json.c/strings.c - Add a couple of utility functions

Added 'ast_json_object_string_get' to the JSON wrapper in order to make it a
little easier to retrieve a string field from the JSON object.

Also added an 'ast_strings_equal' function that safely checks (checks for NULLs)
for equality between two strings.

Change-Id: I26f0a16d61537505eb41b4b05ef2e6d67fc2541b

include/asterisk/json.h

@@ -561,6 +561,17 @@ size_t ast_json_object_size(struct ast_json *object);
  */
 struct ast_json *ast_json_object_get(struct ast_json *object, const char *key);
 
+/*!
+ * \brief Get a string field from a JSON object.
+ * \since 16.3.0
+ *
+ * \param object JSON object.
+ * \param key Key of string field to look up.
+ * \return String value of given \a key.
+ * \return \c NULL on error, or key value is not a string.
+ */
+#define ast_json_object_string_get(object, key) ast_json_string_get(ast_json_object_get(object, key))
+
 /*!
  * \brief Set a field in a JSON object.
  * \since 12.0.0

include/asterisk/strings.h

@@ -1297,6 +1297,21 @@ void ast_str_container_remove(struct ao2_container *str_container, const char *r
  */
 char *ast_generate_random_string(char *buf, size_t size);
 
+/*!
+ * \brief Compare strings for equality checking for NULL.
+ * \since 16.3.0
+ *
+ * This function considers NULL values as non-strings, thus a false condition.
+ * This means that it will return false if one, or both of the given values are
+ * NULL (i.e. two NULLs are not equal strings).
+ *
+ * \param str1 The string to compare to str2
+ * \param str2 The string to compare to str1
+ *
+ * \return true if valid strings and equal, false otherwise.
+ */
+int ast_strings_equal(const char *str1, const char *str2);
+
 /*!
  * \brief Compares 2 strings using realtime-style operators
  * \since 13.9.0

main/strings.c

@@ -236,6 +236,15 @@ char *ast_generate_random_string(char *buf, size_t size)
 	return buf;
 }
 
+int ast_strings_equal(const char *str1, const char *str2)
+{
+	if (!str1 || !str2) {
+		return 0;
+	}
+
+	return str1 == str2 || !strcmp(str1, str2);
+}
+
 int ast_strings_match(const char *left, const char *op, const char *right)
 {
 	char *internal_op = (char *)op;