Add support for TLS to HTTP servers

We support TLS for gRPC clients, gRPC servers, and HTTP clients. HTTP
servers are the only ones for which TLS is not supported. This is
typically not an issue, because people generally put ingress controllers
in front of HTTP servers. However, if people want to run Buildbarn in
bare metal environments it may be useful to support this.

pkg/http/server.go

@@ -21,17 +21,30 @@ func NewServersFromConfigurationAndServe(configurations []*configuration.ServerC
 				return err
 			}
 			authenticatedHandler := NewAuthenticatingHandler(handler, authenticator)
+
+			tlsConfig, err := util.NewTLSConfigFromServerConfiguration(configuration.Tls)
+			if err != nil {
+				return err
+			}
+
 			for _, listenAddress := range configuration.ListenAddresses {
 				server := http.Server{
-					Addr:    listenAddress,
-					Handler: authenticatedHandler,
+					Addr:      listenAddress,
+					Handler:   authenticatedHandler,
+					TLSConfig: tlsConfig,
 				}
 				group.Go(func(ctx context.Context, siblingsGroup, dependenciesGroup program.Group) error {
 					<-ctx.Done()
 					return server.Close()
 				})
 				group.Go(func(ctx context.Context, siblingsGroup, dependenciesGroup program.Group) error {
-					if err := server.ListenAndServe(); err != http.ErrServerClosed {
+					var err error
+					if tlsConfig == nil {
+						err = server.ListenAndServe()
+					} else {
+						err = server.ListenAndServeTLS("", "")
+					}
+					if err != http.ErrServerClosed {
 						return util.StatusWrapf(err, "Failed to launch HTTP server %#v", server.Addr)
 					}
 					return nil