Add std::net::lookup_addr for reverse DNS lookup

[murarth]

Closes #22608

[rust-highfive]

r? @aturon

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[aturon]

cc @alexcrichton

[bors]

☔ The latest upstream changes (presumably #23352) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

For now we may want to hold off adding this as std::net is still shaking out. I've added this to the wishlist issue, however. There are extra options and parameters to getnameinfo, so we may want to think a little longer about what this interface should look like. This is pretty minor though, so I think I'd be fine with it for now.

[murarth]

I'm not sure how to amend this if IpAddr has been removed. Is there not going to be a standard type to express "either an IPv4 or an IPv6 address?"

[alexcrichton]

I think this should take a SocketAddr now instead of an IpAddr (and better yet a &SocketAddr)

[alexcrichton]

Could a test be added for this?

[murarth]

Rebased after recent changes to std::net. I created the SocketAddr::new method because I needed to use something to that effect. If we still want to hold off on a public constructor for SocketAddr, I can make it a private local function instead.

@alexcrichton: I don't know if a test can be written for it. getnameinfo looks at system files and the outside network, neither of which can be relied upon in a test. I tested it myself, if that's at all reassuring. :P

[daboross]

Just wondering, would it be at all possible to test this using localhost, or are there some targets where that wouldn't work?

[murarth]

@daboross: Generally, that would work, but it's possible that the /etc/hosts file (or non-Unix equivalent) may not contain an entry for 127.0.0.1 or it could have a different name. But perhaps the definition of localhost is a reasonable assumption made by other tests.

[alexcrichton]

I think this is separable enough from the ip_addr feature that this may want to be called lookup_addr instead.

[alexcrichton]

Ok, this looks good to me, thanks @murarth! r=me with a separate feature name for this API.

[murarth]

@alexcrichton: Done.

[alexcrichton]

@bors: r+ c0dd239

[bors]

⌛ Testing commit c0dd239 with merge b27c254...

[bors]

💔 Test failed - auto-win-64-nopt-t

[alexcrichton]

@bors: retry

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[image: 💔] Test failed - auto-win-64-nopt-t
http://buildbot.rust-lang.org/builders/auto-win-64-nopt-t/builds/2726

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#23419 (comment).

[bors]

⌛ Testing commit c0dd239 with merge 0c9de81...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt