Do not assume that CopyOnWrite arguments reside on the root filesystem.
We can only do a CoW if the source and destination paths are on the same
APFS volume. Checking that the root volume is APFS is not sufficient.
TESTED=Manual testing on a MacBook with an APFS root volume and a FUSE
mount; we cannot easily automate a test since we can't assume the types
and mount points of filesystems on our automated test machines.
PiperOrigin-RevId: 273558974
diff --git a/src/TulsiGenerator/Scripts/apfs_clone_copy.py b/src/TulsiGenerator/Scripts/apfs_clone_copy.py
index b86d3a6..43159ab 100644
--- a/src/TulsiGenerator/Scripts/apfs_clone_copy.py
+++ b/src/TulsiGenerator/Scripts/apfs_clone_copy.py
@@ -15,6 +15,7 @@
"""Copy on write with similar behavior to shutil.copy2, when available."""
+import errno
import os
import re
import shutil
@@ -44,9 +45,35 @@
return True
+def _IsOnDevice(path, st_dev):
+ """Checks if a given path belongs to a FS on a given device.
+
+ Args:
+ path: a filesystem path, possibly to a non-existent file or directory.
+ st_dev: the ID of a device with a filesystem, as in os.stat(...).st_dev.
+
+ Returns:
+ True if the path or (if the path does not exist) its closest existing
+ ancestor exists on the device.
+ False if not.
+ """
+ if not os.path.isabs(path):
+ path = os.path.abspath(path)
+ try:
+ return os.stat(path).st_dev == st_dev
+ except OSError as err:
+ if err.errno == errno.ENOENT:
+ dirname = os.path.dirname(path)
+ if len(dirname) < len(path):
+ return _IsOnDevice(dirname, st_dev)
+ return False
+
# At launch, determine if the root filesystem is APFS.
IS_ROOT_APFS = _APFSCheck('/')
+# At launch, determine the root filesystem device ID.
+ROOT_ST_DEV = os.stat('/').st_dev
+
def CopyOnWrite(source, dest, tree=False):
"""Invokes cp -c to perform a CoW copy2 of all files, like clonefile(2).
@@ -61,8 +88,11 @@
#
# Identical to shutil's copy2 method, used by shutil's move and copytree.
cmd = ['cp']
- if IS_ROOT_APFS:
- # Assume copy on write (clone) is possible if the root FS is APFS.
+ if IS_ROOT_APFS and _IsOnDevice(source, ROOT_ST_DEV) and _IsOnDevice(
+ dest, ROOT_ST_DEV):
+ # Copy on write (clone) is possible if both source and destination reside in
+ # the same APFS volume. For simplicity, and since checking FS type can be
+ # expensive, allow CoW only for the root volume.
cmd.append('-c')
if tree:
# Copy recursively if indicated.
