bug#74907: 31.0.50; nsm-check-local-subnet-ipv4 test fails on macOS with VPN enabled

[Stefan Kangas <stefankangas@gmail.com>]

Robert Pluim <rpluim@gmail.com> writes:

>>>>>> On Mon, 16 Dec 2024 21:55:33 +0000, Stefan Kangas <stefankangas@gmail.com> said:
>
>     Stefan> I don't see a difference there.
>
>     Stefan> ;; With VPN
>     Stefan>     (network-lookup-address-info "localhost")
>     Stefan>     => ([0 0 0 0 0 0 0 1 0] [127 0 0 1 0])
>
>     Stefan>     (network-lookup-address-info "127.0.0.1")
>     Stefan>     => ([127 0 0 1 0])
>
>     Stefan> ;; Without VPN
>     Stefan>     (network-lookup-address-info "localhost")
>     Stefan>     => ([0 0 0 0 0 0 0 1 0] [127 0 0 1 0])
>
>     Stefan>     (network-lookup-address-info "127.0.0.1")
>     Stefan>     => ([127 0 0 1 0])
>
> In an interactive session or -batch?

In an interactive session.  I tried now in batch too, using this:

./src/emacs -Q -batch -eval '(progn (princ (network-lookup-address-info
"localhost")) (terpri) (princ (network-lookup-address-info
"127.0.0.1")))'

;; With VPN
    ([0 0 0 0 0 0 0 1 0] [127 0 0 1 0])
    ([127 0 0 1 0])

;; Without VPN
    ([0 0 0 0 0 0 0 1 0] [127 0 0 1 0])
    ([127 0 0 1 0])

> Anyway, hereʼs a wild stab in the dark based on the only code path I
> could see that would give your original output. If that works Iʼd like
> to know which VPN client youʼre using so I can avoid it 😀

It didn't work, unfortunately.

However, I now see that I made a mistake in my original recipe to
reproduce this: I didn't bind 'nsm-trust-local-network'.

I can reproduce the backtrace consistently by evaluating this in an
interactive session:

    (let ((nsm-trust-local-network t))
      (should (eq t (nsm-should-check "example.org"))))

This allowed edebugging this, and I see that the backtrace comes from
nsm.el:238, where we do:

    (substring (nth 3 info) 0 -1)

With a VPN, `info` is bound to this when I get the backtrace:

    ("utun0"
     [10 0 0 1 0]
     (0 . [0 0 10 255 255 255 16 2 0 0 10 0 0 1])
     (0 . [0 0 255 0 0 0 16 2 0 0 10 0 0 1]))

Without a VPN, `info` is bound to this instead:

    ("utun6"
     [65153 0 0 0 6123 19123 32345 45123 0]
     [65153 0 0 0 65535 65535 65535 65535 0]
     [65535 65535 65535 65535 0 0 0 0 0])

Clearly, this will not work:

    (substring '(0 . [1 2 3 4]) 0 -1)

So the question is why `network-interface-list` would return such an
unusual value here.