send(2) with AF_UNIX SOCK_DGRAM blocks upon full send buffer; MSG_NOWAIT ignored

[kohlschuetter]

When sending datagrams via AF_UNIX, a buffer-full condition prevents "send" from returning immediately with an error, even when MSG_NOWAIT is set.

For comparison, macOS always returns with errno=ENOBUFS in that situation, regardless whether MSG_NOWAIT was set or not. Linux returns with "EAGAIN" only with MSG_NOWAIT set, blocking if not.

This prevents code relying on these features from working as expected.

A workaround is to define a send timeout on the socket, however that may be brittle when receiving an existing socket via file descriptors, for example.

[kohlschuetter]

Example code demonstrating the bug

[korli]

​https://review.haiku-os.org/c/haiku/+/6796

[korli]

Committed in hrev57200