Summary:
Fsnodes have a lot of data about files, but right now we can't access it
through a Fsnode lookup or a manifest walk, because the LeafId for a Fsnode is
just the content id and the file type.
This is a bit sad, because it means we e.g. cannot dump a manifest with file
sizes (D23471561 (179e4eb80e)).
Just changing the LeafId is easy, but that brings a new problem with Fsnode
derivation.
Indeed, deriving manifests normally expects us to have the "derive leaf"
function produce a LeafId (so we'd want to produce a `FsnodeFile`), but in
Fsnodes, this currently happens in deriving trees instead.
Unfortunately, we cannot easily just move the code that produces `FsnodeFile`
from the tree derivation to the leaf derivation, that is, do:
```
fn check_fsnode_leaf(
leaf_info: LeafInfo<FsnodeFile, (ContentId, FileType)>,
) -> impl Future<Item = (Option<FsnodeSummary>, FsnodeFile), Error = Error>
```
Indeed, the performance of Fsnode derivation relies on all the leaves for a
given tree being derived together with the tree and its parents in context.
So, we'd need the ability for deriving a new leaf to return something different
from the actual leaf id. This means we want to return a `(ContentId,
FileType)`, even though our `LeafId` is a `FsnodeFile`.
To do this, this diff introduces a new `IntermediateLeafId` type in the
derivation. This represents the type of the leaf that is passed from deriving a
leaf to deriving a tree. We need to be able to turn a real `LeafId` into it,
because sometimes we don't re-derive leaves.
I think we could also refactor some of the code that passes a context here to
just do this through the `IntermediateLeafId`, but I didn't look into this too
much.
So, this diff does that, and uses it in Mononoke Admin so we can print file
sizes.
Reviewed By: StanislavGlebik
Differential Revision: D23497754
fbshipit-source-id: 2fc480be0b1e4d3d261da1d4d3dcd9c7b8501b9b
f1e4f62e2d