[tracker/wip/carlosg/better-errors: 2/2] libtracker-data: Provide snippet of parsing failures

[Carlos Garnacho <carlosg src gnome org>]

commit 3fdb6888491187d4199d4a7f8a58d0830f4e3a6f
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed May 27 23:33:39 2020 +0200

    libtracker-data: Provide snippet of parsing failures
    
    This makes it crystal clear where the parser is having trouble with.

 src/libtracker-data/tracker-sparql-parser.c | 52 +++++++++++++++++++++++++++--
 1 file changed, 49 insertions(+), 3 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql-parser.c b/src/libtracker-data/tracker-sparql-parser.c
index 73eca8c53..0c4ab28e9 100644
--- a/src/libtracker-data/tracker-sparql-parser.c
+++ b/src/libtracker-data/tracker-sparql-parser.c
@@ -33,6 +33,7 @@ typedef struct _TrackerGrammarParser TrackerGrammarParser;
 
 #define NODES_PER_CHUNK 128
 #define RULE_STATE_DEFAULT_SIZE 128
+#define SNIPPET_LENGTH 30
 
 struct _TrackerRuleState {
        const TrackerGrammarRule *rule;
@@ -669,13 +670,54 @@ rule_equals (gconstpointer a,
        }
 }
 
+static gchar *
+get_error_snippet (TrackerGrammarParser *parser,
+                   TrackerParserState   *state)
+{
+       gssize start, end, error_pos;
+       gchar *sample, *snippet;
+
+       start = state->error_len - SNIPPET_LENGTH / 2;
+       end = state->error_len + SNIPPET_LENGTH / 2;
+       error_pos = SNIPPET_LENGTH / 2;
+
+       if (start < 0) {
+               end += ABS (start);
+               error_pos += start;
+               start = 0;
+       }
+
+       if (end > parser->query_len) {
+               gint move = end - parser->query_len;
+
+               if (start >= move) {
+                       start -= move;
+                       error_pos += move;
+               }
+
+               end -= move;
+       }
+
+       sample = g_strndup (&parser->query[start], end - start);
+       snippet = g_strdup_printf ("%s%s%s\n%*c‸",
+                                  start == 0 ? " " : "…",
+                                  sample,
+                                  end == parser->query_len ? " " : "…",
+                                  (gint) error_pos + 1, ' ');
+       g_free (sample);
+
+       return snippet;
+}
+
 static void
-tracker_parser_state_propagate_error (TrackerParserState  *state,
-                                      GError             **error)
+tracker_parser_state_propagate_error (TrackerGrammarParser  *parser,
+                                      TrackerParserState    *state,
+                                      GError               **error)
 {
        const TrackerGrammarRule *rule;
        GString *str = g_string_new (NULL);
        GHashTable *repeated;
+       gchar *snippet;
 
        if (state->current > state->error_len) {
                /* The errors gathered are outdated, but we are propagating
@@ -714,6 +756,10 @@ tracker_parser_state_propagate_error (TrackerParserState  *state,
                g_hash_table_unref (repeated);
        }
 
+       snippet = get_error_snippet (parser, state);
+       g_string_append_printf (str, ":\n%s", snippet);
+       g_free (snippet);
+
        g_set_error (error,
                     TRACKER_SPARQL_ERROR,
                     TRACKER_SPARQL_ERROR_PARSE,
@@ -740,7 +786,7 @@ tracker_grammar_parser_apply (TrackerGrammarParser      *parser,
        state.node_tree->root = tracker_parser_state_transact_match (&state);
 
        if (!tracker_grammar_parser_read (parser, &state)) {
-               tracker_parser_state_propagate_error (&state, error);
+               tracker_parser_state_propagate_error (parser, &state, error);
                tracker_node_tree_free (state.node_tree);
                g_free (state.rule_states.rules);
                return NULL;