btrfs hack: shrink the allocated size

We are using read-only btrfs compressed for the /usr partition,
and sometimes the free space shown by `btrfs fi usage` and the `df` IS diverging,
with `df` showing the free space from the `btrfs fi usage`'s `Device unallocated`.

I have found to fix the allocation, by decreasing the size to more than the btrfs file system
allows, but curiosly the allocation is getting reset
during the failed attempt and the issue is fixed.

This is of course, a _HACK_, and can at any time produce unwanted behaviours.

Note that the issue cannot be reliably reproduced.

See Flatcar issue: flatcar/Flatcar#1473

See issue upstream:
https://bugzilla.kernel.org/show_bug.cgi?id=219167

Signed-off-by: Adrian Vladu <[email protected]>

build_library/build_image_util.sh

@@ -806,6 +806,8 @@ EOF
     sudo fstrim "${root_fs_dir}/usr" || true
   fi
 
+  "${BUILD_LIBRARY_DIR}/disk_util" --disk_layout="${disk_layout}" btrfsresetallocation \
+      "${root_fs_dir}/usr"
   # Make the filesystem un-mountable as read-write and setup verity.
   if [[ ${disable_read_write} -eq ${FLAGS_TRUE} ]]; then
     # Unmount /usr partition

build_library/disk_util

@@ -768,6 +768,25 @@ def Tune(options):
   if not action_done:
     raise Exception("No options specified!")
 
+def SudoOutputToConsole(cmd):
+  print(SudoOutput(cmd).decode())
+
+def LogBtrfsUsage(mount_point, section=None):
+  print('Btrfs usage for mount point `%s` %s' % (mount_point, section))
+  try:
+      SudoOutputToConsole(['btrfs', 'fi', 'usage', mount_point])
+  except Exception as ex:
+      print('Btrfs usage could not be retrieved for mount point `%s` %s' % (mount_point, section))
+      print(ex)
+
+def BtrfsResetAllocation(options):
+
+    LogBtrfsUsage(options.disk_image, 'before disk resize')
+    try:
+        Sudo(['btrfs', 'fi', 'resize', '-500m', options.disk_image])
+    except Exception as ex:
+        print(ex)
+    LogBtrfsUsage(options.disk_image, 'after disk resize')
 
 def Verity(options):
   """Hash verity protected filesystems.
@@ -1119,6 +1138,10 @@ def main(argv):
   a = actions.add_parser('parseonly', help='validate config')
   a.set_defaults(func=DoParseOnly)
 
+  a = actions.add_parser('btrfsresetallocation', help='btrfs reset allocation')
+  a.add_argument('disk_image', help='path to disk image file')
+  a.set_defaults(func=BtrfsResetAllocation)
+
   options = parser.parse_args(argv[1:])
   options.func(options)