Reject a certificate whose verify callback throws

An exception from a verify callback unwound through the TLS library,
which is C: OpenSSL leaked the objects of the handshake, and a server
worker thread ended the process. It also left the handshake count that
update_certs_pem() waits on raised, so with a task queue that caught the
exception, every later update waited forever. Take the exception as a
rejection, and lower the count on every way out of the handshake.

test/test.cc

@@ -4933,6 +4933,46 @@ TEST(SSLClientServerTest, VerifyCallbackKeepsClientCertificateRequired) {
   EXPECT_EQ("ok", res->body);
 }
 
+TEST(SSLClientServerTest, ThrowingVerifyCallbackRejectsTheCertificate) {
+  SSLServer svr(SERVER_CERT_FILE, SERVER_PRIVATE_KEY_FILE, CLIENT_CA_CERT_FILE);
+  ASSERT_TRUE(svr.is_valid());
+  tls::set_verify_callback(svr.tls_context(),
+                           [](const tls::VerifyContext &) -> bool {
+                             throw std::runtime_error("verify");
+                           });
+  svr.Get("/test", [](const Request &, Response &res) {
+    res.set_content("ok", "text/plain");
+  });
+
+  thread t = thread([&]() { ASSERT_TRUE(svr.listen(HOST, PORT)); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+  svr.wait_until_ready();
+
+  auto get = [] {
+    SSLClient cli(HOST, PORT, CLIENT_CERT_FILE, CLIENT_PRIVATE_KEY_FILE);
+    cli.enable_server_certificate_verification(false);
+    return cli.Get("/test");
+  };
+  EXPECT_FALSE(get());
+
+  // The handshake is over, so an update does not wait for it.
+  std::string cert_pem, key_pem;
+  read_file(SERVER_CERT_FILE, cert_pem);
+  read_file(SERVER_PRIVATE_KEY_FILE, key_pem);
+  EXPECT_TRUE(svr.update_certs_pem(cert_pem.c_str(), key_pem.c_str()));
+
+  tls::set_verify_callback(svr.tls_context(), [](const tls::VerifyContext &v) {
+    return v.preverify_ok;
+  });
+  auto res = get();
+  ASSERT_TRUE(res) << "Error: " << to_string(res.error());
+  EXPECT_EQ("ok", res->body);
+}
+
 #ifdef CPPHTTPLIB_OPENSSL_SUPPORT
 // A client that resumes its session gets through, but not once the client CA
 // has changed, as a resumed session does not show the certificate again.