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Wed, Feb 1 2006 8:14 AM | Permanent Link |
Tim Young [Elevate Software] Elevate Software, Inc. timyoung@elevatesoft.com | André,
<< But for me it doesn't look like a feature. It's not a bug? It's a feature? >> It's neither. It's a problem caused by another piece of software. For example, if someone wrote a piece of software that simply monitored the hard drive and deleted or moved any .dat file that was created in the temporary files directory, would you consider that a DBISAM bug ? That's essentially what we're dealing with here, and the evidence that it isn't a bug is in the changes that we made in 4.x - we simply allow the developer to change the file extension that we're using for temporary tables. -- Tim Young Elevate Software www.elevatesoft.com |
Wed, Feb 1 2006 8:36 AM | Permanent Link |
André Kleinschmidt | Tim,
From the posts before, I understood the problem as if it's caused by the creation-flag of the files .. (delete on close) couldn't this be left away, in this case it should be unimportant if any other software is monitoring and opening tempfiles. Because even if the temporaryfileextension is changed to *.tmp or whatever another monitoring software could harm those too. Even if you allow the developer in the 4.xx versions to change the extension wasn't it possible that it will happen with any extension if a monitoring software is hitting just all instead only .dat-files? What about changing the handling of tempfile creation and deletion? So that a tempfile is created without any winAPIs that causes flags to delete a temporary created file again after beeing opened by another application or service. And on a point where the tempfile is surely not nessessary anymore, it could get deleted explicitly. greetings André "Tim Young [Elevate Software]" <timyoung@elevatesoft.com> wrote: André, << But for me it doesn't look like a feature. It's not a bug? It's a feature? >> It's neither. It's a problem caused by another piece of software. For example, if someone wrote a piece of software that simply monitored the hard drive and deleted or moved any .dat file that was created in the temporary files directory, would you consider that a DBISAM bug ? That's essentially what we're dealing with here, and the evidence that it isn't a bug is in the changes that we made in 4.x - we simply allow the developer to change the file extension that we're using for temporary tables. -- Tim Young Elevate Software www.elevatesoft.com |
Wed, Feb 1 2006 7:16 PM | Permanent Link |
> Temporary tables are opened exclusively. There should be no way that an
> external process could interfere with the files once they are open. Indeed. I was not implying that. Here are the scenarios I had in mind... The AV software has requested Windows to notify it when new files are created. It has a thread sitting there idle just waiting to act on that notification, so it will get in on the act very early on in the lifetime of the file. Scenario 1. The AV opens the file, scans it, finds no virus, and closes it. If this happens at a time when the file is not currently open in DBISAM, but when it does have a delete on close flag associated with it, then Windows will delete the file. Scenario 2. The AV opens the file, and begins to scan it. Before the AV closes it, DBISAM tries to open it in exclusive mode but fails because it is still open. This failure could look like the file has been deleted - especially if it does disappear shortly afterwards as a result of the delete on close flag. Both of the above scenarios could be "faked" by running DBISAM under a debugger, stepping it through the initial OS calls to create/close/open the file, and simultaneously opening and closing the file in another process (scenario 1), or opening and leaving it open (scenario 2). It strikes me that the AV software is probably not doing anything different with files in the temporary directory compared to files in the application directory. The difference between the two types of file is more likely down to the difference in how they are created/opened. The software doing the deleting is probably Windows itself. --Bill Sparrow-- | |
Thu, Feb 2 2006 8:36 AM | Permanent Link |
Tim Young [Elevate Software] Elevate Software, Inc. timyoung@elevatesoft.com | André,
<< From the posts before, I understood the problem as if it's caused by the creation-flag of the files .. (delete on close) >> That is not correct. << Because even if the temporaryfileextension is changed to *.tmp or whatever another monitoring software could harm those too. >> Sure. But now you're getting into the territory where there's nothing you can do short of disabling the AV software on the desired directory. << Even if you allow the developer in the 4.xx versions to change the extension wasn't it possible that it will happen with any extension if a monitoring software is hitting just all instead only .dat-files? >> Yes. << What about changing the handling of tempfile creation and deletion? So that a tempfile is created without any winAPIs that causes flags to delete a temporary created file again after beeing opened by another application or service. >> As stated before, we don't use the delete-on-close flag for the creation of the temporary table, only for the open after the creation (for obvious reasons - you can't use the flag if you intend to close the file right after creating it). The problem is that the AV software is deleting/moving the file before the open ever occurs. -- Tim Young Elevate Software www.elevatesoft.com |
Thu, Feb 2 2006 8:50 AM | Permanent Link |
Tim Young [Elevate Software] Elevate Software, Inc. timyoung@elevatesoft.com | Bill,
<< The AV opens the file, scans it, finds no virus, and closes it. If this happens at a time when the file is not currently open in DBISAM, but when it does have a delete on close flag associated with it, then Windows will delete the file. >> I understand, but DBISAM does not use the delete-on-close flag during file creation. Only during the opens. In addition, the delete-on-close flag is a Win32 API call parameter, not a file attribute. It does not persist beyond the current API call, therefore there is no way for Windows to know that DBISAM wants this parameter until it gets the CreateFile call with the parameter. Finally, you cannot logically have a file that is not open with the delete-on-close flag. << The AV opens the file, and begins to scan it. Before the AV closes it, DBISAM tries to open it in exclusive mode but fails because it is still open. This failure could look like the file has been deleted - especially if it does disappear shortly afterwards as a result of the delete on close flag. >> I've never encountered a case where Windows reports a "file not found" issue as a "access error" issue, or vice-versa. If DBISAM says that the file is not there, then it means that Windows told it that the file isn't there. In this particular case, it means that the AV software moved it or deleted it. If DBISAM says that it can't access the file, then it's most likely due to the AV software having the file open after DBISAM created it and interfering with the exclusive open (again, DBISAM is informed by Windows that there is an access error). << The difference between the two types of file is more likely down to the difference in how they are created/opened. >> DBISAM doesn't create temporary tables any differently than it does normal tables. The fact that normal tables work is due to the fact that the AV software scans particular directories, like the local hard drive directories or the temporary files directory. << The software doing the deleting is probably Windows itself. >> Absolutely not. What you're describing completely fails to account for the fact that it works fine in situations where the AV software is not present. DBISAM only uses the delete-on-close flag for the exclusive opens, hence it should never fail or have an issue unless something messes with the file after the create/close but before the exclusive open. -- Tim Young Elevate Software www.elevatesoft.com |
Thu, Feb 2 2006 11:13 AM | Permanent Link |
Ah. It looks like I have misunderstood some Windows fundamentals. Thanks
for putting me right, Tim, and apologies for casting aspersions on your coding! I thought the delete on close was an attribute of the file on disk, now I see it is an attribute stored in the singleton associated with the open file. Looks like I was getting mixed up with FILE_ATTRIBUTE_TEMPORARY which can be specified as a parameter to SetFileAttributes. | |
Fri, Feb 3 2006 5:21 PM | Permanent Link |
Tim Young [Elevate Software] Elevate Software, Inc. timyoung@elevatesoft.com | Bill,
<< Ah. It looks like I have misunderstood some Windows fundamentals. Thanks for putting me right, Tim, and apologies for casting aspersions on your coding! >> No problem. And I'm not perfect, so it certainly pays to ask questions when things look like they might be incorrect. Sometimes that jogs my noggin in a way that reveals a solution. -- Tim Young Elevate Software www.elevatesoft.com |
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