Replace some uses of the deprecated mktemp function #1770

The tempfile.mktemp function is deprecated, because of a race condition where the file may be concurrently created between when its name is generated and when it is opened. Other faciliies in the tempfile module overcome this by generating a name, attempting to create the file or directory in a way that guarantees failure if it already existed, and, in the occasional case that it did already exist, generating another name and trying again (stopping after a predefined limit). For further information on mktemp deprecation: - https://docs.python.org/3/library/tempfile.html#tempfile.mktemp - gitpython-developers/smmap#41 The security risk of calls to mktemp in this project's test suite is low. However, it is still best to avoid using it, because it is deprecated, because it is (at least slightly) brittle, and because any use of mktemp looks like a potential security risk and thereby imposes a burden on working with the code (which could potentially be addressed with detailed comments analyzing why it is believed safe in particular cases, but this would typically be more verbose, and at least as challenging to add, as replacing mktemp with a better alternative). This commit replaces *some* uses of mktemp in the test suite: those where it is readily clear how to do so in a way that preserves the code's intent: - Where a name for a temporary directory is generated with mktemp and os.mkdir is called immediately, mkdtemp is now used. - Where a name for a temporary file that is not customized (such as with a prefix) is generated with mktemp, such that the code under test never uses the filename but only the already-open file-like object, TemporaryFile is now used. As the name isn't customized, the test code in these cases does not express an intent to allow the developer to inspect the file after a test failure, so even if the file wasn't guaranteed to be deleted with a finally block or context manager, it is fine to do so. TemporaryFile supports this use case well on all systems including Windows, and automatically deletes the file. - Where a name for a temporary file that *is* customized (such as with a prefix) to reflect the way the test uses it is generated with mktemp, and the test code does not attempt deterministic deletion of the file when an exception would make the test fail, NamedTemporaryFile with delete=False is now used. The original code to remove the file when the test succeeds is modified accordingly to do the same job, and also commented to explain that it is being handled this way to allow the file to be kept and examined when a test failure occurs. Other cases in the test suite should also be feasible to replace, but are left alone for now. Some of them are ambiguous in their intent, with respect to whether the file should be retained after a test failure. Others appear deliberately to avoid creating a file or directory where the code under test should do so, possibly to check that this is done properly. (One way to preserve that latter behavior, while avoiding the weakness of using mktemp and also avoiding inadverently reproducing that weakness by other means, could be to use a path in a temporary directory made for the test.) This commit also doesn't address the one use of mktemp in the code under test (i.e., outside the test suite, inside the git module).

This makes two related changes to git.index.util.TemporaryFileSwap: - Replace mktemp with mkstemp and then immediately closing the file. This avoids two possible name clashes: the usual name clash where the file may be created by another process between when mktemp generates the name and when the file is created, and the problem that mktemp does not check for files that contain the generated name as a part. The latter is specific to the use here, where a string generated by mktemp was manually concatenated to the base filename. This addresses that by passing the filename as the prefix for mkstemp to use. - Replace os.remove with os.replace and simplify. This is made necessary on Windows by the file already existing, due to mkstemp creating it. Deleting the file and allowing it to be recreated would reproduce the mktemp race condition in full (obscured and with extra steps). But os.replace supports an existing target file on all platforms. It is now also used in __exit__, where it allows the Windows check for the file to be removed, and (in any OS) better expresses the intent of the code to human readers. Furthermore, because one of the "look before you leap" checks in __exit__ is removed, the minor race condition in cleaning up the file is somewhat decreased. A small amount of related refactoring is included. The interface is not changed, even where it could be simplified (by letting __exit__ return None), and resource acquisition remains done on construction rather than in __enter__, because changing those aspects of the design could break some existing uses. Although the use of mktemp replaced here was in the git module and not the test suite, its use was to generate filenames for use in a directory that would ordinarily be under the user's control, such that the ability to carry out typical mktemp-related attacks would already require the ability to achieve the same goals more easily and reliably. Although TemporaryFileSwap is a public class that may be used directly by applications, it is only useful for making a temporary file in the same directory as an existing file. Thus the intended benefits of this change are mainly to code quality and a slight improvement in robustness.

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Replace some uses of the deprecated mktemp function #1770

Replace some uses of the deprecated mktemp function #1770

Commits on Dec 13, 2023