setkeepalive < 1000 silently ignored

Version

22.14.0

Platform

N/A

Subsystem

No response

What steps will reproduce the bug?

Create a TCP socket and call socket.setKeepAlive(true, 400).

How often does it reproduce? Is there a required condition?

Any keepalive less than 1 second.

What is the expected behavior? Why is that the expected behavior?

This should send keepalive packets every 400 ms.

The documentation says as much:

keepAliveInitialDelay {number} If set to a positive number, it sets the
initial delay before the first keepalive probe is sent on an idle socket.
Default: 0.

What do you see instead?

The code truncates to the nearest 1000ms, so the socket doesn't send keepalive packets and no error is issued.

Additional information

It seems this parameter is even broken in the tests.

https://github.com/nodejs/node/blob/3db54912fa2e4a696d4e2ec27dfb2aeecfc65191/test/parallel/test-net-keepalive.js#L42
https://github.com/nodejs/node/blob/3db54912fa2e4a696d4e2ec27dfb2aeecfc65191/test/parallel/test-net-persistent-keepalive.js#L30

Ideally, the keepalive interval would be respected in milliseconds. Assuming that's infeasible,

1. a runtime warning should be issued if setKeepAlive is given a value <1000.
2. the documentation should be updated to reflect that, even though the value is given in milliseconds, it will only be respected to the second.

Start at the socket.setKeepAlive(true, 400) entry point and inspect the behavior covered by test/parallel/test-net-keepalive.js and test/parallel/test-net-persistent-keepalive.js. Determine whether subsecond delays should be supported or rejected, then update the relevant tests and documentation so the chosen behavior is explicit and verified.