x/net/proxy: provide DialContext

[chowey]

With go1.7, the net package now provides a DialContext function. This is the recommended way to use http.Transport.

x/net/proxy should do the same so we can use it when creating our http.Transports for use with SOCKS proxies.

[bradfitz]

I don't remember that code, but sounds reasonable.

[artyom]

I need this as well so I can try to implement context support for cancelation in this package.

I'd like to discuss API changes before doing any implementation. My conservative proposal (hopefully completely backwards-compatible) is to leave Dialer interface unchanged and introduce new interface:

type ContextDialer interface {
	Dialer
	DialContext(ctx context.Context, network, address string) (net.Conn, error)
}

Then all functions of this package that take Dialer argument and return Dialer may be changed to return ContextDialer type instead and do type assertion on passed Dialer argument value to check whether it implements ContextDialer.

As example, FromURL signature would change from:

func FromURL(u *url.URL, forward proxy.Dialer) (proxy.Dialer, error)

to

func FromURL(u *url.URL, forward proxy.Dialer) (proxy.ContextDialer, error)

[Fardinak]

Possible duplicate of #19354.
This seems to be covered by the issue above, which is in the Go1.9 milestone and also in review at https://golang.org/cl/37641.

/cc @bradfitz

[gopherbot]

CL https://golang.org/cl/38278 mentions this issue.

[mjgarton]

https://go-review.googlesource.com/c/net/+/37641

[gopherbot]

Change https://golang.org/cl/168921 mentions this issue: proxy: add Dial (with context)