From 0dccdae4510ff5603247be871e64a6119647ea2a Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Wed, 5 Feb 2025 16:53:27 -0500
Subject: 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/
---
 src/mtab.c                        |  4 ++--
 tests/gnu/fstype_btrfs_subvol.out |  4 ++++
 tests/gnu/fstype_btrfs_subvol.sh  | 25 +++++++++++++++++++++++++
 3 files changed, 31 insertions(+), 2 deletions(-)
 create mode 100644 tests/gnu/fstype_btrfs_subvol.out
 create mode 100644 tests/gnu/fstype_btrfs_subvol.sh

diff --git a/src/mtab.c b/src/mtab.c
index 4cbb9a6..40a9885 100644
--- a/src/mtab.c
+++ b/src/mtab.c
@@ -256,7 +256,7 @@ static int bfs_mtab_fill_types(struct bfs_mtab *mtab) {
 			continue;
 		}
 
-		if (trie_set_mem(&mtab->types, &sb.dev, sizeof(sb.dev), mount->type) != 0) {
+		if (trie_set_mem(&mtab->types, &sb.mnt_id, sizeof(sb.mnt_id), mount->type) != 0) {
 			goto fail;
 		}
 	}
@@ -279,7 +279,7 @@ const char *bfs_fstype(const struct bfs_mtab *mtab, const struct bfs_stat *statb
 		}
 	}
 
-	const char *type = trie_get_mem(&mtab->types, &statbuf->dev, sizeof(statbuf->dev));
+	const char *type = trie_get_mem(&mtab->types, &statbuf->mnt_id, sizeof(statbuf->mnt_id));
 	if (type) {
 		return type;
 	} else {
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'
-- 
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