net/tshttpproxy: don't proxy through ourselves

When running a SOCKS or HTTP proxy, configure the tshttpproxy package to
drop those addresses from any HTTP_PROXY or HTTPS_PROXY environment
variables.

Fixes #7407

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I6cd7cad7a609c639780484bad521c7514841764b

net/tshttpproxy/tshttpproxy.go

@@ -9,11 +9,16 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"net"
 	"net/http"
 	"net/url"
 	"os"
+	"runtime"
+	"strings"
 	"sync"
 	"time"
+
+	"golang.org/x/net/http/httpproxy"
 )
 
 // InvalidateCache invalidates the package-level cache for ProxyFromEnvironment.
@@ -27,9 +32,24 @@ func InvalidateCache() {
 
 var (
 	mu           sync.Mutex
-	noProxyUntil time.Time // if non-zero, time at which ProxyFromEnvironment should check again
+	noProxyUntil time.Time         // if non-zero, time at which ProxyFromEnvironment should check again
+	config       *httpproxy.Config // used to create proxyFunc
+	proxyFunc    func(*url.URL) (*url.URL, error)
 )
 
+func getProxyFunc() func(*url.URL) (*url.URL, error) {
+	// Create config/proxyFunc if it's not created
+	mu.Lock()
+	defer mu.Unlock()
+	if config == nil {
+		config = httpproxy.FromEnvironment()
+	}
+	if proxyFunc == nil {
+		proxyFunc = config.ProxyFunc()
+	}
+	return proxyFunc
+}
+
 // setNoProxyUntil stops calls to sysProxyEnv (if any) for the provided duration.
 func setNoProxyUntil(d time.Duration) {
 	mu.Lock()
@@ -39,6 +59,59 @@ func setNoProxyUntil(d time.Duration) {
 
 var _ = setNoProxyUntil // quiet staticcheck; Windows uses the above, more might later
 
+// SetSelfProxy configures this package to avoid proxying through any of the
+// provided addresses–e.g. if they refer to proxies being run by this process.
+func SetSelfProxy(addrs ...string) {
+	mu.Lock()
+	defer mu.Unlock()
+
+	// Ensure we have a valid config
+	if config == nil {
+		config = httpproxy.FromEnvironment()
+	}
+
+	normalizeHostPort := func(s string) string {
+		host, portStr, err := net.SplitHostPort(s)
+		if err != nil {
+			return s
+		}
+
+		// Normalize the localhost IP into "localhost", to avoid IPv4/IPv6 confusion.
+		if host == "127.0.0.1" || host == "::1" {
+			return "localhost:" + portStr
+		}
+
+		// On Linux, all 127.0.0.1/8 IPs are also localhost.
+		if runtime.GOOS == "linux" && strings.HasPrefix(host, "127.0.0.") {
+			return "localhost:" + portStr
+		}
+
+		return s
+	}
+
+	normHTTP := normalizeHostPort(config.HTTPProxy)
+	normHTTPS := normalizeHostPort(config.HTTPSProxy)
+
+	// If any of our proxy variables point to one of the configured
+	// addresses, ignore them.
+	for _, addr := range addrs {
+		normAddr := normalizeHostPort(addr)
+		if normHTTP != "" && normHTTP == normAddr {
+			log.Printf("tshttpproxy: skipping HTTP_PROXY pointing to self: %q", addr)
+			config.HTTPProxy = ""
+			normHTTP = ""
+		}
+		if normHTTPS != "" && normHTTPS == normAddr {
+			log.Printf("tshttpproxy: skipping HTTPS_PROXY pointing to self: %q", addr)
+			config.HTTPSProxy = ""
+			normHTTPS = ""
+		}
+	}
+
+	// Invalidate to cause it to get re-created
+	proxyFunc = nil
+}
+
 // sysProxyFromEnv, if non-nil, specifies a platform-specific ProxyFromEnvironment
 // func to use if http.ProxyFromEnvironment doesn't return a proxy.
 // For example, WPAD PAC files on Windows.
@@ -48,7 +121,8 @@ var sysProxyFromEnv func(*http.Request) (*url.URL, error)
 // but additionally does OS-specific proxy lookups if the environment variables
 // alone don't specify a proxy.
 func ProxyFromEnvironment(req *http.Request) (*url.URL, error) {
-	u, err := http.ProxyFromEnvironment(req)
+	localProxyFunc := getProxyFunc()
+	u, err := localProxyFunc(req.URL)
 	if u != nil && err == nil {
 		return u, nil
 	}