json-parser: Support loading files via memory mapping

Add a new `json_parser_load_from_mapped_file()` to load JSON from
files via memory mapping. It’s otherwise similar to
`json_parser_load_from_file()`. It’s in the right position to be able
to memory map the file it’s reading from: it reads the input once
before building a `JsonNode` structure to represent it, doesn’t write
to the file, and often deals with large input files.

This should speed things up slightly due to reducing time spent
allocating a large chunk of heap memory to load the file into, if a
caller can support that.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/json-glib/json-parser.c b/json-glib/json-parser.c
index a90bbd8..3d77975 100644
--- a/json-glib/json-parser.c
+++ b/json-glib/json-parser.c
@@ -1089,6 +1089,9 @@ json_parser_load (JsonParser   *parser,
  * Loads a JSON stream from the content of @filename and parses it. See
  * json_parser_load_from_data().
  *
+ * If the file is large or shared between processes,
+ * json_parser_load_from_mapped_file() may be a more efficient way to load it.
+ *
  * Return value: %TRUE if the file was successfully loaded and parsed.
  *   In case of error, @error is set accordingly and %FALSE is returned
  */
@@ -1132,6 +1135,61 @@ json_parser_load_from_file (JsonParser   *parser,
 }
 
 /**
+ * json_parser_load_from_mapped_file:
+ * @parser: a #JsonParser
+ * @filename: the path for the file to parse
+ * @error: return location for a #GError, or %NULL
+ *
+ * Loads a JSON stream from the content of @filename and parses it. Unlike
+ * json_parser_load_from_file(), @filename will be memory mapped as read-only
+ * and parsed. @filename will be unmapped before this function returns.
+ *
+ * If mapping or reading the file fails, a %G_FILE_ERROR will be returned.
+ *
+ * Return value: %TRUE if the file was successfully loaded and parsed.
+ *   In case of error, @error is set accordingly and %FALSE is returned
+ * Since: 1.6
+ */
+gboolean
+json_parser_load_from_mapped_file (JsonParser   *parser,
+                                   const gchar  *filename,
+                                   GError      **error)
+{
+  JsonParserPrivate *priv;
+  GError *internal_error = NULL;
+  gboolean retval = TRUE;
+  GMappedFile *mapped_file = NULL;
+
+  g_return_val_if_fail (JSON_IS_PARSER (parser), FALSE);
+  g_return_val_if_fail (filename != NULL, FALSE);
+
+  priv = parser->priv;
+
+  mapped_file = g_mapped_file_new (filename, FALSE, &internal_error);
+  if (mapped_file == NULL)
+    {
+      g_propagate_error (error, internal_error);
+      return FALSE;
+    }
+
+  g_free (priv->filename);
+
+  priv->is_filename = TRUE;
+  priv->filename = g_strdup (filename);
+
+  if (!json_parser_load (parser, g_mapped_file_get_contents (mapped_file),
+                         g_mapped_file_get_length (mapped_file), &internal_error))
+    {
+      g_propagate_error (error, internal_error);
+      retval = FALSE;
+    }
+
+  g_clear_pointer (&mapped_file, g_mapped_file_unref);
+
+  return retval;
+}
+
+/**
  * json_parser_load_from_data:
  * @parser: a #JsonParser
  * @data: the buffer to parse