[MirageOS-devel] basic support for transactions in irminsule

[Thomas Gazagnaire <thomas@xxxxxxxxxxxxxx>]

Hi all,

just a quick update on irminsule.

I've added last week a support of "views", which are efficient in-memory lazy 
cache for subtrees, without history. Reads are done only when needed and all 
the serialisation cost of computing SHA1 from values is done only when you 
commit the full view to store.

There are two use-cases for such views:
- you need a staging area, ie. you don't want to commit every operation on the 
store: you create a view for a given path, do some stuff, and when you are 
happy, you create one big commit in the store with the changes
- you want to map a sub-tree to an OCaml value: in that case you want to be 
able to store the subtree in different parts of your store if needed or you 
don't have a specific path in mind when you create the initial view.

When you want the view to be committed, you can simply replace the current 
sub-tree by your view using the `update_view` function.

But there are use-cases where it's not exactly what you need, for instance: 
transactions. In that case, you want to replay all the updates on top of the 
HEAD version of the tree, and check that the reads returns the good result. 
I've just added that to Irminsule, with the 'merge_view' function. In bonus, 
you have all the operations done on the view which now appears in the commit 
message.

I'm still not totally happy with the atomicity of such operations (ie. there is 
no guarantee than nobody else is modifying the HEAD tag when you merge a 
transaction, which can lead to transaction loss if not careful enough). 

As usual, feel free to play with the new feature and give some feedback on the 
mailing list or on the Github issue tracker.

Best,
Thomas
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