate
the events into a single struct kevent.
kevents being placed on the kqueue; instead, the filter wiApril 21, 2020the events into a single struct kevent.
Calling
.Fn close
on a file descriptor will remove any kevents that reference the descriptor.
@ -233,6 +232,11 @@ to always be returned.
When a filter is successfully added the
.Va data
field will be zero.
Note that if this flag is encountered and there is no remaining space in
.Fa eventlist
to hold the
.Dv EV_ERROR
event, then subsequent changes will not get processed.
.It Dv EV_ONESHOT
Causes the event to return only the first occurrence of the filter
being triggered.