Don't just wait for a response because NLM_F_REQUEST is set

[Toorero]

It is incorrect assuming a response is guaranteed solely because NLM_F_REQUEST is set.

NLM_F_REQUEST by itself only marks a message as coming from user space and is a requirement to make successful requests to the kernel as an application.¹

We can only assume a response (in form of an ACK or error) is guaranteed if NLM_F_ACK is set.²
Then either an error or an ack is getting returned.

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Further research is needed to determine if a response is guaranteed on NLM_F_ECHO as the kernel doc states it isn't universally implemented³. In my mind sending a NLM_F_REQUEST | NLM_F_ECHO request could produce no response if the message is successful (no ACK is issued, because NLM_F_ACK is unset) but the addressed system doesn't implement echo or the request changes nothing.

Footnotes

1. https://github.com/torvalds/linux/blob/f5bbbfec59b4e2fb7520a91de3df8a6174325d6a/net/netlink/af_netlink.c#L2548-L2550 ↩

2. https://github.com/torvalds/linux/blob/f5bbbfec59b4e2fb7520a91de3df8a6174325d6a/net/netlink/af_netlink.c#L2560-L2562 ↩

3. https://github.com/torvalds/linux/blob/f5bbbfec59b4e2fb7520a91de3df8a6174325d6a/Documentation/userspace-api/netlink/intro.rst#notification-echo ↩

[Toorero]

The current behaviour could lead to a bunch of stale PendingRequests accumulating in Protocol even if you'd use ConnectionHandle::notify without NLM_F_ACK set on success.

[Toorero]

This would also allow to work around Kernels not acknowledging BATCH_BEGIN and BATCH_END messages prior to bf2ac49 by not supplying them with an NLM_F_ACK flag.

On kernels prior to bf2ac49 (e.g. Ubuntu 24.04 LTS) the newly introduced method request_batched by #45 would hang forever, even if NLM_F_ACK is set on every message.

In general, it is not recommended [1] to force the kernel to acknowledge every message. If you batch messages, requesting and receiving a single acknowledgement should be enough, since it's an all-or-nothing-operation.