Allow caching requests to endpoints that serve immutable content

[rudolfs]

We could potentially allow browsers caching the following endpoint responses indefinitely:

/v1/projects/:project/commits/:sha
/v1/projects/:project/tree/:sha/*path
/v1/projects/:project/blob/:sha/*path
/v1/projects/:project/readme/:sha

Because the content is addressed by a SHA and shouldn't change (unless there's a collision).

But unfortunately the responses contain a lastCommit attribute which can vary. Looking at the front-end code, we only use this attribute when querying the /v1/projects/:project/blob/:sha/*path endpoint. And even that could be fetched in a separate request.

Therefore I propose we remove the lastCommit attribute from all the requests and create a separate endpoint for getting the lastCommit info for a blob and update the front-end. And finally add caching headers to all the endpoints.

[rudolfs]

Here's a snippet for adding the cache headers:

diff --git a/http-api/Cargo.toml b/http-api/Cargo.toml
index 006cd1f..50f5794 100644
--- a/http-api/Cargo.toml
+++ b/http-api/Cargo.toml
@@ -34,7 +34,7 @@ chrono = { version = "0.4.19", features = ["serde"] }
 axum = { version = "0.5.3", default-features = false, features = ["json", "headers", "query"] }
 axum-server = { version = "0.3", default-features = false, features = ["tls-rustls"] }
 hyper = { version ="0.14.17", default-features = false, features = ["server"] }
-tower-http = { version = "0.3.0", default-features = false, features = ["trace", "cors"] }
+tower-http = { version = "0.3.0", default-features = false, features = ["trace", "cors", "set-header"] }
 
 [dev-dependencies]
 tower = { version = "0.4", features = ["util"] }
diff --git a/http-api/src/v1/projects.rs b/http-api/src/v1/projects.rs
index c600c91..25037a7 100644
--- a/http-api/src/v1/projects.rs
+++ b/http-api/src/v1/projects.rs
@@ -2,6 +2,8 @@ use std::collections::HashMap;
 use std::convert::{TryFrom, TryInto};
 use std::str::FromStr;
 
+use axum::handler::Handler;
+use axum::http::{header, HeaderValue};
 use axum::response::IntoResponse;
 use axum::routing::get;
 use axum::{Extension, Json, Router};
@@ -9,6 +11,7 @@ use hyper::StatusCode;
 use librad::identities::Project;
 use serde::Deserialize;
 use serde_json::json;
+use tower_http::set_header::SetResponseHeaderLayer;
 
 use librad::collaborative_objects::ObjectId;
 use librad::git::identities::{self, SomeIdentity};
@@ -30,18 +33,53 @@ use crate::commit::{Commit, CommitContext, CommitTeaser, CommitsQueryString, Com
 use crate::project::{self, Info};
 use crate::{get_head_commit, Context, Error};
 
+const CACHE_STATIC: &str = "public, max-age=31536000, immutable";
+const CACHE_1_HOUR: &str = "public, max-age=3600, must-revalidate";
+
 pub fn router(ctx: Context) -> Router {
     Router::new()
         .route("/projects", get(project_root_handler))
         .route("/projects/:project", get(project_alias_or_urn_handler))
         .route("/projects/:project/commits", get(history_handler))
-        .route("/projects/:project/commits/:sha", get(commit_handler))
-        .route("/projects/:project/activity", get(activity_handler))
-        .route("/projects/:project/tree/:sha/*path", get(tree_handler))
+        .route(
+            "/projects/:project/commits/:sha",
+            get(commit_handler.layer(SetResponseHeaderLayer::if_not_present(
+                header::CACHE_CONTROL,
+                HeaderValue::from_static(CACHE_STATIC),
+            ))),
+        )
+        .route(
+            "/projects/:project/activity",
+            get(
+                activity_handler.layer(SetResponseHeaderLayer::if_not_present(
+                    header::CACHE_CONTROL,
+                    HeaderValue::from_static(CACHE_1_HOUR),
+                )),
+            ),
+        )
+        .route(
+            "/projects/:project/tree/:sha/*path",
+            get(tree_handler.layer(SetResponseHeaderLayer::if_not_present(
+                header::CACHE_CONTROL,
+                HeaderValue::from_static(CACHE_STATIC),
+            ))),
+        )
         .route("/projects/:project/remotes", get(remotes_handler))
         .route("/projects/:project/remotes/:peer", get(remote_handler))
-        .route("/projects/:project/blob/:sha/*path", get(blob_handler))
-        .route("/projects/:project/readme/:sha", get(readme_handler))
+        .route(
+            "/projects/:project/blob/:sha/*path",
+            get(blob_handler.layer(SetResponseHeaderLayer::if_not_present(
+                header::CACHE_CONTROL,
+                HeaderValue::from_static(CACHE_STATIC),
+            ))),
+        )
+        .route(
+            "/projects/:project/readme/:sha",
+            get(readme_handler.layer(SetResponseHeaderLayer::if_not_present(
+                header::CACHE_CONTROL,
+                HeaderValue::from_static(CACHE_STATIC),
+            ))),
+        )
         .route("/projects/:project/patches", get(patches_handler))
         .route("/projects/:project/patches/:id", get(patch_handler))
         .route("/projects/:project/issues", get(issues_handler))