Getaddrinfo: always retrieve the canonical name

[picnoir]

Nscd always sets the AI_CANONNAME flag for a getaddrinfo request. When this flag is on, the canonical name (~ FQDN) for the requested address is retrieved.

We found this issue through the nixosTests.hostname.explicitDomain NixOS VM test. It went unnoticed in the wild probably because the nscd client tend to fill canonical name in the request itself once it retrieved it once.

While investigating this issue, I realized that setting the SOCK_STREAM flag gets rid of the duplicate addrs. Meaning that we do not need to filter them ourselves with a HashSet anymore.

[picnoir]

Not too proud of this part. Can you think of a more rust-idiomatic way to retrieve the addrs and the canonical name in one go?

As a reminder, the canonical name is stored in the first Airesponse struct.

[flokli]

I'd write this block like this:

            let ai_resp: AiResponse = match resp {
                Ok(ai_resp_iter) => {
                    // filter out bad responses.
                    let mut ai_resp_iter = ai_resp_iter.filter_map(|e| e.ok()).peekable();
                    // According to man 3 getaddrinfo, the resulting
                    // canonical name should be stored in the first
                    // addrinfo struct.
                    // Re-using the request hostname if we don't get a
                    // canonical name.
                    let canon_name = ai_resp_iter
                        .peek()
                        .and_then(|e| e.canonname.to_owned())
                        .unwrap_or(hostname.to_string());

                    let addrs = ai_resp_iter
                        .map(|e| e.sockaddr.ip())
                        .collect::<Vec<_>>();

                    AiResponse {
                        canon_name,
                        addrs,
                    }
                }
                Err(_) => ai_resp_empty,
            };

[picnoir]

Nice! Peek is exactly what I was looking for! <3

[picnoir]

With this PR, nix-build -A nixosTests.hostname.explicitDomain succeeds 🎉