net/tshttpproxy: aggressively rate-limit error logs in Transport.Proxy path

Otherwise log upload HTTP requests generate proxy errrors which
generate logs which generate HTTP requests which generate proxy
errors which generate more logs, etc.

Fixes #879

net/tshttpproxy/tshttpproxy_windows.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/alexbrainman/sspi/negotiate"
 	"golang.org/x/sys/windows"
+	"tailscale.com/types/logger"
 )
 
 var (
@@ -38,6 +39,13 @@ var cachedProxy struct {
 	val *url.URL
 }
 
+// proxyErrorf is a rate-limited logger specifically for errors asking
+// WinHTTP for the proxy information. We don't want to log about
+// errors often, otherwise the log message itself will generate a new
+// HTTP request which ultimately will call back into us to log again,
+// forever. So for errors, we only log a bit.
+var proxyErrorf = logger.RateLimitedFn(log.Printf, 10*time.Minute, 2 /* burst*/, 10 /* maxCache */)
+
 func proxyFromWinHTTPOrCache(req *http.Request) (*url.URL, error) {
 	if req.URL == nil {
 		return nil, nil
@@ -79,7 +87,14 @@ func proxyFromWinHTTPOrCache(req *http.Request) (*url.URL, error) {
 			setNoProxyUntil(10 * time.Second)
 			return nil, nil
 		}
-		log.Printf("tshttpproxy: winhttp: GetProxyForURL(%q): %v/%#v", urlStr, err, err)
+		if err == windows.ERROR_INVALID_PARAMETER {
+			// Seen on Windows 8.1. (https://github.com/tailscale/tailscale/issues/879)
+			// TODO(bradfitz): figure this out.
+			setNoProxyUntil(time.Hour)
+			proxyErrorf("tshttpproxy: winhttp: GetProxyForURL(%q): ERROR_INVALID_PARAMETER [unexpected]", urlStr)
+			return nil, nil
+		}
+		proxyErrorf("tshttpproxy: winhttp: GetProxyForURL(%q): %v/%#v", urlStr, err, err)
 		if err == syscall.Errno(ERROR_WINHTTP_UNABLE_TO_DOWNLOAD_SCRIPT) {
 			setNoProxyUntil(10 * time.Second)
 			return nil, nil
@@ -88,7 +103,7 @@ func proxyFromWinHTTPOrCache(req *http.Request) (*url.URL, error) {
 	case <-ctx.Done():
 		cachedProxy.Lock()
 		defer cachedProxy.Unlock()
-		log.Printf("tshttpproxy: winhttp: GetProxyForURL(%q): timeout; using cached proxy %v", urlStr, cachedProxy.val)
+		proxyErrorf("tshttpproxy: winhttp: GetProxyForURL(%q): timeout; using cached proxy %v", urlStr, cachedProxy.val)
 		return cachedProxy.val, nil
 	}
 }
@@ -96,7 +111,7 @@ func proxyFromWinHTTPOrCache(req *http.Request) (*url.URL, error) {
 func proxyFromWinHTTP(ctx context.Context, urlStr string) (proxy *url.URL, err error) {
 	whi, err := winHTTPOpen()
 	if err != nil {
-		log.Printf("winhttp: Open: %v", err)
+		proxyErrorf("winhttp: Open: %v", err)
 		return nil, err
 	}
 	defer whi.Close()