Atomic Changes

[vcsjones]

Ideally OVST would be atomic - that is, if some part of the sign process does not work, then it should appear as if no attempt was made to the package.

This is slightly difficult because applying a signature usually means updating or creating a handful of files.

1. [Content Types.xml] this already supports the notion of commit changes.
2. _rels/.rels this already supports the notion of commit changes.
3. origin.psdor creating parts is NOT currently atomic.
4. Relationship for origin.psdor this is already atomic in the sense that it is transactional because origin.psdor is immediately written to disk.
5. signature.psdsxs creating parts is NOT currently atomic.

The difficult part here is we need a stream to write to - and read from - without updating the ZIP.

Options:

1. Shadow copy the ZIP to %TEMP% first, then atomically switch them out. I don't like this idea
2. New parts can be backed by a memory stream at first, then become real part streams. I like this idea more.

We already keep a running list of OpcPart objects by URI: we would need to virtualize the access to the OpcPackage to return a virtual part with a memory stream. When the signature process is complete, flush all of the virtual parts in to real parts and swap out the memory stream for a part stream.