snapshot: enforce keeping same backing store type (v2)

src/lxc/bdev.c

@@ -476,6 +476,7 @@ static const struct bdev_ops dir_ops = {
 	.clone_paths = &dir_clonepaths,
 	.destroy = &dir_destroy,
 	.create = &dir_create,
+	.can_snapshot = false,
 };
 
 
@@ -733,6 +734,7 @@ static const struct bdev_ops zfs_ops = {
 	.clone_paths = &zfs_clonepaths,
 	.destroy = &zfs_destroy,
 	.create = &zfs_create,
+	.can_snapshot = true,
 };
 
 //
@@ -1132,6 +1134,7 @@ static const struct bdev_ops lvm_ops = {
 	.clone_paths = &lvm_clonepaths,
 	.destroy = &lvm_destroy,
 	.create = &lvm_create,
+	.can_snapshot = true,
 };
 
 //
@@ -1429,6 +1432,7 @@ static const struct bdev_ops btrfs_ops = {
 	.clone_paths = &btrfs_clonepaths,
 	.destroy = &btrfs_destroy,
 	.create = &btrfs_create,
+	.can_snapshot = true,
 };
 
 //
@@ -1699,6 +1703,7 @@ static const struct bdev_ops loop_ops = {
 	.clone_paths = &loop_clonepaths,
 	.destroy = &loop_destroy,
 	.create = &loop_create,
+	.can_snapshot = false,
 };
 
 //
@@ -1932,6 +1937,7 @@ static const struct bdev_ops overlayfs_ops = {
 	.clone_paths = &overlayfs_clonepaths,
 	.destroy = &overlayfs_destroy,
 	.create = &overlayfs_create,
+	.can_snapshot = true,
 };
 
 static const struct bdev_type bdevs[] = {
@@ -2012,11 +2018,14 @@ struct bdev *bdev_init(const char *src, const char *dst, const char *data)
  */
 struct bdev *bdev_copy(const char *src, const char *oldname, const char *cname,
 			const char *oldpath, const char *lxcpath, const char *bdevtype,
-			int snap, const char *bdevdata, unsigned long newsize,
+			int flags, const char *bdevdata, unsigned long newsize,
 			int *needs_rdep)
 {
 	struct bdev *orig, *new;
 	pid_t pid;
+	bool snap = flags & LXC_CLONE_SNAPSHOT;
+	bool maybe_snap = flags & LXC_CLONE_MAYBE_SNAPSHOT;
+	bool keepbdevtype = flags & LXC_CLONE_KEEPBDEVTYPE;
 
 	/* if the container name doesn't show up in the rootfs path, then
 	 * we don't know how to come up with a new name
@@ -2050,9 +2059,17 @@ struct bdev *bdev_copy(const char *src, const char *oldname, const char *cname,
 	}
 
 	/*
+	 * special case for snapshot - if caller requested maybe_snapshot and
+	 * keepbdevtype and backing store is directory, then proceed with a copy
+	 * clone rather than returning error
+	 */
+	if (maybe_snap && keepbdevtype && !bdevtype && !orig->ops->can_snapshot)
+		snap = false;
+
+	/*
 	 * If newtype is NULL and snapshot is set, then use overlayfs
 	 */
-	if (!bdevtype && snap && strcmp(orig->type , "dir") == 0)
+	if (!bdevtype && !keepbdevtype && snap && strcmp(orig->type , "dir") == 0)
 		bdevtype = "overlayfs";
 
 	*needs_rdep = 0;

src/lxc/bdev.h

@@ -62,6 +62,7 @@ struct bdev_ops {
 	int (*clone_paths)(struct bdev *orig, struct bdev *new, const char *oldname,
 			const char *cname, const char *oldpath, const char *lxcpath,
 			int snap, unsigned long newsize);
+	bool can_snapshot;
 };
 
 /*
@@ -99,7 +100,7 @@ struct bdev *bdev_init(const char *src, const char *dst, const char *data);
 
 struct bdev *bdev_copy(const char *src, const char *oldname, const char *cname,
 			const char *oldpath, const char *lxcpath, const char *bdevtype,
-			int snap, const char *bdevdata, unsigned long newsize,
+			int flags, const char *bdevdata, unsigned long newsize,
 			int *needs_rdep);
 struct bdev *bdev_create(const char *dest, const char *type,
 			const char *cname, struct bdev_specs *specs);

src/lxc/lxccontainer.c

@@ -2282,7 +2282,7 @@ static int copy_storage(struct lxc_container *c0, struct lxc_container *c,
 	int need_rdep;
 
 	bdev = bdev_copy(c0->lxc_conf->rootfs.path, c0->name, c->name,
-			c0->config_path, c->config_path, newtype, !!(flags & LXC_CLONE_SNAPSHOT),
+			c0->config_path, c->config_path, newtype, flags,
 			bdevdata, newsize, &need_rdep);
 	if (!bdev) {
 		ERROR("Error copying storage");
@@ -2624,7 +2624,12 @@ static int lxcapi_snapshot(struct lxc_container *c, const char *commentfile)
 	if (ret < 0 || ret >= 20)
 		return -1;
 
-	flags = LXC_CLONE_SNAPSHOT | LXC_CLONE_KEEPMACADDR | LXC_CLONE_KEEPNAME;
+	/*
+	 * We pass LXC_CLONE_SNAPSHOT to make sure that a rdepends file entry is
+	 * created in the original container
+	 */
+	flags = LXC_CLONE_SNAPSHOT | LXC_CLONE_KEEPMACADDR | LXC_CLONE_KEEPNAME |
+		LXC_CLONE_KEEPBDEVTYPE | LXC_CLONE_MAYBE_SNAPSHOT;
 	c2 = c->clone(c, newname, snappath, flags, NULL, NULL, 0, NULL);
 	if (!c2) {
 		ERROR("clone of %s:%s failed\n", c->config_path, c->name);

src/lxc/lxccontainer.h

@@ -36,7 +36,9 @@ extern "C" {
 #define LXC_CLONE_KEEPNAME        (1 << 0) /*!< Do not edit the rootfs to change the hostname */
 #define LXC_CLONE_KEEPMACADDR     (1 << 1) /*!< Do not change the MAC address on network interfaces */
 #define LXC_CLONE_SNAPSHOT        (1 << 2) /*!< Snapshot the original filesystem(s) */
-#define LXC_CLONE_MAXFLAGS        (1 << 3) /*!< Number of \c LXC_CLONE_* flags */
+#define LXC_CLONE_KEEPBDEVTYPE    (1 << 3) /*!< Use the same bdev type */
+#define LXC_CLONE_MAYBE_SNAPSHOT  (1 << 4) /*!< Snapshot only if bdev supports it, else copy */
+#define LXC_CLONE_MAXFLAGS        (1 << 5) /*!< Number of \c LXC_CLONE_* flags */
 #define LXC_CREATE_QUIET          (1 << 0) /*!< Redirect \c stdin to \c /dev/zero and \c stdout and \c stderr to \c /dev/null */
 #define LXC_CREATE_MAXFLAGS       (1 << 1) /*!< Number of \c LXC_CREATE* flags */