proxy: add Dial (with context)

[dweomer]

The existing API does not allow client code to take advantage of Dialer implementations that implement DialContext receivers. This a familiar API, see net.Dialer.

Fixes golang/go#27874
Fixes golang/go#19354
Fixes golang/go#17759
Fixes golang/go#13455

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Patch Set 3:

Please note that I was aware of https://go-review.googlesource.com/c/net/+/37641 before submitting this PR/CR. With this in mind I chose to merely add the proxy.Dial, proxy.DialTimeout, and proxy.DialContext functions along with the proxy.ContextDialer single-method interface. The proxy.direct and proxy.PerHost types have been modified to also implement proxy.ContextDialer so as to curry the context as deeply into the stack as possible and therefore minimize goroutine leakage. Legacy implementations of proxy.Dialer, when invoked from DialTimeout or DialContext, will temporarily leak a goroutine for as long as the underlying dialer implementation takes to timeout. Without changing the public API this seemed to me the best that could be hoped for.

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Message from Brad Fitzpatrick:

Patch Set 4:

(6 comments)

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Patch Set 7: Commit message was updated.

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Patch Set 7:

(6 comments)

Patch Set 4:

(6 comments)

Brad, thank you for the feedback. I have incorporated your suggestions.

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Patch Set 9:

(1 comment)

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Patch Set 9: Run-TryBot+1

(4 comments)

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Patch Set 9:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=ca2c88a5

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Patch Set 9: TryBot-Result+1

TryBots are happy.

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Patch Set 10:

(2 comments)

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Patch Set 10:

See my other comments on dial.go and socks5.go?

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Patch Set 10:

(1 comment)

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Message from Jacob Blain Christen:

Patch Set 10:

Patch Set 10:

See my other comments on dial.go and socks5.go?

Yes, I will make the argument ordering change but I have a reply re: your func ProxyDial comment

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Message from Jacob Blain Christen:

Patch Set 11:

(1 comment)

adjusted argument order, as suggested, for dialContext()

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Patch Set 11:

(1 comment)

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Patch Set 11:

(1 comment)

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Patch Set 12: Run-TryBot+1 Code-Review+2

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Patch Set 12:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=9a1750e0

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Patch Set 12:

(1 comment)

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Patch Set 12: TryBot-Result+1

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This PR is being closed because golang.org/cl/168921 has been merged.