Upon HTTP_1_1_REQUIRED, retry the request with HTTP/1.1

This is a companion patch to cbea2fd2c (NTLM: force the connection to
HTTP/1.1, 2018-12-06): with NTLM, we can switch to HTTP/1.1
preemptively. However, with other (Negotiate) authentication it is not
clear to this developer whether there is a way to make it work with
HTTP/2, so let's try HTTP/2 first and fall back in case we encounter the
error HTTP_1_1_REQUIRED.

Note: we will still keep the NTLM workaround, as it avoids an extra
round trip.

Daniel Stenberg helped a lot with this patch, in particular by
suggesting to introduce the Curl_h2_http_1_1_error() function.

Closes #3349

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

3 files changed, 32 insertions, 0 deletions

diff --git a/lib/http2.c b/lib/http2.c
index c33bee3eb..a61d8c240 100644
--- a/lib/http2.c
+++ b/lib/http2.c
@@ -2367,6 +2367,14 @@ void Curl_http2_cleanup_dependencies(struct Curl_easy *data)
     Curl_http2_remove_child(data->set.stream_depends_on, data);
 }
 
+/* Only call this function for a transfer that already got a HTTP/2
+   CURLE_HTTP2_STREAM error! */
+bool Curl_h2_http_1_1_error(struct connectdata *conn)
+{
+  struct http_conn *httpc = &conn->proto.httpc;
+  return (httpc->error_code == NGHTTP2_HTTP_1_1_REQUIRED);
+}
+
 #else /* !USE_NGHTTP2 */
 
 /* Satisfy external references even if http2 is not compiled in. */
diff --git a/lib/http2.h b/lib/http2.h
index 4492ec211..67db3dffb 100644
--- a/lib/http2.h
+++ b/lib/http2.h
@@ -59,6 +59,9 @@ CURLcode Curl_http2_add_child(struct Curl_easy *parent,
 void Curl_http2_remove_child(struct Curl_easy *parent,
                              struct Curl_easy *child);
 void Curl_http2_cleanup_dependencies(struct Curl_easy *data);
+
+/* returns true if the HTTP/2 stream error was HTTP_1_1_REQUIRED */
+bool Curl_h2_http_1_1_error(struct connectdata *conn);
 #else /* USE_NGHTTP2 */
 #define Curl_http2_init(x) CURLE_UNSUPPORTED_PROTOCOL
 #define Curl_http2_send_request(x) CURLE_UNSUPPORTED_PROTOCOL
@@ -74,6 +77,7 @@ void Curl_http2_cleanup_dependencies(struct Curl_easy *data);
 #define Curl_http2_add_child(x, y, z)
 #define Curl_http2_remove_child(x, y)
 #define Curl_http2_cleanup_dependencies(x)
+#define Curl_h2_http_1_1_error(x) 0
 #endif
 
 #endif /* HEADER_CURL_HTTP2_H */
diff --git a/lib/multi.c b/lib/multi.c
index cca138005..4ba22c46e 100644
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -46,6 +46,7 @@
 #include "vtls/vtls.h"
 #include "connect.h"
 #include "http_proxy.h"
+#include "http2.h"
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
 #include "curl_memory.h"
@@ -1955,6 +1956,25 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
           done = TRUE;
         }
       }
+      else if((CURLE_HTTP2_STREAM == result) &&
+                Curl_h2_http_1_1_error(data->easy_conn)) {
+        CURLcode ret = Curl_retry_request(data->easy_conn, &newurl);
+
+        infof(data, "Forcing HTTP/1.1 for NTLM");
+        data->set.httpversion = CURL_HTTP_VERSION_1_1;
+
+        if(!ret)
+          retry = (newurl)?TRUE:FALSE;
+        else
+          result = ret;
+
+        if(retry) {
+          /* if we are to retry, set the result to OK and consider the
+             request as done */
+          result = CURLE_OK;
+          done = TRUE;
+        }
+      }
 
       if(result) {
         /*