Client: Allow SMB2 create contexts on CreateFile, and add SMB2_CREATE_TIMEWARP_TOKEN

[smbecker]

What this does

Lets a client send SMB2 create contexts with CreateFile and read the ones the server answers
with, and adds the pieces needed to open a previous version (shadow copy) of a file.

Closes #371.

Background

CreateRequest.CreateContexts and CreateResponse.CreateContexts were already implemented; the client API simply had no way to reach them. A shadow copy is addressed with the
SMB2_CREATE_TIMEWARP_TOKEN create context.

Compatibility

The existing CreateFile overload is unchanged and now delegates to the new one passing null contexts, so behavior for current callers is identical. Nothing is added to ISMBFileStore since create-contexts are SMB2-specific, and that interface is also implemented by SMB1 and server-side stores. A caller that needs contexts uses SMB2FileStore directly.

[TalAloni]

Thanks Shaun,
Given that this is a niche use case, I'm not sure how I want to proceed here,
The CreateFile API is hard-to-understand as it is, so even if I had gone with the CreateFile overload you suggested in the pull request, having it marked as "protected" rather than "public" would have made more sense (to avoid cluttering the public API)
In that alternative, the CreateContextName class and the CreateContextHelper class should be part of the solution that use the library and not part of the library itself.
To put it more bluntly: I have no interest in exposing an API that allows a client to open a previous version of a file, but I am open to enable developers to utilize such capabilities.

[smbecker]

Thanks Shaun, Given that this is a niche use case, I'm not sure how I want to proceed here, The CreateFile API is hard-to-understand as it is, so even if I had gone with the CreateFile overload you suggested in the pull request, having it marked as "protected" rather than "public" would have made more sense (to avoid cluttering the public API) In that alternative, the CreateContextName class and the CreateContextHelper class should be part of the solution that use the library and not part of the library itself. To put it more bluntly: I have no interest in exposing an API that allows a client to open a previous version of a file, but I am open to enable developers to utilize such capabilities.

I can certainly remove the CreateContextName and CreateContextHelper classes as those are probably a bit too niche to include in the public API as part of this project. However, without the public overload for request/response create-context, how would a developer be able to utilize such capabilities? Given that ISMBClient.TreeConnect is what is responsible for discovering what implementation of ISMBFileStore to return and developers have no ability to override that today, making the overload protected would still render it pretty much inaccessible.

[smbecker]

To put it more bluntly: I have no interest in exposing an API that allows a client to open a previous version of a file, but I am open to enable developers to utilize such capabilities.

The current approach of using a public method on SMB2FileStore was my attempt at not messing with the public API that most developers would generally be consuming (utilizing ISMBFileStore) while still making it available for developers to utilize such capabilities.

[TalAloni]

without the public overload for request/response create-context, how would a developer be able to utilize such capabilities? Given that ISMBClient.TreeConnect is what is responsible for discovering what implementation of ISMBFileStore to return and developers have no ability to override that today, making the overload protected would still render it pretty much inaccessible.

Thanks, you make a good point, there would need to be a "protected virtual" CreateFileStore method in SMB2Client for that to work.
Not loving this but definitely better than exposing CreateContext as part of a public API.
(CreateContext is an SMB2 concept, not a file store concept, so it feels wrong even before we consider how complicated the CreateFile API is)