Decomposedfs fix revision download

[butonic]

We rewrote the finish upload code to use a write lock when creating and updating node metadata. This prevents some corner cases and allows us to calculate the size diff atomically for a subsequent PR where we will propagate that size diff.

We now also check the correct permissions when downloading a revision. Downloading revisions only magically works because we are using a suffix that survives being listed at the ocdav meta versions endpoint, initiating a normal file download via the CS3 api and then the dataprovider being able to directly access that version. The DownloadRevision code was never used and was in fact implemented wrong. It would always have returned an empty file.

The workaround is that we can detect revision downloads using the .REV. delimiter that we put between the nodeid and the revision timestamp. When Download detects this delimiter it will now delegate to the fixed DownloadRevision.

fixes owncloud/ocis#3868
fixes owncloud/ocis#765

I will postpone the size diff propagation to a subsequent PR, as this already changes quite a bit.

[lgtm-com]

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[lgtm-com]

This pull request introduces 1 alert when merging 4dae4b6 into 3dbdcc5 - view on LGTM.com

new alerts:

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[lgtm-com]

This pull request introduces 1 alert when merging 11fc7c4 into 3dbdcc5 - view on LGTM.com

new alerts:

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[butonic]

versions can still not be downloaded, it seemst the resource id is constructed wrong ... 👀

[dragonchaser]

lgtm, if ci gets green :)

[butonic]

ocis si is green: https://drone.owncloud.com/owncloud/ocis/16907

[butonic]

superseded by #3482 as it contains the necessary test fixes