net: add Resolver type, Dialer.Resolver, and DefaultResolver
The new Resolver type (a struct) has 9 Lookup methods, all taking a context.Context. There's now a new DefaultResolver global, like http's DefaultTransport and DefaultClient. net.Dialer now has an optional Resolver field to set the Resolver. This also does finishes some resolver cleanup internally, deleting lookupIPMerge and renaming lookupIPContext into Resolver.LookupIPAddr. The Resolver currently doesn't let you tweak much, but it's a struct specifically so we can add knobs in the future. Currently I just added a bool to force the pure Go resolver. In the future we could let people provide an interface to implement the methods, or add a Timeout time.Duration, which would wrap all provided contexts in a context.WithTimeout. Fixes #16672 Change-Id: I7ba1f886704f06def7b6b5c4da9809db51bc1495 Reviewed-on: https://go-review.googlesource.com/29440 Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
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