loop: helper to get a loop and bind it

Add a new helper, `loop_for_fd` that will get (or create) an
unbounded loop device, bind it to an fd and then set its
status. Since this is racy and can fail the method does these
steps in a retry-loop.

osbuild/loop.py

@@ -1,5 +1,6 @@
 import contextlib
 import ctypes
+import errno
 import fcntl
 import os
 import stat
@@ -383,3 +384,43 @@ class LoopControl:
 
         self._check_open()
         return fcntl.ioctl(self.fd, self.LOOP_CTL_GET_FREE)
+
+    def loop_for_fd(self, fd: int, **kwargs):
+        """
+        Get or create an unbound loopback device and bind it to an fd
+
+        Getting an unbound loopback device, attaching a backing file
+        descriptor and setting the loop device status is racy so this
+        method will retry until it succeeds or it fails to get an
+        unbound loop device.
+
+        All given keyword arguments are forwarded to `Loop.set_status`.
+        """
+
+        self._check_open()
+
+        if fd < 0:
+            raise ValueError(f"Invalid file descriptor '{fd}'")
+
+        while True:
+            lo = Loop(self.get_unbound())
+
+            try:
+                lo.set_fd(fd)
+            except OSError as e:
+                lo.close()
+                if e.errno == errno.EBUSY:
+                    continue
+                raise e
+
+            # `set_status` returns EBUSY when the pages from the
+            # previously bound file have not been fully cleared yet.
+            try:
+                lo.set_status(**kwargs)
+            except BlockingIOError:
+                lo.clear_fd()
+                lo.close()
+                continue
+            break
+
+        return lo