tools/osbuild-image-info: handle root on subvol

Btrfs volumes typically only contain subvolumes instead of (parts of)
the OS tree directly.  In our images in particular, this is always the
case.  When searching for root to find /etc/fstab, search through the
subvolumes on a btrfs volume for the file and return the path to the
root subvolume.

Co-authored-by: Michael Vogt <michael.vogt@gmail.com>

tools/osbuild-image-info

@@ -2571,6 +2571,39 @@ def partition_is_lvm(part: Dict) -> bool:
     return part["type"].upper() in ["E6D6D379-F507-44C2-A23C-238F2A3DF928", "8E"]
 
 
+def parse_subvol_list(output):
+    """
+    Parse the output of 'btrfs subvolume list' and return just the subvolume names/paths.
+    """
+    paths = []
+    for line in output.strip().split("\n"):
+        # subvolume names can have spaces in them, but they are the last field and they are preceded by the word
+        # path
+        parts = line.partition(" path ")
+
+        # str.partition() always returns a 3-tuple, but will return (str, "", "") if the separator is not found
+        if parts[2] == "":
+            raise RuntimeError(f"failed to parse output line from 'btrfs subvolume list': {line}")
+
+        paths.append(parts[2])
+    return paths
+
+
+def find_root_subvol(root):
+    """
+    Given a btrfs volume root, find the subvolume that contains the root OS tree.
+    """
+    subvols = subprocess_check_output(["btrfs", "subvolume", "list", root], parse_fn=parse_subvol_list)
+
+    # look through each subvol for /etc/fstab
+    for subvol in subvols:
+        path = os.path.join(root, subvol)
+        if os.path.exists(os.path.join(path, "etc/fstab")):
+            return path
+
+    return None
+
+
 # pylint: disable=too-many-branches disable=too-many-statements
 def append_partitions(report, image):
     partitions = report["partitions"]
@@ -2630,6 +2663,11 @@ def append_partitions(report, image):
                     if os.path.exists(f"{tree}/etc/fstab"):
                         fstab.extend(read_fstab(tree))
                         break
+                    if fs["type"] == "btrfs":
+                        root_subvol = find_root_subvol(tree)
+                        if root_subvol:
+                            fstab.extend(read_fstab(root_subvol))
+                            break
             else:
                 raise RuntimeError("no fstab file found")