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Re: [ISSUE] Cancelling svn checkout leaves an open file handle

From: Branko Čibej <brane_at_apache.org>
Date: Wed, 9 Sep 2020 09:39:50 +0200

On 09.09.2020 08:18, Daniel Sahlberg wrote:
> Den ons 9 sep. 2020 kl 06:44 skrev Nathan Hartman
> <hartman.nathan_at_gmail.com <mailto:hartman.nathan_at_gmail.com>>:
> On Tue, Sep 8, 2020 at 6:08 AM Uroš Jovanović <urosh3d_at_gmail.com
> <mailto:urosh3d_at_gmail.com>> wrote:
> >
> > Then, mid downloading some of the larger files a temp file will
> appear in .svn\tmp. Once that happens, hit the Cancel button.
> > It will signal the cancellation to the svn client and it will
> throw the SvnOperationCanceledException, the SvnClient gets
> disposed BUT an open file handle remains on ".svn\tmp\svn-XYZ123"
> file.
> > If you try to delete it, Windows will complain that it is used
> by our test app. :(
> Moving this to the dev@ list...
> Potentially long-running APIs such as 'checkout' allow the client to
> provide a 'cancel_func' callback, which is called at various strategic
> places to ask the client whether the operation should be canceled.
> It sounds to me like one of those places sees a cancel request and
> returns to its caller, forgetting to do some cleanup.
> Last night I tried to find such a place by reading code.
> The 'checkout' command sets up a working copy (if necessary) and then
> calls the 'update' logic to do the heavy lifting.
> The 'update' logic is quite involved as it handles all sorts of
> possibilities, which means the number of branches of the call tree
> that need to be checked are too numerous for my code reading approach
> to be sensible.
> My thoughts for an automated approach, provided there is a way for a
> process to inquire how many open file handles it has (I assume there
> is a way; I've just never had to do this): The idea is to write a
> minimal client that does the following (on a ramdrive):
> 1. Check out a working copy of a repository, giving a cancel_func 'A'
>    that increments a global variable 'n' each time it is called and
>    always returns "don't cancel."
> 2. Loop n times, the loop counter being a global variable 'x':
>    2.1: Delete the working copy.
>    2.2: Check out a working copy of the same repository, giving a
>         different cancel_func 'B' that returns "don't cancel" the
>         first (x - 1) times it is called, and returns "cancel" the
>         x-th time it is called.
>    2.3: Test whether there are open file handles. If there are, we
>         know at which iteration the cleanup is not done, and we break
>         out of the loop.
> 3. If x >= n, quit; we didn't find the problem.
> 4. Delete the working copy.
> 5. Check out a working copy of the same repository, giving a different
>    cancel_func 'C' that returns "don't cancel" the first (x - 1) times
>    it is called, and traps the x-th time it is called, allowing the
>    call stack to be examined.
> Notes and caveats:
> 1. This could run for days (or years).
> 2. Then again, if it can be exposed pretty reliably by a user hitting
>    a Cancel button in a GUI, that means cancel_func is called
>    frequently enough from the offending location that it should
>    (hopefully) be caught relatively soon in the process.
> 3. I think a huge repository isn't needed. The Greek Tree used by the
>    test suite may suffice. If it doesn't expose the bug, I'd retry
>    with a larger file thrown in. If that doesn't expose it, add
>    increasing complexity such as externals, etc.
> 4. This relies on the logic being executed identically for each
>    checkout (i.e., cancel_func is called the same number of times from
>    the same call sites).
> 5. No idea how this could be turned into a regression test.
> 6. If there's a better way, I'd love to hear it!
> For a regression test (as well as trying to pinpoint what goes wrong),
> wouldn't it be enough if the cancel_func check for the presence of a
> file in .svn/tmp (maybe even checking if it is open - in Linux that
> should be easy enough to check in /proc/$PID/fd) and then signal to
> cancel. That would "only" need a repository/file that is large enough
> to trigger calling the cancel_func.

Not even that. The cancel check function is provided by the caller (in
the client context) and can return 'true' when the conditions are right.
Finding the correct temp directory is a bit more involved though because
it's platform-dependent (i.e., I'm not sure if APR can reliably tell us

> I checked quickly and I also see the open file when checking out using
> TortoiseSVN and cancelling and it seems to occur all the time.

It could be a but in our library, or it could be a bug in TortoiseSVN,
since the API caller controls the lifetime of pools. The easiest way to
check would be to run a checkout from the command line client built with
APR pool debugging enabled (that means a custom build of APR is needed,
too) and examining the debug output.

-- Brane
Received on 2020-09-09 09:39:55 CEST

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