Prevent deletion of file when it is being copied

[Emmet Hikory]

Nimit Shah wrote:
> While copying a file from my computer to external disk, I by mistake
> shift+deleted the file. But still the file transfer dialog showed that it
> was continuing. At the end of the transfer it failed.
> 
> Hence i request you to add a check for file transfer before deleting the
> file.

    As much as this would be a lovely feature, I don't believe that it is
something that we could implement in Ubuntu.

    When copying a file, there are usually two ways to go about it: either
open the entire file, and write it to a new location, or open a series of
sections of the file, and write them each to a new location.  There are a
very large number of programs that provide both of these mechanisms in the
archive.  In the majority of cases, the first potential solution is not
used, because it limits file copies to files that fit entirely in memory
(with everything else), and requires a longer-running algorithm, but
when the second method is used, the file cannot be allowed to be deleted
before the file transfer is confirmed as complete.

    When deleting a file, the usual practice is to remove the reference
from the directory definitions (unlinking), leaving the underlying filesystem
to manage recovery of the newly available space.  Again, there are a vast
number of packages in the archive providing programs that do this.

    In order to implement the feature you describe, we would have to either
provide some systems interface that traps all calls to unlink() and checks
some external reference to determine if it is being copied or patch all
software that could potentially delete files to check the reference, whilst
simultaneously patching every package that provides a means to copy a file
to populate this reference during the file copy, which would make all such
operations considerably slower, with potentially massive impact on server
capacities, interactive response times, and battery life.

    Further, it is unlikely that the developers and maintainers of most of
the software in our archive would be willing to accept such patches, given
the potential complications and incompatibilities with other systems, such
that the result of this vast undertaking would require considerable ongoing
development effort to port these patches for each new upstream release.

    Lastly, in the event that any of the programs providing file copy
functionality were to crash, they may not properly clear the reference
indicating files whose deletion need block on the transfer completion.
As a result of such a crash (or any other bug when updating references),
a user's system may end up having any number of files that cannot be
deleted without manual intervention into the file transfer reference.

-- 
Emmet HIKORY