Fix hard link issues if we have several rw branches. Debian Bug#509516.

COW functions always use the highest level rw branch, but
if we have several rw branches and the file to link from
is located on a lower rw branch, cow functions decided to use
the highest rw branch to link the file to. However, if those
branches are different filesystems, that is not allowed

How to reproduce:

ro/destdir
rw_low/fromdir/testfile
/tmp/rw_hi/ (/tmp is a different filesystem)
union/

unionfs-fuse -d -o cow /tmp/rw_hi=RW:rw_low=RW:ro=RO union/

link union/fromdir/testfile union/destdir/testfile


==> link: cannot create link `union/destdir/testfile' to `union/fromdir/testfile': Invalid cross-device link


This patch fixed that issue.

3 files changed, 20 insertions, 4 deletions

diff --git a/src/findbranch.c b/src/findbranch.c
index 145edf6..9581c3a 100644
--- a/src/findbranch.c
+++ b/src/findbranch.c
@@ -107,8 +107,10 @@ int find_rorw_branch(const char *path) {
 /**
  * Find a writable branch. If file does not exist, we check for
  * the parent directory.
+ * @path 	- the path to find or to copy (with last element cut off)
+ * @ rw_hint	- the rw branch to copy to, set to -1 to autodetect it
  */
-int find_rw_branch_cutlast(const char *path) {
+int __find_rw_branch_cutlast(const char *path, int rw_hint) {
 	DBG_IN();
 
 	int branch = find_rw_branch_cow(path);
@@ -136,8 +138,12 @@ int find_rw_branch_cutlast(const char *path) {
 		goto out;
 	}
 
+	int branch_rw;
 	// since it is a directory, any rw-branch is fine
-	int branch_rw = find_lowest_rw_branch(uopt.nbranches);
+	if (rw_hint == -1)
+		branch_rw = find_lowest_rw_branch(uopt.nbranches);
+	else
+		branch_rw = rw_hint;
 
 	// no writable branch found, we must return an error
 	if (branch_rw < 0) {
@@ -155,6 +161,14 @@ out:
 }
 
 /**
+ * Call __find_rw_branch_cutlast()
+ */
+int find_rw_branch_cutlast(const char *path) {
+	int rw_hint = -1; // autodetect rw_branch
+	return __find_rw_branch_cutlast(path, rw_hint);
+}
+
+/**
  * copy-on-write
  * Find path in a union branch and if this branch is read-only, 
  * copy the file to a read-write branch.
diff --git a/src/findbranch.h b/src/findbranch.h
index 6378d86..56cfc08 100644
--- a/src/findbranch.h
+++ b/src/findbranch.h
@@ -15,6 +15,7 @@ typedef enum searchflag {
 int find_rorw_branch(const char *path);
 int find_lowest_rw_branch(int branch_ro);
 int find_rw_branch_cutlast(const char *path);
+int __find_rw_branch_cutlast(const char *path, int rw_hint);
 int find_rw_branch_cow(const char *path);
 
 #endif
diff --git a/src/unionfs.c b/src/unionfs.c
index cd58a83..091f00d 100644
--- a/src/unionfs.c
+++ b/src/unionfs.c
@@ -241,10 +241,11 @@ static int unionfs_link(const char *from, const char *to) {
 	int i = find_rw_branch_cow(from);
 	if (i == -1) return -errno;
 
-	// FIXME, we actually MUST COW to i
-	int j = find_rw_branch_cutlast(to);
+	int j = __find_rw_branch_cutlast(to, i);
 	if (j == -1) return -errno;
 
+	DBG("from branch: %d to branch: %d\n", i, j);
+
 	char f[PATHLEN_MAX], t[PATHLEN_MAX];
 	if (BUILD_PATH(f, uopt.branches[i].path, from)) return -ENAMETOOLONG;
 	if (BUILD_PATH(t, uopt.branches[j].path, to)) return -ENAMETOOLONG;