lib-index: Track .log.2 rotation time in index header This avoids unnecessarily stat()ing the file. Also it's a bit better since it's tracking the actual rotation time, not the mtime of what the .log file happened to have at the time of rotation. The initial rotation timestamp is written only to the dovecot.index header without going through dovecot.index.log. This works, because the dovecot.index is written practically always after a log rotation. For the rare cases when it doesn't happen, the dovecot.index.log.2 just gets deleted later after the next log rotation.

--- a/src/lib-index/mail-index-private.h	Tue Jul 11 15:33:56 2017 +0300
+++ b/src/lib-index/mail-index-private.h	Tue Jul 11 15:35:16 2017 +0300
@@ -174,6 +174,7 @@
 	uoff_t log_rotate_min_size, log_rotate_max_size;
 	unsigned int log_rotate_min_created_ago_secs;
 	unsigned int log_rotate_log2_stale_secs;
+	uint32_t pending_log2_rotate_time;
 
 	pool_t extension_pool;
 	ARRAY(struct mail_index_registered_ext) extensions;