Re: [PATCH] GIT_ONE_FILESYSTEM: flip the default to stop at filesystem boundaries

Junio C Hamano [off-list ref] writes:

   I wonder if "git add" and friends should also notice it and warn.  If
   you have more than one values of ce->ce_dev in the index, it means that
   the working tree spans more than one filesystem and from a subdirectory
   with an entry that has a ce->ce_dev different from the value for a path
   at the top of the work tree, you will not be able to discover the top
   of the tree without GIT_ONE_FILESYSTEM set to true.  A likely scenario
   for this to happen would be:

    (1) You have a tarball of some sort; you extract it $there;

	$ mkdir $there && cd $there
        $ tar xf /var/tmp/tarball.tar

    (2) You notice the filesystem lacks enough free space, and move some
        part (say "images/") to a separate filesystem, and bind-mount;

        $ mv images $another/. && rm -fr images && mkdir images
        $ mount --bind $another/images images

    (3) You add everything to start the project;

        $ git init && git add .

        Up to this point it would work (you are at the top of the working
        tree).  And this is the point we _could_ notice and warn that you
        will have trouble in step (4).

    (4) Go down to a subdirectory and start futzing;

	$ cd images && gimp naughty.jpg && git add -u

And this adds such a check.

 read-cache.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index f1f789b..486bb2a 100644
--- a/read-cache.c
+++ b/read-cache.c

@@ -1550,6 +1550,7 @@ int write_index(struct index_state *istate, int newfd)
 	struct cache_entry **cache = istate->cache;
 	int entries = istate->cache_nr;
 	struct stat st;
+	int more_than_one_dev;
 
 	for (i = removed = extended = 0; i < entries; i++) {
 		if (cache[i]->ce_flags & CE_REMOVE)

@@ -1572,6 +1573,7 @@ int write_index(struct index_state *istate, int newfd)
 	if (ce_write(&c, newfd, &hdr, sizeof(hdr)) < 0)
 		return -1;
 
+	more_than_one_dev = 0;
 	for (i = 0; i < entries; i++) {
 		struct cache_entry *ce = cache[i];
 		if (ce->ce_flags & CE_REMOVE)

@@ -1580,8 +1582,15 @@ int write_index(struct index_state *istate, int newfd)
 			ce_smudge_racily_clean_entry(ce);
 		if (ce_write_entry(&c, newfd, ce) < 0)
 			return -1;
+		if (i && ce->ce_dev != cache[0]->ce_dev)
+			more_than_one_dev = 1;
 	}
 
+	if (more_than_one_dev &&
+	    !git_env_bool("GIT_DISCOVERY_ACROSS_FILESYSTEM", 0))
+		warning("working tree spans across filesystems but "
+			"GIT_DISCOVERY_ACROSS_FILESYSTEM is not set.");
+
 	/* Write extension data here */
 	if (istate->cache_tree) {
 		struct strbuf sb = STRBUF_INIT;