[nginx] SSL: support ALPN (IETF's successor to NPN).

[Ruslan Ermilov]

On Wed, Jan 29, 2014 at 03:51:50PM +0000, Valentin Bartenev wrote:
> details:   http://hg.nginx.org/nginx/rev/01e2a5bcdd8f
> branches:  
> changeset: 5545:01e2a5bcdd8f
> user:      Piotr Sikora <piotr at cloudflare.com>
> date:      Tue Jan 28 15:33:49 2014 -0800
> description:
> SSL: support ALPN (IETF's successor to NPN).
> 
> Signed-off-by: Piotr Sikora <piotr at cloudflare.com>

This change breaks compilation with clang on systems
where OpenSSL doesn't have ALPN support:

: cc -c -pipe  -O -Wall -Wextra -Wpointer-arith -Wconditional-uninitialized -Wno-unused-parameter -Werror -g  -I src/core -I src/event -I src/event/modules -I src/os/unix -I /opt/local/include -I objs -I src/http -I src/http/modules \
:                 -o objs/src/http/ngx_http_request.o \
:                 src/http/ngx_http_request.c
: src/http/ngx_http_request.c:728:44: error: variable 'data' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
:         if (len == spdy.len && ngx_strncmp(data, spdy.data, spdy.len) == 0) {
:                                            ^~~~
: src/core/ngx_string.h:53:56: note: expanded from macro 'ngx_strncmp'
: #define ngx_strncmp(s1, s2, n)  strncmp((const char *) s1, (const char *) s2, n)
:                                                        ^
: src/http/ngx_http_request.c:713:38: note: initialize the variable 'data' to silence this warning
:         const unsigned char     *data;
:                                      ^
:                                       = NULL
: 1 error generated.
: make[1]: *** [objs/src/http/ngx_http_request.o] Error 1
: make: *** [build] Error 2

clang(1) mistakenly thinks that "len" can be non-zero, and under
this condition "data" may be uninitialized.

It's possible to rewrite the code such as that it doesn't trigger
the warning:

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -713,11 +713,8 @@ ngx_http_ssl_handshake_handler(ngx_conne
         const unsigned char     *data;
         static const ngx_str_t   spdy = ngx_string(NGX_SPDY_NPN_NEGOTIATED);
 
-        len = 0;
-
 #ifdef TLSEXT_TYPE_application_layer_protocol_negotiation
         SSL_get0_alpn_selected(c->ssl->connection, &data, &len);
-#endif
 
 #ifdef TLSEXT_TYPE_next_proto_neg
         if (len == 0) {
@@ -725,6 +722,10 @@ ngx_http_ssl_handshake_handler(ngx_conne
         }
 #endif
 
+#else /* TLSEXT_TYPE_next_proto_neg */
+        SSL_get0_next_proto_negotiated(c->ssl->connection, &data, &len);
+#endif
+
         if (len == spdy.len && ngx_strncmp(data, spdy.data, spdy.len) == 0) {
             ngx_http_spdy_init(c->read);
             return;