Add support for SSL_SESSION_is_resumable()

Provide a way to test whether the SSL_SESSION object can be used to resume a
sesion or not.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3008)

include/openssl/ssl.h

@@ -1502,6 +1502,7 @@ __owur int SSL_SESSION_set1_id_context(SSL_SESSION *s, const unsigned char *sid_
                                 unsigned int sid_ctx_len);
 __owur int SSL_SESSION_set1_id(SSL_SESSION *s, const unsigned char *sid,
                                unsigned int sid_len);
+__owur int SSL_SESSION_is_resumable(const SSL_SESSION *s);
 
 __owur SSL_SESSION *SSL_SESSION_new(void);
 const unsigned char *SSL_SESSION_get_id(const SSL_SESSION *s,

ssl/ssl_sess.c

@@ -46,12 +46,12 @@ static void SSL_SESSION_list_add(SSL_CTX *ctx, SSL_SESSION *s);
 static int remove_session_lock(SSL_CTX *ctx, SSL_SESSION *c, int lck);
 
 /*
- * TODO(TLS1.3): SSL_get_session() and SSL_get1_session() are problematic in
- * TLS1.3 because, unlike in earlier protocol versions, the session ticket
- * may not have been sent yet even though a handshake has finished. The session
- * ticket data could come in sometime later...or even change if multiple session
- * ticket messages are sent from the server. We need to work out how to deal
- * with this.
+ * SSL_get_session() and SSL_get1_session() are problematic in TLS1.3 because,
+ * unlike in earlier protocol versions, the session ticket may not have been
+ * sent yet even though a handshake has finished. The session ticket data could
+ * come in sometime later...or even change if multiple session ticket messages
+ * are sent from the server. The preferred way for applications to obtain
+ * a resumable session is to use SSL_CTX_sess_set_new_cb().
  */
 
 SSL_SESSION *SSL_get_session(const SSL *ssl)
@@ -929,6 +929,16 @@ int SSL_SESSION_set1_id_context(SSL_SESSION *s, const unsigned char *sid_ctx,
     return 1;
 }
 
+int SSL_SESSION_is_resumable(const SSL_SESSION *s)
+{
+    /*
+     * In the case of EAP-FAST, we can have a pre-shared "ticket" without a
+     * session ID.
+     */
+    return !s->not_resumable
+           && (s->session_id_length > 0 || s->ext.ticklen > 0);
+}
+
 long SSL_CTX_set_timeout(SSL_CTX *s, long t)
 {
     long l;

ssl/statem/statem_clnt.c

@@ -1049,13 +1049,9 @@ int tls_construct_client_hello(SSL *s, WPACKET *pkt)
         return 0;
     }
 
-    if ((sess == NULL) || !ssl_version_supported(s, sess->ssl_version) ||
-        /*
-         * In the case of EAP-FAST, we can have a pre-shared
-         * "ticket" without a session ID.
-         */
-        (!sess->session_id_length && !sess->ext.tick) ||
-        (sess->not_resumable)) {
+    if (sess == NULL
+            || !ssl_version_supported(s, sess->ssl_version)
+            || !SSL_SESSION_is_resumable(sess)) {
         if (!ssl_get_new_session(s, 0))
             return 0;
     }

util/libssl.num

@@ -440,3 +440,4 @@ SSL_get0_peer_CA_list                   440	1_1_1	EXIST::FUNCTION:
 SSL_CTX_add1_CA_list                    441	1_1_1	EXIST::FUNCTION:
 SSL_CTX_get0_CA_list                    442	1_1_1	EXIST::FUNCTION:
 SSL_CTX_add_custom_ext                  443	1_1_1	EXIST::FUNCTION:
+SSL_SESSION_is_resumable                444	1_1_1	EXIST::FUNCTION: