lightningd: unescape log message strings received from plugins

Plugins can send arbitrary strings in "log" notifications. Lightningd
was logging these strings without unescaping them, leading to ugly
backslashes in the log. Unescape the strings before logging them.

Changelog-Fixed: Log messages from plugins no longer contain an extra level of escaping.

Author: whitslack

lightningd/plugin.c

@@ -3,6 +3,7 @@
 #include <ccan/ccan/tal/grab_file/grab_file.h>
 #include <ccan/crc32c/crc32c.h>
 #include <ccan/io/io.h>
+#include <ccan/json_escape/json_escape.h>
 #include <ccan/mem/mem.h>
 #include <ccan/opt/opt.h>
 #include <ccan/pipecmd/pipecmd.h>
@@ -487,6 +488,7 @@ static const char *plugin_log_handle(struct plugin *plugin,
 {
 	const jsmntok_t *msgtok, *leveltok;
 	enum log_level level;
+	const char *msg;
 	bool call_notifier;
 	msgtok = json_get_member(plugin->buffer, paramstok, "message");
 	leveltok = json_get_member(plugin->buffer, paramstok, "level");
@@ -511,10 +513,15 @@ static const char *plugin_log_handle(struct plugin *plugin,
 			       json_tok_full(plugin->buffer, leveltok));
 	}
 
+	msg = json_escape_unescape_len(tmpctx, plugin->buffer + msgtok->start,
+				       msgtok->end - msgtok->start);
+	if (!msg)
+		return tal_fmt(plugin, "Log notification from plugin has a \"message\" "
+			       "string containing an invalid escape sequence.");
+
 	call_notifier = (level == LOG_BROKEN || level == LOG_UNUSUAL)? true : false;
 	/* FIXME: Let plugin specify node_id? */
-	log_(plugin->log, level, NULL, call_notifier, "%.*s", msgtok->end - msgtok->start,
-	     plugin->buffer + msgtok->start);
+	log_(plugin->log, level, NULL, call_notifier, "%s", msg);
 	return NULL;
 }