https://issues.apache.org/bugzilla/show_bug.cgi?id=47657
Summary: Make WebDAV MOVE atomic when replacing files on same
device
Product: Apache httpd-2
Version: 2.3-HEAD
Platform: All
OS/Version: All
Status: NEW
Severity: normal
Priority: P2
Component: mod_dav
AssignedTo: [email protected]
ReportedBy: [email protected]
--- Comment #0 from [email protected] 2009-08-06 14:55:34 PDT ---
When WebDAV MOVE is used to replace an existing file with a new one, mod_dav
systematically deletes the destination file before moving the new file to the
destination filename. This non-atomicity leads to a race condition whereby GET
requests can return 404 Not Found for a short time. When the source and
destination are on the same filesystem, it should be possible to use standard
OS APIs for file renaming (which generally support atomic file replacement)
without deleting the destination file first.
See line 2873 of modules/dav/main/mod_dav.c
(http://svn.apache.org/viewvc/httpd/httpd/trunk/modules/dav/main/mod_dav.c?revision=644050):
/* If destination is being replaced, remove it first
* (we know Ovewrite must be TRUE). Then try to copy/move the resource.
*/
if (replace_dest)
err = (*resnew->hooks->remove_resource)(resnew, &multi_response);
if (err == NULL) {
if (is_move)
err = (*resource->hooks->move_resource)(resource, resnew,
&multi_response);
else
err = (*resource->hooks->copy_resource)(resource, resnew, depth,
&multi_response);
}
It may be sufficient to simply suppress the call to remove_resource to fix this
bug, although I haven't tried it...
The original WebDAV specification (RFC 2518) clearly says that the replacement
should occur atomically:
8.9 MOVE Method
The MOVE operation on a non-collection resource is the logical
equivalent of a copy (COPY), followed by consistency maintenance
processing, followed by a delete of the source, where all three
actions are performed atomically.
The more recent specification (RFC 4918) changes the wording slightly: "all
three actions are performed in a single operation". Whether this means "atomic"
is debatable. Nevertheless, it is highly useful in practice for the operation
to be atomic whenever possible.
Note: To ensure atomic file replacement on Win32, it is apparently necessary to
use MoveFileTransacted to ensure atomicity (see
http://msdn.microsoft.com/en-us/library/aa365241%28VS.85%29.aspx), although
this API is only supported on Vista and later. For other OSes, POSIX rename
should do the job.
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