mt/dir-iterator-updates, was Re: What's cooking in git.git (Jul 2019, #01; Wed, 3)

From

Johannes Schindelin <johannes.schindelin@gmx.de>

Date

Jul 4, 2019, 10:03 UTC

Message-ID

<nycvar.QRO.7.76.6.1907041136530.44@tvgsbejvaqbjf.bet>

In-Reply-To

<xmqqpnmqzrbg.fsf@gitster-ct.c.googlers.com>

Hi Junio,

On Wed, 3 Jul 2019, Junio C Hamano wrote:

> * mt/dir-iterator-updates (2019-06-25) 10 commits
>  - clone: replace strcmp by fspathcmp
>  - clone: use dir-iterator to avoid explicit dir traversal
>  - clone: extract function from copy_or_link_directory
>  - clone: copy hidden paths at local clone
>  - dir-iterator: add flags parameter to dir_iterator_begin
>  - dir-iterator: refactor state machine model
>  - dir-iterator: use warning_errno when possible
>  - dir-iterator: add tests for dir-iterator API
>  - clone: better handle symlinked files at .git/objects/
>  - clone: test for our behavior on odd objects/* content
>
>  Adjust the dir-iterator API and apply it to the local clone
>  optimization codepath.
>
>  Is this ready for 'next'?

-- snip --

diff --git a/dir-iterator.c b/dir-iterator.c
index 52db87bdc99f..85cd04b7b571 100644
--- a/dir-iterator.c
+++ b/dir-iterator.c
@@ -8,6 +8,7 @@ struct dir_iterator_level {

 	/* The inode number of this level's directory. */
 	ino_t ino;
+	dev_t dev;

 	/*
 	 * The length of the directory part of path at this level
@@ -63,6 +64,7 @@ static int push_level(struct dir_iterator_int *iter)
 		strbuf_addch(&iter->base.path, '/');
 	level->prefix_len = iter->base.path.len;
 	level->ino = iter->base.st.st_ino;
+	level->dev = iter->base.st.st_dev;

 	level->dir = opendir(iter->base.path.buf);
 	if (!level->dir) {
@@ -138,11 +140,14 @@ static int find_recursive_symlinks(struct dir_iterator_int *iter)
 	int i;

 	if (!(iter->flags & DIR_ITERATOR_FOLLOW_SYMLINKS) ||
-	    !S_ISDIR(iter->base.st.st_mode))
+	    !S_ISDIR(iter->base.st.st_mode) ||
+	    /* On Windows, st_ino is always set to 0 */
+	    !iter->base.st.st_ino)
 		return 0;

 	for (i = 0; i < iter->levels_nr; ++i)
-		if (iter->base.st.st_ino == iter->levels[i].ino)
+		if (iter->base.st.st_ino == iter->levels[i].ino &&
+		    iter->base.st.st_dev == iter->levels[i].dev)
 			return 1;
 	return 0;
 }
-- snap --

Duy had also suggested to guard part of this using `USE_STDEV`, but as
Matheus figured out that would not make sense, as the `USE_STDEV` flag
really is meant to work around issues with network filesystems where
`st_dev` can be unreliable.

However, in the meantime I thought about this a bit more and I remembered
how this is done elsewhere: I saw many recursive symlink resolvers that
just have an arbitrary cut-off after following, say, 32 links.

In fact, Git itself already has this in abspath.c:

	/* We allow "recursive" symbolic links. Only within reason, though. */
	#ifndef MAXSYMLINKS
	#define MAXSYMLINKS 32
	#endif

But then, the patch in question uses `stat()` instead of `lstat()`, so we
would not have any way to count the number of symbolic links we followed.

Do we _have_ to, though? At some stage the path we come up with is beyond
`PATH_MAX` and we can stop right then and there.

Besides, the way `find_recursive_symlinks()` is implemented adds quadratic
behavior.

So I would like to submit the idea of simplifying the code drastically,
by skipping the `find_recursive_symlinks()` function altogether.

This would solve another issue I have with that function, anyway: The name
suggests, to me at least, that we follow symlinks recursively. It does
not. I think that could have been addressed by using the adjective
"circular" instead of "recursive". But I also think there are enough
reasons to do away with this function in the first place.

Ciao,
Dscho