http: attempt Negotiate auth in http.emptyAuth=auto mode

When a server advertises Negotiate (SPNEGO) authentication, the
"auto" mode of http.emptyAuth should detect this as an "exotic"
method and proactively send empty credentials, allowing libcurl to
use the system Kerberos ticket without prompting the user.

However, two features interact to prevent this from working:

The Negotiate-stripping logic, introduced in 4dbe664
(remote-curl: fall back to Basic auth if Negotiate fails,
2015-01-08), removes CURLAUTH_GSSNEGOTIATE from the allowed
methods on the first 401 response. The empty-auth auto-detection,
introduced in 40a18fc (http: add an "auto" mode for
http.emptyauth, 2017-02-25), then checks the remaining methods
for anything "exotic" -- but Negotiate has already been removed,
so auto mode never activates for servers whose only non-Basic/Digest
method is Negotiate (e.g., Apache with mod_auth_kerb offering
Basic + Negotiate).

Fix this by delaying the Negotiate stripping in auto mode: on the
first 401, keep Negotiate in the allowed methods so that auto mode
can detect it and retry with empty credentials. If that attempt
fails (no valid Kerberos ticket), strip Negotiate on the second 401
and fall through to credential_fill() as usual.

To support this, also teach http_reauth_prepare() to skip
credential_fill() when empty auth is about to be attempted, since
filling real credentials would bypass the empty-auth mechanism.

The true and false modes are unchanged: true sends empty credentials
on the very first request (before any 401), and false never sends
them.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

http.c

@@ -138,6 +138,7 @@ static unsigned long empty_auth_useless =
 	CURLAUTH_BASIC
 	| CURLAUTH_DIGEST_IE
 	| CURLAUTH_DIGEST;
+static int empty_auth_try_negotiate;
 
 static struct curl_slist *pragma_header;
 static struct string_list extra_http_headers = STRING_LIST_INIT_DUP;
@@ -667,6 +668,17 @@ static void init_curl_http_auth(CURL *result)
 
 void http_reauth_prepare(int all_capabilities)
 {
+	/*
+	 * If we deferred stripping Negotiate to give empty auth a
+	 * chance (auto mode), skip credential_fill on this retry so
+	 * that init_curl_http_auth() sends empty credentials and
+	 * libcurl can attempt Negotiate with the system ticket cache.
+	 */
+	if (empty_auth_try_negotiate &&
+	    !http_auth.password && !http_auth.credential &&
+	    (http_auth_methods & CURLAUTH_GSSNEGOTIATE))
+		return;
+
 	credential_fill(the_repository, &http_auth, all_capabilities);
 }
 
@@ -1895,7 +1907,18 @@ static int handle_curl_result(struct slot_results *results)
 				http_proactive_auth = PROACTIVE_AUTH_NONE;
 			return HTTP_NOAUTH;
 		} else {
-			http_auth_methods &= ~CURLAUTH_GSSNEGOTIATE;
+			if (curl_empty_auth == -1 &&
+			    !empty_auth_try_negotiate &&
+			    (results->auth_avail & CURLAUTH_GSSNEGOTIATE)) {
+				/*
+				 * In auto mode, give Negotiate a chance via
+				 * empty auth before stripping it. If it fails,
+				 * we will strip it on the next 401.
+				 */
+				empty_auth_try_negotiate = 1;
+			} else {
+				http_auth_methods &= ~CURLAUTH_GSSNEGOTIATE;
+			}
 			if (results->auth_avail) {
 				http_auth_methods &= results->auth_avail;
 				http_auth_methods_restricted = 1;