[bug-diffutils] bug#21715: feat req: an option to skip directory inode comparison

[Aaron Davies]

i have a situation where diff -r of two directories fails to report differences 
that diff of two files in those directories shows

i haven't dug into the code yet, but the directories involved are on an NFS 
mount of a netapp filer, one is the copy-on-write snapshot counterpart of the 
other, they show up as the same filesystem, and are the same inode

so my guess is that there's something in the recursive traversal that says that 
any two directories that are on the same fs and are the same inode must have 
identical descendent contents and can be skipped completely

this fs probably violates POSIX by doing this, but there's no easy way around 
it[1], so it would be very useful if diff had a command-line option to turn 
that optimization off

i've heard anecdotal reports of other filesystems that cause the same problem

here's a sample session:

$ mkdir foo
$ echo baz >foo/bar
$ # wait until the next snapshot is created
$ echo quux >foo/bar
$ diff -r .snapshot/nightly.0/foo foo
$ diff .snapshot/nightly.0/foo/bar foo/bar
1c1
< baz
---
> quux
$ stat -c %i .snapshot/nightly.0/foo foo
69403847
69403847
$ 
-- 
Aaron Davies
address@hidden

[1] when i ran into this, i ended up using something like diff <(find 
.snapshot/nightly.0/foo -type f|sort|xargs cat) <(find foo -type f|sort|xargs 
cat) to do the comparison -- sufficient for my needs at the time, but cleaning 
it up for general purpose use would essentially be rewriting the -r part of 
diff from scratch