util/mnt: extract MountGuard into new module

Extract the `MountGuard` class from all stages that defined it into
a new `mnt` utility module.

osbuild/util/mnt.py

@@ -0,0 +1,40 @@
+"""Mount utilities
+"""
+
+import contextlib
+import subprocess
+
+
+class MountGuard(contextlib.AbstractContextManager):
+    def __init__(self):
+        self.mounts = []
+
+    def mount(self, source, target, bind=True, ro=False, mode="0755"):
+        options = []
+        if bind:
+            options += ["bind"]
+        if ro:
+            options += ["ro"]
+        if mode:
+            options += [mode]
+
+        args = ["--make-private"]
+        if options:
+            args += ["-o", ",".join(options)]
+
+        subprocess.run(["mount"] + args + [source, target], check=True)
+        self.mounts += [{"source": source, "target": target}]
+
+    def umount(self):
+
+        while self.mounts:
+            mount = self.mounts.pop()  # FILO: get the last mount
+            target = mount["target"]
+            # The sync should in theory not be needed but in rare
+            # cases `target is busy` error has been spotted.
+            # Calling  `sync` does not hurt so we keep it for now.
+            subprocess.run(["sync", "-f", target], check=True)
+            subprocess.run(["umount", target], check=True)
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.umount()