openssl: Advertise NPN protocols for server sockets

If a server socket is created with a context on which the
`set_npn_protocols` method has been called, during TLS connection
establishment, the server will advertise the list of protocols given to
the method, in case the client indicates that it supports the NPN TLS
extension.

openssl/src/ssl/mod.rs

@@ -249,6 +249,36 @@ extern fn raw_next_proto_select_cb(ssl: *mut ffi::SSL,
     ffi::SSL_TLSEXT_ERR_OK
 }
 
+/// The function is given as the callback to `SSL_CTX_set_next_protos_advertised_cb`.
+///
+/// It causes the parameter `out` to point at a `*const c_uchar` instance that
+/// represents the list of protocols that the server should advertise as those
+/// that it supports.
+/// The list of supported protocols is found in the extra data of the OpenSSL
+/// context.
+#[cfg(feature = "npn")]
+extern fn raw_next_protos_advertise_cb(ssl: *mut ffi::SSL,
+                                       out: *mut *const c_uchar, outlen: *mut c_uint,
+                                       _arg: *mut c_void) -> c_int {
+    unsafe {
+        // First, get the list of (supported) protocols saved in the context extra data.
+        let ssl_ctx = ffi::SSL_get_SSL_CTX(ssl);
+        let protocols = ffi::SSL_CTX_get_ex_data(ssl_ctx, get_npn_protos_idx());
+        if protocols.is_null() {
+            *out = b"".as_ptr();
+            *outlen = 0;
+        } else {
+            // If the pointer is valid, put the pointer to the actual byte array into the
+            // output parameter `out`, as well as its length into `outlen`.
+            let protocols: &Vec<u8> = mem::transmute(protocols);
+            *out = protocols.as_ptr();
+            *outlen = protocols.len() as c_uint;
+        }
+    }
+
+    ffi::SSL_TLSEXT_ERR_OK
+}
+
 /// The signature of functions that can be used to manually verify certificates
 pub type VerifyCallback = fn(preverify_ok: bool,
                              x509_ctx: &X509StoreContext) -> bool;
@@ -426,6 +456,9 @@ impl SslContext {
             // matching based on the client-supported list of protocols that
             // has been saved.
             ffi::SSL_CTX_set_next_proto_select_cb(*self.ctx, raw_next_proto_select_cb, ptr::null_mut());
+            // Also register the callback to advertise these protocols, if a server socket is
+            // created with the context.
+            ffi::SSL_CTX_set_next_protos_advertised_cb(*self.ctx, raw_next_protos_advertise_cb, ptr::null_mut());
         }
     }
 }