net/http: provide ProxyFromEnvironment functionality without requiring environment

[rogpeppe]

It is not possible to use ProxyFromEnvironment if the proxy settings may ever change, because it uses once-only-cached values from the environment variables.

For example, when running a server that reads its proxy settings from a config that may be updated at runtime, it would be useful to be able to create a new Transport.Proxy value that uses the new settings.

It is non-trivial, so just reimplementing it outside the standard library is undesirable.

I propose an addition to net/http:

// ProxyConfig holds configuration for HTTP proxy settings.
type ProxyConfig struct {
	RequestMethod string
	HTTPProxy     string
	HTTPSProxy    string
	NoProxy       string
}

// ProxyForRequest determines the URL to use for the given request.
// This method may be used as a value for Transport.Proxy.
func (cfg *ProxyConfig) ProxyForRequest(req *http.Request) (*url.URL, error)

// ProxyConfigFromEnvironment returns a ProxyConfig
// instance populated from environment variables.
// ProxyFromEnvironment is equivalent to ProxyConfigFromEnvironment().Proxy
// except that it only reads the environment variables once,
// the first time it is called.
func ProxyConfigFromEnvironment() *ProxyConfig {
	return &ProxyConfig{
		RequestMethod: os.Getenv("REQUEST_METHOD"),
		HTTPProxy: os.Getenv("HTTP_PROXY"),
		HTTPSProxy: os.Getenv("HTTPS_PROXY"),
		NoProxy: os.Getenv("NO_PROXY"),
	}
}

[rogpeppe]

@bradfitz ?

[ianlancetaylor]

CC @tombergan

[tombergan]

This sounds entirely reasonable to me. For reference, "NoProxy" is the non-trivial part of the implementation. @rogpeppe, want to send a CL?

For completeness, another option is to add http.ReloadProxyConfigFromEnvironment() to reload the environment variables. This is not a good idea because it will be hard to write that function in such a way that it won't race with concurrent calls to RoundTrip (which may be calling ProxyFromEnvironment).

[rogpeppe]

@tombergan Done

https://go-review.googlesource.com/#/c/go/+/68091/

[tombergan]

@rogpeppe, can you flesh out how you're going to use this feature? Specifically, why does ProxyConfigFromEnvironment() return the current values of the environment variables, rather than read the environment once and cache the result?

I did not read your proposed definition of ProxyConfigFromEnvironment carefully enough the first time I read your proposal. I'm not sure how you will update ProxyConfig safely when there are concurrent RoundTrip calls.

[rogpeppe]

To guard against races, it's not hard to do something like:

transport.Proxy = func(req *http.Request) (*url.URL, error) {
	myType.mu.Lock()
	defer myType.mu.Unlock()
	return myType.proxyConfig.ProxyForRequest(req)
}

An alternative might be to put a mutex into ProxyConfig, unexport the fields, and provide a way of updating it. That provides a more complex API though, and I'm not sure it's worth it - there are plenty of exported fields in net/http that are vulnerable to concurrency issues if modified without care.

[tombergan]

I agree adding a mutex to ProxyConfig is not worth it. The part that confused me is this:

// ProxyFromEnvironment is equivalent to ProxyConfigFromEnvironment().Proxy
// except that it only reads the environment variables once,
// the first time it is called.

Why does ProxyFromEnvironment need to re-read the environment variables each time it is called? It seems simpler to assume that the environment variables are fixed at startup. If the config needs to change, the caller can change the struct using a lock as in your above example.

[rogpeppe]

Why does ProxyFromEnvironment need to re-read the environment variables each time it is called? It seems simpler to assume that the environment variables are fixed at startup. If the config needs to change, the caller can change the struct using a lock as in your above example.

It doesn't. I think my explanation was confusing. I've updated the explanation and added a note about the need for a mutex. PTAL.

[gopherbot]

Change https://golang.org/cl/68091 mentions this issue: net/http: add ProxyConfig and NewProxyConfigFromEnvironment