Re: [PATCH] new: Don't scan unchanged directories with no sub-directories

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+Date: Thu, 24 Oct 2013 17:08:37 -0400\r
+From: Austin Clements <amdragon@MIT.EDU>\r
+To: notmuch@notmuchmail.org\r
+Subject: Re: [PATCH] new: Don't scan unchanged directories with no\r
+       sub-directories\r
+Message-ID: <20131024210837.GH20337@mit.edu>\r
+References: <1382646822-24556-1-git-send-email-amdragon@mit.edu>\r
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+\r
+There might be a problem with this patch.  Directory entries that are\r
+*symlinks* to other directories do not increase the containing\r
+directory's link count, but we do count them as directories in\r
+add_files pass 1 and traverse in to them.  Hence, if you had a\r
+directory that contained no sub-directories, but did contain symlinks\r
+to other directories, we would fail to notice changes in the symlinked\r
+directories.\r
+\r
+We could check if the database thinks there are sub-directories and\r
+only bail early if the directory is unchanged and *both* the file\r
+system and the database think there are no sub-directories.\r
+\r
+Quoth myself on Oct 24 at  4:33 pm:\r
+> This can substantially reduce the cost of notmuch new in some\r
+> situations, such as when the file system cache is cold or when the\r
+> Maildir is on NFS.\r
+> ---\r
+>  notmuch-new.c | 20 ++++++++++++++++++++\r
+>  1 file changed, 20 insertions(+)\r
+> \r
+> diff --git a/notmuch-new.c b/notmuch-new.c\r
+> index faa33f1..364c73a 100644\r
+> --- a/notmuch-new.c\r
+> +++ b/notmuch-new.c\r
+> @@ -323,6 +323,26 @@ add_files (notmuch_database_t *notmuch,\r
+>      }\r
+>      db_mtime = directory ? notmuch_directory_get_mtime (directory) : 0;\r
+>  \r
+> +    /* If the directory is unchanged from our last scan and has no\r
+> +     * sub-directories, then return without scanning it at all.  In\r
+> +     * some situations, skipping the scan can substantially reduce the\r
+> +     * cost of notmuch new, especially since the huge numbers of files\r
+> +     * in Maildirs make scans expensive, but all files live in leaf\r
+> +     * directories.\r
+> +     *\r
+> +     * To check for sub-directories, we borrow a trick from find,\r
+> +     * kpathsea, and many other UNIX tools: since a directory's link\r
+> +     * count is the number of sub-directories (specifically, their\r
+> +     * '..' entries) plus 2 (the link from the parent and the link for\r
+> +     * '.').  This check is safe even on weird file systems, since\r
+> +     * file systems that can't compute this will return 0 or 1.  This\r
+> +     * is safe even on *really* weird file systems like HFS+ that\r
+> +     * mistakenly return the total number of directory entries, since\r
+> +     * that only inflates the count beyond 2.\r
+> +     */\r
+> +    if (directory && fs_mtime == db_mtime && st.st_nlink == 2)\r
+> +    goto DONE;\r
+> +\r
+>      /* If the database knows about this directory, then we sort based\r
+>       * on strcmp to match the database sorting. Otherwise, we can do\r
+>       * inode-based sorting for faster filesystem operation. */\r