[Python-Dev] Remove tempfile.mktemp()

[eryk sun]

On 3/19/19, Victor Stinner <vstinner at redhat.com> wrote:
>
> When I write tests, I don't really care of security, but
> NamedTemporaryFile caused me many troubles on Windows: you cannot
> delete a file if it's still open in a another program. It's way more
> convenient to use tempfile.mktemp().

Opening the file again for normal access is problematic.
NamedTemporaryFile opens it with delete access, but Python's open()
function doesn't support delete-access sharing unless an opener is
used that calls CreateFileW.

NamedTemporaryFile does open files with delete-access sharing, so any
process can delete the file if it's allowed by the file's security and
attributes. You may be thinking of unlinking. In Windows versions
prior to 10, that's always a two-step process. A file with its delete
disposition set doesn't get unlinked until all references for all open
instances are closed.

In Windows 10 (release 1709+), we have the option of using
SetFileInformationByHandle: FileDispositionInfoEx (21) with
FILE_DISPOSITION_FLAG_POSIX_SEMANTICS (2) and
FILE_DISPOSITION_FLAG_DELETE (1). The online documentation hasn't been
updated to include this, but it's supported in the headers for
_WIN32_WINNT_WIN10_RS1 and later. This operation unlinks the file as
soon as we close our handle, even if it has existing references. This
is explained in the remarks for the underlying NT system call [1]. In
particular this resolves the race condition related to handles opened
by anti-malware programs.

It may be worth adding support for deleting files by handle that tries
FileDispositionInfoEx in 1709+. This will work in about half of all
Windows systems. (About 40% still run Windows 7.) It's not a panacea
for Windows file-deleting woes. We still need to be able to open the
file with delete access, which requires existing opens to share delete
access.

[1]: https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/content/ntddk/ns-ntddk-_file_disposition_information_ex