From 43fb8698602e63b39b376d15c24adedcca17d63f Mon Sep 17 00:00:00 2001
From: Christian Kellner <christian@kellner.me>
Date: Fri, 13 Aug 2021 10:02:45 +0000
Subject: [PATCH] loop: helper to call `ioctl_blockdev_flushbuf`

Add a small new helper that calls `linux.ioctl_blockdev_flushbuf`
for the block device.
---
 osbuild/loop.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/osbuild/loop.py b/osbuild/loop.py
index 3710c3d3..0fd0f4e9 100644
--- a/osbuild/loop.py
+++ b/osbuild/loop.py
@@ -6,6 +6,7 @@ import os
 import stat
 import time
 
+from .util import linux
 
 __all__ = [
     "Loop",
@@ -163,6 +164,19 @@ class Loop:
 
         fcntl.flock(self.fd, op)
 
+    def flush_buf(self) -> None:
+        """Flush the buffer cache of the loopback device
+
+        This function might be required to be called before the usage
+        of `clear_fd`. It seems that the kernel (as of version 5.13.8)
+        is not clearing the buffer cache of the block device layer in
+        case the fd is manually cleared.
+
+        NB: This function needs the `CAP_SYS_ADMIN` capability.
+        """
+
+        linux.ioctl_blockdev_flushbuf(self.fd)
+
     def set_fd(self, fd):
         """Bind a file descriptor to the loopback device