netlink: suppress sending NLMSG_ERROR if NLMSG_DONE is already sent

[melifaro]

Summary

Netlink has a confirmation/error reporting mechanism for the sent
messages. Kernel explicitly acks each messages if requested (NLM_F_ACK)
or if message processing results in an error.
Similarly, for multipart messages - typically dumps, where each message
represents a single object like an interface or a route - another
message, NLMSG_DONE is used to indicate the end of dump and the
resulting status.
As a result, successfull dump ends with both NLMSG_DONE and NLMSG_ERROR
messages.
RFC 3549 does not say anything specific about such case.
Linux adopted an optimisation which suppresses NLMSG_ERROR message
when NLMSG_DONE is already sent. Certain libraries/applications like
libnl depends on such behavior.

Suppress sending NLMSG_ERROR if NLMSG_DONE is already sent, by
setting newly-added 'suppress_ack' flag in the writer and checking
this flag when generating ack.

This change restores libnl compatibility.

Test Plan

Before:

~ nl-link-list
Error: Unable to allocate link cache: Message sequence number mismatch
`

After:

~ nl-link-list
vtnet0 ether 52:54:00:14:e3:19 <broadcast,multicast,up,running>
lo0 ieee1394 <loopback,multicast,up,running>

[melifaro]

add manpage update.

[pauamma_gundo.com]

share/man/man4/netlink.4

215 ↗

(On Diff #113670)

Missed that one before.

220–224 ↗

(On Diff #113670)

Rewrite for clarity (but check this is what you meant) and consistency with use of "group" just below.

If you decide not to go with it, s/multipath/multipart/ on the last line.

[bapt]

tested with success!

[melifaro]

Address (most of) the comments.

[pauamma_gundo.com]

Other than that nit fixable on commit, manual page English LGTM and appears consistent with change summary.

share/man/man4/netlink.4

224 ↗

(On Diff #113671)