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From Mark Hammond <skippy.hamm...@gmail.com>
Subject Re: Windows support
Date Sun, 14 Jun 2009 22:45:15 GMT
On 15/06/2009 7:49 AM, Damien Katz wrote:
>
> On Jun 10, 2009, at 7:56 PM, Mark Hammond wrote:
>
>> On 4/06/2009 3:27 AM, Damien Katz wrote:
>>> I was once again reminded we need to add proper Windows support for
>>> CouchDB when Mark Hammond provided a very nice patch to provide a
>>> windows version of a couchdb unix script. (Thanks Mark!)
>>
>> Thanks! Sorry I was a bit slow to see this thread.
>>
>>> But we also have some broken tests. Other than the issue Mark's patch
>>> fixes, I think it's only compaction that's still broken.
>>
>> compaction and deletion are broken. I've opened a bug on the delete
>> problem at https://issues.apache.org/jira/browse/COUCHDB-326, as
>> alisdair seemed to think that couch could have a simple change made to
>> allow 'delete' to work without changing how the files are opened.
>>
>> > I think the
>>> cleanest way to fix that is to change the erlang VM to open db files
>>> using flags to give them unix like semantics.
>>
>> I've learnt something about Windows file-systems today :) My
>> experiments show that when using FILE_SHARE_DELETE:
>>
>> * A file can be deleted while it is in use - but another file of that
>> name can not be created until the handles are closed.
>>
>> * A file can however be renamed while it is in use, thereby allowing
>> the original name to be re-opened.
>>
>> If we consider the 'delete' case, then FILE_SHARE_DELETE may still end
>> up with a 'race' if an attempt to delete the DB is quickly followed by
>> a request to create it - the delete should succeed, but the create
>> will not.
>>
>> If we consider the 'compact' case, I *suspect* that couch will need to
>> rely on the ability to rename and replace rather than a simple delete
>> - so that might work fine.
>>
>> I'll have a bit of a dig around in the erlang sources and see what I
>> find, but of you have any other thoughts or pointers in the meantime,
>> please let me know.
>>
>> Cheers,
>>
>> Mark
>
>
>
> Perhaps all files that get deleted (via explicit deletion or due to
> compaction) should first be renamed or moved to special trash dir just
> before being actually deleted. The file should be given a random name to
> avoid conflicting with previous deletions.

I don't see how this will fix things for delete without also changing 
the open mode of the file - if the file is still opened by one of the 
couchdb processes, the rename to a temp file will still fail.  If we 
change couch such that the couch process is terminated before deleting 
there should be no need to also change the file mode, nor to use a temp 
dir.  If we do change the file mode, there should be no need to use a 
temp dir either.

IOW, using a temp dir doesn't change the fact that either (a) all 
handles must be closed before rename/delete or (b) we change the open 
mode of the file.  Unless of course I'm missing something.

Is it possible that for the delete case we can do as suggested in 
COUCHDB-326 - ie, after we signal the couchdb processes to terminate, 
actually wait for termination to complete?

I understand compaction may be trickier still when it comes to swapping 
the original and compacted files over, but the same basic invariants 
apply - either we investigate changing the mode of the file, or we take 
steps to ensure the relevant couch processes have terminated at that time.

My apologies if I'm missing something obvious,

Cheers,

Mark

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