GIT: unionfs2-2.6.27.y: Unionfs: document reasons for opaque directories

[Erez Zadok]

commit a19978fd5e4a0de11d349e2fb2b2f9f9e0022071
Author: Erez Zadok <ezk at cs.sunysb.edu>
Date:   Sun Mar 23 01:16:33 2008 -0400

    Unionfs: document reasons for opaque directories
    
    Signed-off-by: Erez Zadok <ezk at cs.sunysb.edu>

diff --git a/Documentation/filesystems/unionfs/concepts.txt b/Documentation/filesystems/unionfs/concepts.txt
index 0bc1a19..b853788 100644
--- a/Documentation/filesystems/unionfs/concepts.txt
+++ b/Documentation/filesystems/unionfs/concepts.txt
@@ -52,6 +52,42 @@ Later, when Unionfs traverses branches (due to lookup or readdir), it
 eliminate 'foo' from the namespace (as well as the whiteout itself.)
 
 
+Opaque Directories:
+===================
+
+Assume we have a unionfs mount comprising of two branches.  Branch 0 is
+empty; branch 1 has the directory /a and file /a/f.  Let's say we mount a
+union of branch 0 as read-write and branch 1 as read-only.  Now, let's say
+we try to perform the following operation in the union:
+
+	rm -fr a
+
+Because branch 1 is not writable, we cannot physically remove the file /a/f
+or the directory /a.  So instead, we will create a whiteout in branch 0
+named /.wh.a, masking out the name "a" from branch 1.  Next, let's say we
+try to create a directory named "a" as follows:
+
+	mkdir a
+
+Because we have a whiteout for "a" already, Unionfs behaves as if "a"
+doesn't exist, and thus will delete the whiteout and replace it with an
+actual directory named "a".
+
+The problem now is that if you try to "ls" in the union, Unionfs will
+perform is normal directory name unification, for *all* directories named
+"a" in all branches.  This will cause the file /a/f from branch 1 to
+re-appear in the union's namespace, which violates Unix semantics.
+
+To avoid this problem, we have a different form of whiteouts for
+directories, called "opaque directories" (same as BSD Union Mount does).
+Whenever we replace a whiteout with a directory, that directory is marked as
+opaque.  In Unionfs 2.x, it means that we create a file named
+/a/.wh.__dir_opaque in branch 0, after having created directory /a there.
+When unionfs notices that a directory is opaque, it stops all namespace
+operations (including merging readdir contents) at that opaque directory.
+This prevents re-exposing names from masked out directories.
+
+
 Duplicate Elimination:
 ======================