Increment session hit counter for ticket resumptions (#1320)

OpenSSL increments session hits in two places:

1. Handshake finish [on the client side](https://github.com/openssl/openssl/blob/1750689767cc922bdbe73358f7256475f0838c67/ssl/statem/statem_lib.c#L1494)
2. Fetching potentially pre-existing session [on the server side](https://github.com/openssl/openssl/blob/1750689767cc922bdbe73358f7256475f0838c67/ssl/ssl_sess.c#L665)

Prior to this change, we failed to increment the session "cache" hit counter for TLS 1.3 servers. I put "cache" in scare-quotes because whether that behavior is correct depends on how you define a cache, as TLS 1.3 ticket resumption is stateless and doesn't require a true session cache. Semantics aside, the change in this PR conforms to OpenSSL's behavior. OpenSSL [documents](https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_sess_hits.html) session hit counts as:

> SSL_CTX_sess_hits() returns the number of successfully reused sessions. In client mode a session set with [SSL_set_session(3)](https://www.openssl.org/docs/man1.1.1/man3/SSL_set_session.html) successfully reused is counted as a hit. In server mode a session successfully retrieved from internal or external cache is counted as a hit.

include/openssl/ssl.h

@@ -5016,7 +5016,8 @@ OPENSSL_EXPORT int SSL_CTX_sess_accept_renegotiate(const SSL_CTX *ctx);
 // SSL/TLS sessions in server mode.
 OPENSSL_EXPORT int SSL_CTX_sess_accept_good(const SSL_CTX *ctx);
 
-// SSL_CTX_sess_hits returns the number of successfully reused sessions.
+// SSL_CTX_sess_hits returns the number of successfully reused sessions, from
+// both session cache and session tickets.
 OPENSSL_EXPORT int SSL_CTX_sess_hits(const SSL_CTX *ctx);
 
 // SSL_CTX_sess_cb_hits returns the number of successfully retrieved sessions

ssl/extensions.cc

@@ -4019,6 +4019,11 @@ enum ssl_ticket_aead_result_t ssl_process_ticket(
   // Other consumers may expect a non-empty session ID to indicate resumption.
   session->session_id_length = SHA256_DIGEST_LENGTH;
 
+  // Ticket-based session resumption bypasses the session cache, but counts as
+  // session reuse, so update the session's "session hit" counter.
+  ssl_update_counter(ssl->session_ctx.get(), ssl->session_ctx->stats.sess_hit,
+                     true);
+
   *out_session = std::move(session);
   return ssl_ticket_aead_success;
 }

ssl/ssl_test.cc

@@ -9223,6 +9223,11 @@ TEST_P(SSLVersionTest, SameKeyResume) {
   ClientConfig config;
   config.session = session.get();
 
+  // No hits before we resume the connection
+  EXPECT_EQ(SSL_CTX_sess_hits(client_ctx_.get()), 0);
+  EXPECT_EQ(SSL_CTX_sess_hits(server_ctx_.get()), 0);
+  EXPECT_EQ(SSL_CTX_sess_hits(server_ctx2.get()), 0);
+
   // Resuming with |server_ctx_| again works.
   bssl::UniquePtr<SSL> client, server;
   ASSERT_TRUE(ConnectClientAndServer(&client, &server, client_ctx_.get(),
@@ -9236,8 +9241,11 @@ TEST_P(SSLVersionTest, SameKeyResume) {
   EXPECT_TRUE(SSL_session_reused(client.get()));
   EXPECT_TRUE(SSL_session_reused(server.get()));
 
-  // By this point, the session has been resumed twice on the client side.
+  // By this point, the session has been resumed twice on the client side and
+  // once for each server context.
   EXPECT_EQ(SSL_CTX_sess_hits(client_ctx_.get()), 2);
+  EXPECT_EQ(SSL_CTX_sess_hits(server_ctx_.get()), 1);
+  EXPECT_EQ(SSL_CTX_sess_hits(server_ctx2.get()), 1);
 }
 
 TEST_P(SSLVersionTest, DifferentKeyNoResume) {