fix(image): clean partial mounts and refuse mounted export deletion

hydra/build.py

@@ -58,6 +58,21 @@ sys.exit(code)
 '''
 
 
+def rootfs_retry_guard_script():
+    """Refuse deletion of a still-mounted export, including nested mountpoints."""
+    return r"""mounts=$(findmnt --list --raw --noheadings --output TARGET)
+while IFS= read -r mountpoint; do
+  case "$mountpoint" in
+    "$root"|"$root/"*) echo 'Refusing to remove a mounted rootfs export' >&2; exit 1 ;;
+  esac
+done <<< "$mounts"
+if [ -e "$root" ] || [ -L "$root" ]; then
+  [ ! -L "$root" ] && [ -f "$root/.hydra-export-owned" ] || { echo 'Unowned rootfs output' >&2; exit 1; }
+  rm -rf -- "$root"
+fi
+"""
+
+
 def verify_hash(path, expected):
     digest = hashlib.sha256()
     with Path(path).open('rb') as stream:
@@ -421,12 +436,7 @@ fi'''
         snapshot = image.digest(original)
         assembler = self.assembly(snapshot)
         commit = self.run(['git', '-C', str(self.source_dir), 'rev-parse', 'HEAD'], capture=True).stdout.strip()
-        script = r"""root=/hydra-output/rootfs
-if [ -e "$root" ] || [ -L "$root" ]; then
-  [ ! -L "$root" ] && [ -f "$root/.hydra-export-owned" ] || { echo 'Unowned rootfs output' >&2; exit 1; }
-  rm -rf -- "$root"
-fi
-mkdir -p "$root"
+        script = 'root=/hydra-output/rootfs\n' + rootfs_retry_guard_script() + r"""mkdir -p "$root"
 touch "$root/.hydra-export-owned"
 tar --extract --preserve-permissions --numeric-owner --file /hydra-output/artifacts/builder-rootfs.tar --directory "$root"
 /hydra-source/image/configure-rootfs.sh "$root"