Added getnameinfo

[ahgamut]

implementation of getnameinfo is similar to getaddrinfo, with additional internal functions. I tested it with an example program on 1.1.1.1 and 8.8.4.4 and the output matches glibc.

Two helper functions are currently stub:

When performing an rDNS lookup, I have to look through /etc/hosts first, but struct HostsTxt is already sorted according to name. Should I do a linear search or re-sort it according to address (and then sort back)?

The service name is currently mapped to 0 always. I'll try to add a struct ServicesTxt similar to struct HostsTxt if I figure it out, or just a simple parse of /etc/services.

[jart]

Thanks for sending this! The /etc/services stuff looks orthogonal to this change. Could we remove that and do it as a separate PR if you need it?

[ahgamut]

Ok, I'll look to add a separate PR for parsing /etc/services later (maybe along with the struct servent stuff).

What do you think about ResolveHostsReverse? Should I implement that in this PR or leave it as a stub?

[jart]

getnameinfo is a reverse lookup function so that other function seems on-topic.

[ahgamut]

Removed the services lookup stub.

I used the HostsTxt internal structure for the address -> name lookup, so the logarithmic-time lookup of names is lost if one keeps alternating between name and address lookups.

[jart]

We can always make it bidirectional if performance ends up being an issue for someone's use case. Will review as soon as I get a chance.

[jart]

LGTM. Thank you!

[ahgamut]

I tested getnameinfo on the system glibc and looks like it performs a linear lookup: if I have multiple entries with the same IP, glibc picks the first one that matches, but this implementation picks a different one because it starts at the middle.

[jart]

You'd have to replace qsort with a stable sorting algorithm and then replace bsearch with the leftmost variant of binary search. I'd recommend doing brute force linear since we're only comparing on uint32 ip addresses.

[ahgamut]

I'll submit a PR that uses just brute-force linear search for ResolveHostsReverse.

Even then, the getnameinfo output would differ between glibc and cosmopolitan when there are multiple entries mapped to the same IP address. Consider the below /etc/hosts file:

localhost    127.0.0.1
ahost        127.0.0.1
bhost        127.0.0.1
mhost        127.0.0.1
xhost        127.0.0.1
yhost        127.0.0.1
zhost        127.0.0.1

Now if I call getnameinfo to find the result for 127.0.0.1, localhost is the expected return value, but I will get either ahost or mhost (depending on where the search starts) because SortHostsTxt has sorted the entries by name, and localhost is no longer first.