mtab: Use mount IDs instead of dev_t for bfs_fstype()

This fixes -fstype on btrfs subvolumes, which previously reported
"unknown" due to their dev_t being potentially different from the mount
point.

Link: https://savannah.gnu.org/bugs/?50859
Link: https://lwn.net/Articles/866582/

2 files changed, 29 insertions, 0 deletions

diff --git a/tests/gnu/fstype_btrfs_subvol.out b/tests/gnu/fstype_btrfs_subvol.out
new file mode 100644
index 0000000..8871fb9
--- /dev/null
+++ b/tests/gnu/fstype_btrfs_subvol.out
@@ -0,0 +1,4 @@
+mnt
+mnt/file
+mnt/subvol
+mnt/subvol/file
diff --git a/tests/gnu/fstype_btrfs_subvol.sh b/tests/gnu/fstype_btrfs_subvol.sh
new file mode 100644
index 0000000..71df45c
--- /dev/null
+++ b/tests/gnu/fstype_btrfs_subvol.sh
@@ -0,0 +1,25 @@
+# Test that -fstype works in btrfs subvolumes
+
+command -v btrfs &>/dev/null || skip
+
+cd "$TEST"
+
+# Make a btrfs filesystem image
+truncate -s128M img
+mkfs.btrfs img >&2
+
+# Mount it
+mkdir mnt
+bfs_sudo mount img mnt || skip
+defer bfs_sudo umount mnt
+
+# Make it owned by us
+bfs_sudo chown "$(id -u):$(id -g)" mnt
+
+# Create a subvolume inside it
+btrfs subvolume create mnt/subvol >&2
+
+# Make a file in and outside the subvolume
+"$XTOUCH" mnt/file mnt/subvol/file
+
+bfs_diff mnt -fstype btrfs -print -o -printf '%p %F\n'