net: no way to set timeout for LookupAddr

[Tasssadar]

On Go 1.6, amd64 linux.

My application does a lot of lookups via net.LookupAddr. Sometimes, it gets stuck forever:

goroutine 114 [IO wait]:
net.runtime_pollWait(0x7f4e03df6000, 0x72, 0xc821d94400)
        /home/tassadar/go-linux-amd64-bootstrap/src/runtime/netpoll.go:160 +0x60
net.(*pollDesc).Wait(0xc8234f8140, 0x72, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/fd_poll_runtime.go:73 +0x3a
net.(*pollDesc).WaitRead(0xc8234f8140, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/fd_poll_runtime.go:78 +0x36
net.(*netFD).Read(0xc8234f80e0, 0xc821d94400, 0x200, 0x200, 0x0, 0x7f4e03e0e028, 0xc8200b20a0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/fd_unix.go:250 +0x23a
net.(*conn).Read(0xc8200bc080, 0xc821d94400, 0x200, 0x200, 0x200, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/net.go:172 +0xe4
net.(*UDPConn).readDNSResponse(0xc8200bc080, 0xc821b7f100, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/dnsclient_unix.go:52 +0xa5
net.exchange(0xc8223f4393, 0xd, 0xc823484380, 0x1a, 0xc8223f000c, 0x12a05f200, 0x0, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/dnsclient_unix.go:155 +0x4ea
net.tryOneName(0xc821b4e080, 0xc823484380, 0x1a, 0xc8223f000c, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/dnsclient_unix.go:181 +0x2f3
net.lookup(0xc823484380, 0x1a, 0xc82348000c, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/dnsclient_unix.go:318 +0x2eb
net.goLookupPTR(0xc8211d5bc4, 0xc, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/dnsclient_unix.go:528 +0x167
net.lookupAddr(0xc8211d5bc4, 0xc, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/lookup_unix.go:158 +0xfc
net.LookupAddr(0xc8211d5bc4, 0xc, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/tassadar/go-linux-amd64-bootstrap/src/net/lookup.go:183 +0x4c

I tried to follow the code inside go, but there doesn't seem to be a way to set a timeout for this request.

[mdlayher]

Since there are nine net.Lookup* methods, I assume that adding a timeout for one of them would mean adding a timeout for each one?

[bradfitz]

Yes. And we can't modify any existing API; we can only add.

I think the most future-proof option is to add a new type Resolver struct and add the 9 Lookup* methods to it, but taking a context.Context.

And then we can add options in the future to the Resolver type, like which upstream DNS server to use, whether to force cgo or not, etc.

/cc @ianlancetaylor @mdempsky @mikioh

[bradfitz]

See also: #12503

[mdlayher]

I think the most future-proof option is to add a new type Resolver struct and add the 9 Lookup* methods to it, but taking a context.Context.

And then we can add options in the future to the Resolver type, like which upstream DNS server to use, whether to force cgo or not, etc.

I'd be happy to draft a proposal for this. Should I file a new issue when it's ready?

[bradfitz]

@mdlayher, I sent https://golang.org/cl/29440 ... can you review that?

[mdlayher]

Done. Thanks for that!

[gopherbot]

CL https://golang.org/cl/29440 mentions this issue.