storemodel: define traits to decouple heavyweight implementation details

Summary:
Similar to `configmodel` introduced by D27641666 (a25ec579bf). Introduce a trait so we can
decouple from various storage implementation details.

This helps in many ways:
- De-couple store details. For example, `checkout` should not care about how to strip hg filelog header.
- Adds flexibility. For example, adopting the eagerepo store, or a git store, and they "just work" in various places (ex. in `checkout`, and the Python world).

For now, it only defines the pure content reading trait. The interface is now chosen to
match `checkout`'s needs. `(Key, Result<Bytes>)` might be arguably better than
`Result<(Key, Bytes)>` or `Result<(Bytes, Key)>`. But that can be changed later
after moving code from `checkout` to `revisionstore`.

More traits will be added later as we need them.

Reviewed By: DurhamG

Differential Revision: D33118996

fbshipit-source-id: 7f2b00b5bdefdf5b45ecf581f1586b5216e3e1cb

eden/scm/lib/storemodel/Cargo.toml

@@ -0,0 +1,11 @@
+# @generated by autocargo from //eden/scm/lib/storemodel:storemodel
+[package]
+name = "storemodel"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+async-trait = "0.1.51"
+futures = { version = "0.3.13", features = ["async-await", "compat"] }
+minibytes = { path = "../minibytes" }
+types = { path = "../types" }

eden/scm/lib/storemodel/src/lib.rs

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+//! # storemodel
+//!
+//! Provides trait definitions for storage needs.
+//! Useful to de-couple from heavyweight storage implementations.
+//!
+//! Traits defined by this crate are ideally tech-debt free and
+//! consider both hg and git use-cases. This mainly means:
+//! - APIs do expose hg details like filelog "copy from" or LFS pointer.
+//! - History-related APIs should avoid linkrev or linknode, which do not exist
+//!   in git.
+//!
+//! For flexibility, different features might be defined in different traits.
+//! Traits can be combined later. For example, reading file content, metadata,
+//! and history should probably be 3 different traits.
+
+use async_trait::async_trait;
+use futures::stream::BoxStream;
+pub use minibytes::Bytes;
+use types::Key;
+
+#[async_trait]
+pub trait ReadFileContents {
+    type Error;
+
+    /// Read the content of specified files.
+    ///
+    /// The returned content should be just the file contents. This means:
+    /// - The returned content does not contain the "copy from" header.
+    /// - The returned content does not contain raw LFS content. LFS pointer
+    ///   is resolved transparently.
+    /// - If the file content is redacted, it's an error instead of an explicit
+    ///   instead of a placeholder of dummy data.
+    async fn read_file_contents(
+        &self,
+        keys: Vec<Key>,
+    ) -> BoxStream<Result<(Bytes, Key), Self::Error>>;
+}