[tracker/wip/carlosg/resources-as-cursors: 9/18] libtracker-sparql: Implement service description of HTTP endpoints

[Carlos Garnacho <carlosg src gnome org>]

commit c95840fbd4014de935c5337cec4aac917ab6669d
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Jul 2 20:59:06 2022 +0200

    libtracker-sparql: Implement service description of HTTP endpoints
    
    Implement https://www.w3.org/TR/2013/REC-sparql11-service-description-20130321/
    on our HTTP endpoints, providing some basic information about what features and
    formats does the endpoint support.
    
    This can be accessed using the http://$host:$port/sparql address with no
    additional arguments.

 src/libtracker-sparql/tracker-endpoint-http.c | 122 +++++++++++++++++++++-----
 1 file changed, 99 insertions(+), 23 deletions(-)
---
diff --git a/src/libtracker-sparql/tracker-endpoint-http.c b/src/libtracker-sparql/tracker-endpoint-http.c
index c5829494f..52a19458e 100644
--- a/src/libtracker-sparql/tracker-endpoint-http.c
+++ b/src/libtracker-sparql/tracker-endpoint-http.c
@@ -52,11 +52,22 @@
 #include "config.h"
 
 #include "tracker-endpoint-http.h"
+
+#include "tracker-deserializer-resource.h"
 #include "tracker-serializer.h"
 #include "tracker-private.h"
 
 #include "remote/tracker-http.h"
 
+const gchar *supported_formats[] = {
+       "http://www.w3.org/ns/formats/SPARQL_Results_JSON";,
+       "http://www.w3.org/ns/formats/SPARQL_Results_XML";,
+       "http://www.w3.org/ns/formats/Turtle";,
+       "http://www.w3.org/ns/formats/TriG";,
+};
+
+G_STATIC_ASSERT (G_N_ELEMENTS (supported_formats) == TRACKER_N_SERIALIZER_FORMATS);
+
 struct _TrackerEndpointHttp {
        TrackerEndpoint parent_instance;
        TrackerHttpServer *server;
@@ -164,6 +175,45 @@ pick_format (guint                    formats,
        return FALSE;
 }
 
+static void
+add_supported_formats (TrackerResource *resource,
+                       const gchar     *property)
+{
+       gint i;
+
+       for (i = 0; i < TRACKER_N_SERIALIZER_FORMATS; i++)
+               tracker_resource_add_uri (resource, property, supported_formats[i]);
+}
+
+static TrackerResource *
+create_service_description (TrackerEndpointHttp      *endpoint,
+                            TrackerNamespaceManager **manager)
+{
+       TrackerResource *resource;
+
+       *manager = tracker_namespace_manager_new ();
+       tracker_namespace_manager_add_prefix (*manager, "rdf",
+                                             "http://www.w3.org/1999/02/22-rdf-syntax-ns#";);
+       tracker_namespace_manager_add_prefix (*manager, "sd",
+                                             "http://www.w3.org/ns/sparql-service-description#";);
+       tracker_namespace_manager_add_prefix (*manager, "format",
+                                             "http://www.w3.org/ns/formats/";);
+
+       resource = tracker_resource_new (NULL);
+       tracker_resource_set_uri (resource, "rdf:type", "sd:Service");
+       /* tracker_resource_set_uri (resource, "sd:endpoint", endpoint_uri); */
+       tracker_resource_set_uri (resource, "sd:supportedLanguage", "sd:SPARQL11Query");
+
+       tracker_resource_add_uri (resource, "sd:feature", "sd:EmptyGraphs");
+       tracker_resource_add_uri (resource, "sd:feature", "sd:BasicFederatedQuery");
+       tracker_resource_add_uri (resource, "sd:feature", "sd:UnionDefaultGraph");
+
+       add_supported_formats (resource, "sd:resultFormat");
+       add_supported_formats (resource, "sd:inputFormat");
+
+       return resource;
+}
+
 static void
 http_server_request_cb (TrackerHttpServer  *server,
                         GSocketAddress     *remote_address,
@@ -177,7 +227,7 @@ http_server_request_cb (TrackerHttpServer  *server,
        TrackerSparqlConnection *conn;
        TrackerSerializerFormat format;
        gboolean block = FALSE;
-       const gchar *sparql;
+       const gchar *sparql = NULL;
        Request *data;
 
        if (remote_address) {
@@ -191,30 +241,56 @@ http_server_request_cb (TrackerHttpServer  *server,
                return;
        }
 
-       sparql = g_hash_table_lookup (params, "query");
-       if (!sparql) {
-               tracker_http_server_error (server, request, 500,
-                                          "No query given");
-               return;
-       }
+       if (params)
+               sparql = g_hash_table_lookup (params, "query");
 
-       if (!pick_format (formats, &format)) {
-               tracker_http_server_error (server, request, 500,
-                                          "No recognized accepted formats");
-               return;
-       }
+       if (sparql) {
+               if (!pick_format (formats, &format)) {
+                       tracker_http_server_error (server, request, 500,
+                                                  "No recognized accepted formats");
+                       return;
+               }
 
-       data = g_new0 (Request, 1);
-       data->endpoint = endpoint;
-       data->request = request;
-       data->format = format;
-
-       conn = tracker_endpoint_get_sparql_connection (endpoint);
-       tracker_sparql_connection_query_async (conn,
-                                              sparql,
-                                              NULL,
-                                              query_async_cb,
-                                              data);
+               data = g_new0 (Request, 1);
+               data->endpoint = endpoint;
+               data->request = request;
+               data->format = format;
+
+               conn = tracker_endpoint_get_sparql_connection (endpoint);
+               tracker_sparql_connection_query_async (conn,
+                                                      sparql,
+                                                      NULL,
+                                                      query_async_cb,
+                                                      data);
+       } else {
+               TrackerNamespaceManager *namespaces;
+               TrackerResource *description;
+               TrackerSparqlCursor *deserializer;
+               GInputStream *serializer;
+
+               if (!pick_format (formats, &format))
+                       format = TRACKER_SERIALIZER_FORMAT_TTL;
+
+               /* Requests to with no query return a RDF description
+                * about the HTTP endpoint.
+                */
+               description = create_service_description (TRACKER_ENDPOINT_HTTP (endpoint),
+                                                         &namespaces);
+
+               deserializer = tracker_deserializer_resource_new (description,
+                                                                 namespaces, NULL);
+               serializer = tracker_serializer_new (TRACKER_SPARQL_CURSOR (deserializer),
+                                                    namespaces, format);
+               g_object_unref (deserializer);
+               g_object_unref (description);
+               g_object_unref (namespaces);
+
+               /* Consumes the serializer */
+               tracker_http_server_response (server,
+                                             request,
+                                             format,
+                                             serializer);
+       }
 }
 
 static gboolean