Unlinking Files on Windows

This page describes what happens when Python's 'os.unlink(f)' is called on Windows.

1. File opened using Python's "open"

If the file f itself or any hardlinked copy of f has been opened for reading by another process using Python's 'open()', then calling 'os.unlink(f)' or 'os.rename(f, ..)' will raise

WindowsError: [Error 32] The process cannot access the file because it is being
used by another process: <f>

This could be fixed in Microsoft's C runtime implementation by patching the file open.c (VC8):

diff --git a/open.c b/open.c
--- a/open.c
+++ b/open.c
@@ -395,6 +395,9 @@

         *punlock_flag = 1;

+        if (osplatform == VER_PLATFORM_WIN32_NT )
+            fileshare  |= FILE_SHARE_DELETE;
+
         /*
          * try to open/create the file
          */

and then making sure Python would use that modified C runtime. Python's 'open' would then behave like Mercurial's 'posixfile'.

2. File opened using Mercurial's "posixfile"

If the file f has been opened for reading by another process with 'posixfile(f)', calling 'os.rename(f, ..)' succeeds.

Calling unlink will send that file into a "scheduled delete" state.

Scheduled delete has the following characteristics:

• (a) the entry in the directory for f is still kept

• (b) calling 'fd = posixfile(f, 'w')' will raise 'IOError: [Errno 13] <f>: Access is denied'

• (c) calling 'os.rename(f, f+'.foo')' will raise 'WindowsError: [Error 5] Access is denied'

• (d) calling 'os.lstat(f)' will raise 'WindowsError: [Error 5] Access is denied: <f>'

• (e) calling 'os.path.exists(f)' returns False

Scheduled delete is left as soon as the other process closes the file.

3. See also

• issue2524: "update loses working copy files on Windows for open files"

• issue2543: "os.unlink deletes wrong file on windows share served by Samba"

• How Mercurial could be made working with Virus Scanners on Windows

• "I thought on Windows it is impossible to delete a file which is still open for use. How does AFIO achieve this?"

CategoryInternals