test: check net.ipv4.ip_unprivileged_port_start in parallel/test-cluster-bind-privileged-port

[bnoordhuis]

test/parallel/test-cluster-bind-privileged-port.js expects binding to port 42 to fail but that's not an always-correct assumption.

Check the output of sysctl net.ipv4.ip_unprivileged_port_start because it's possible (albeit unlikely) for that port to be unprivileged.

Example output:

$ sysctl net.ipv4.ip_unprivileged_port_start
net.ipv4.ip_unprivileged_port_start = 1

[7suyash7]

$ sysctl net.ipv4.ip_unprivileged_port_start
net.ipv4.ip_unprivileged_port_start = 1024

Here's the output for me on Ubuntu 22.04.1

[bnoordhuis]

Yes, that's the default (and traditional UNIX behavior.) The test starts failing when it's <= 42.

[7suyash7]

So, you're saying the test should be modified to check the range of reserved ports on the current system, rather than assuming that it will always be 42?

[bnoordhuis]

The test should bail out with common.skip('...') if port 42 is unprivileged, yes.

[7suyash7]

Can I take a try at this? I would write a function to find the first unprivileged port and then try to attach a process to the port just before the first unprivileged port. Then basically the same checks as before (making sure the exit code is 0, etc) Is this implementation fine?
@bnoordhuis

[bnoordhuis]

Go for it but I'd rather it just exits if port 42 is unprivileged. Principle of least surprise and all that.

[7suyash7]

That makes sense, I'll send in a PR in some time!

[7suyash7]

@bnoordhuis PTAL.

[7suyash7]

Please ignore the second PR, I think it got added due to me changing branches locally and pushing.

[bnoordhuis]

Can you link me to your PR? I can't find it for some reason.