From 7ca7675a85095023feb5cc6487b634ba1f5075d1 Mon Sep 17 00:00:00 2001
From: Andreas Schneider <aksdb@gmx.de>
Date: Tue, 3 Apr 2018 12:12:50 +0200
Subject: [PATCH] Simplified authentication negotiation

---
 src/calanonsync/ews.go | 42 ++++++++++++++++++++----------------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/src/calanonsync/ews.go b/src/calanonsync/ews.go
index e44ce43..4b0f797 100644
--- a/src/calanonsync/ews.go
+++ b/src/calanonsync/ews.go
@@ -66,28 +66,24 @@ type EWSRoundTripper struct {
 }
 
 func (er EWSRoundTripper) RoundTrip(r *http.Request) (*http.Response, error) {
-	if er.authType == authTypeBasic {
-		r.SetBasicAuth(er.username, er.password)
-	}
-
 	er.initMutex.Lock()
-	resp, err := er.delegate.RoundTrip(r)
-	if err == nil && resp.StatusCode == http.StatusUnauthorized && er.authType == authTypeUnknown {
-		// This is a good time to find out what the server prefers.
-		authHeaders := resp.Header["Www-Authenticate"]
-		if authHeaders != nil {
-			for _, h := range authHeaders {
-				if strings.HasPrefix(h, "BASIC") {
-					er.authType = authTypeBasic
-				} else if strings.HasPrefix(h, "NTLM") {
-					er.authType = authTypeNTLM
-					break // NTLM is the best we could do
+	if er.authType == authTypeUnknown {
+		// Find authentication scheme.
+		resp, err := http.DefaultClient.Get(r.URL.String())
+		if err == nil && resp.StatusCode == http.StatusUnauthorized {
+			// This is a good time to find out what the server prefers.
+			authHeaders := resp.Header["Www-Authenticate"]
+			if authHeaders != nil {
+				for _, h := range authHeaders {
+					if strings.HasPrefix(h, "BASIC") {
+						er.authType = authTypeBasic
+					} else if strings.HasPrefix(h, "NTLM") {
+						er.authType = authTypeNTLM
+						break // NTLM is the best we could do
+					}
 				}
 			}
-		}
 
-		// So, do we know more than before? If so, try again.
-		if er.authType > authTypeUnknown {
 			if er.authType == authTypeNTLM {
 				// We need to replace the delegator.
 				er.delegate = &httpntlm.NtlmTransport{
@@ -96,13 +92,15 @@ func (er EWSRoundTripper) RoundTrip(r *http.Request) (*http.Response, error) {
 					Password: er.password,
 				}
 			}
-			er.initMutex.Unlock()
-			return er.RoundTrip(r)
 		}
 	}
-
 	er.initMutex.Unlock()
-	return resp, err
+
+	if er.authType == authTypeBasic {
+		r.SetBasicAuth(er.username, er.password)
+	}
+
+	return er.delegate.RoundTrip(r)
 }
 
 func NewEWSCalendar(url, username, password string) *EWSCalendar {