[PATCH v2] new: Don't scan unchanged directories with no sub-directories

[Tomi Ollila]

On Fri, Oct 25 2013, Austin Clements <amdragon at MIT.EDU> wrote:

> This can substantially reduce the cost of notmuch new in some
> situations, such as when the file system cache is cold or when the
> Maildir is on NFS.
> ---

LGTM. The creation and destruction of child directories happens
only if there are symlinks to directories in otherwise leaf directories.

Tomi

>
> This should fix the problem with directories containing symlinks to
> other directories, but no actual sub-directories.
>
>  notmuch-new.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/notmuch-new.c b/notmuch-new.c
> index faa33f1..ba05cb4 100644
> --- a/notmuch-new.c
> +++ b/notmuch-new.c
> @@ -323,6 +323,35 @@ add_files (notmuch_database_t *notmuch,
>      }
>      db_mtime = directory ? notmuch_directory_get_mtime (directory) : 0;
>  
> +    /* If the directory is unchanged from our last scan and has no
> +     * sub-directories, then return without scanning it at all.  In
> +     * some situations, skipping the scan can substantially reduce the
> +     * cost of notmuch new, especially since the huge numbers of files
> +     * in Maildirs make scans expensive, but all files live in leaf
> +     * directories.
> +     *
> +     * To check for sub-directories, we borrow a trick from find,
> +     * kpathsea, and many other UNIX tools: since a directory's link
> +     * count is the number of sub-directories (specifically, their
> +     * '..' entries) plus 2 (the link from the parent and the link for
> +     * '.').  This check is safe even on weird file systems, since
> +     * file systems that can't compute this will return 0 or 1.  This
> +     * is safe even on *really* weird file systems like HFS+ that
> +     * mistakenly return the total number of directory entries, since
> +     * that only inflates the count beyond 2.
> +     */
> +    if (directory && fs_mtime == db_mtime && st.st_nlink == 2) {
> +	/* There's one catch: pass 1 below considers symlinks to
> +	 * directories to be directories, but these don't increase the
> +	 * file system link count.  So, only bail early if the
> +	 * database agrees that there are no sub-directories. */
> +	db_subdirs = notmuch_directory_get_child_directories (directory);
> +	if (!notmuch_filenames_valid (db_subdirs))
> +	    goto DONE;
> +	notmuch_filenames_destroy (db_subdirs);
> +	db_subdirs = NULL;
> +    }
> +
>      /* If the database knows about this directory, then we sort based
>       * on strcmp to match the database sorting. Otherwise, we can do
>       * inode-based sorting for faster filesystem operation. */
> -- 
> 1.8.4.rc3
>
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