validate JSON before loading

This commit also introduces slurp() which reads a file in its entirety. Using
this function instead of doing IO in the functions in load_layout.c again and
again makes the code cleaner (fixing at least two memory leaks) and avoids
re-reading the same file 3 times.

related to #2755

include/load_layout.h

@@ -31,6 +31,12 @@ typedef enum {
  * determine whether the file contains workspaces or regular containers, which
  * is important to know when deciding where (and how) to append the contents.
  * */
-json_content_t json_determine_content(const char *filename);
+json_content_t json_determine_content(const char *buf, const size_t len);
 
-void tree_append_json(Con *con, const char *filename, char **errormsg);
+/**
+ * Returns true if the provided JSON could be parsed by yajl.
+ *
+ */
+bool json_validate(const char *buf, const size_t len);
+
+void tree_append_json(Con *con, const char *buf, const size_t len, char **errormsg);

include/util.h

@@ -164,3 +164,11 @@ void kill_nagbar(pid_t *nagbar_pid, bool wait_for_it);
  * if the number could be parsed.
  */
 bool parse_long(const char *str, long *out, int base);
+
+/**
+ * Slurp reads path in its entirety into buf, returning the length of the file
+ * or -1 if the file could not be read. buf is set to a buffer of appropriate
+ * size, or NULL if -1 is returned.
+ *
+ */
+ssize_t slurp(const char *path, char **buf);