fix(cloudfront): serve DefaultRootObject at the distribution root

crates/fakecloud-cloudfront/src/dataplane.rs

@@ -162,12 +162,20 @@ impl CloudFrontDataPlane {
             ));
         };
 
-        let path_and_query = parts
-            .uri
-            .path_and_query()
-            .map(|p| p.as_str())
-            .unwrap_or("/")
-            .to_string();
+        // A root request is fetched as the distribution's DefaultRootObject; the
+        // query string is preserved, as CloudFront does.
+        let path_and_query = match &route.root_object {
+            Some(object) => match parts.uri.query() {
+                Some(q) => format!("{object}?{q}"),
+                None => object.clone(),
+            },
+            None => parts
+                .uri
+                .path_and_query()
+                .map(|p| p.as_str())
+                .unwrap_or("/")
+                .to_string(),
+        };
         let url = format!("{}{path_and_query}", route.upstream.url_base);
         trace!(%host, path = %parts.uri.path(), origin = %route.upstream.host_header, "CloudFront data plane: proxying");
         let resp = self
@@ -293,6 +301,9 @@ struct RouteResolution {
     default_upstream: UpstreamTarget,
     /// CustomErrorResponses that have a response page path.
     error_rules: Vec<ErrorRule>,
+    /// `DefaultRootObject` as an origin path (`/index.html`), set only when this
+    /// request is for the distribution root and the distribution configures one.
+    root_object: Option<String>,
 }
 
 /// A resolved origin address: the scheme+authority to connect to and the `Host`
@@ -350,10 +361,21 @@ fn resolve_route(
                 .collect()
         })
         .unwrap_or_default();
+    // DefaultRootObject: a request for the distribution ROOT is served the named
+    // object from the origin. AWS applies this to the root only -- a request for a
+    // subdirectory is never rewritten to `<dir>/<object>` -- so the rewrite is
+    // decided here, from the original viewer path, and the cache behavior is still
+    // selected on that original path.
+    let root_object = matches!(path, "" | "/")
+        .then(|| cfg.default_root_object.as_deref().map(str::trim))
+        .flatten()
+        .filter(|o| !o.is_empty())
+        .map(|o| format!("/{}", o.trim_start_matches('/')));
     Some(RouteResolution {
         upstream,
         default_upstream,
         error_rules,
+        root_object,
     })
 }
 
@@ -497,7 +519,9 @@ fn is_hop_by_hop(name: &str) -> bool {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::model::{AliasItems, Aliases, CustomOriginConfig, Origin};
+    use crate::model::{
+        AliasItems, Aliases, CustomOriginConfig, DefaultCacheBehavior, Origin, OriginItems, Origins,
+    };
     use crate::state::StoredDistribution;
     use chrono::Utc;
 
@@ -608,6 +632,61 @@ mod tests {
         assert_eq!(up.host_header, "b.s3-website-us-east-1.amazonaws.com");
     }
 
+    fn cfg_with_root(root: Option<&str>) -> DistributionConfig {
+        DistributionConfig {
+            default_root_object: root.map(Into::into),
+            origins: Origins {
+                quantity: 1,
+                items: Some(OriginItems {
+                    origin: vec![origin("b.s3.us-east-1.amazonaws.com", None)],
+                }),
+            },
+            default_cache_behavior: DefaultCacheBehavior {
+                target_origin_id: "o".into(),
+                ..Default::default()
+            },
+            ..Default::default()
+        }
+    }
+
+    fn root_object_for(cfg: &DistributionConfig, path: &str) -> Option<String> {
+        resolve_route(cfg, path, "127.0.0.1:4566")
+            .expect("route resolves")
+            .root_object
+    }
+
+    #[test]
+    fn default_root_object_applies_to_the_distribution_root() {
+        let cfg = cfg_with_root(Some("index.html"));
+        assert_eq!(root_object_for(&cfg, "/").as_deref(), Some("/index.html"));
+        assert_eq!(root_object_for(&cfg, "").as_deref(), Some("/index.html"));
+    }
+
+    #[test]
+    fn default_root_object_does_not_apply_below_the_root() {
+        // AWS serves the default root object for the distribution root ONLY; a
+        // subdirectory request is never rewritten to `<dir>/<object>`.
+        let cfg = cfg_with_root(Some("index.html"));
+        assert_eq!(root_object_for(&cfg, "/about/"), None);
+        assert_eq!(root_object_for(&cfg, "/about"), None);
+        assert_eq!(root_object_for(&cfg, "/index.html"), None);
+    }
+
+    #[test]
+    fn default_root_object_unset_or_blank_leaves_the_root_alone() {
+        for root in [None, Some(""), Some("   ")] {
+            assert_eq!(root_object_for(&cfg_with_root(root), "/"), None, "{root:?}");
+        }
+    }
+
+    #[test]
+    fn default_root_object_is_normalized_to_a_single_leading_slash() {
+        // AWS rejects a leading slash in the config, but accept one defensively
+        // rather than proxying a `//index.html` path to the origin.
+        let cfg = cfg_with_root(Some("/index.html"));
+        assert_eq!(root_object_for(&cfg, "/").as_deref(), Some("/index.html"));
+    }
+
     #[test]
     fn https_only_custom_origin_uses_https_and_port() {
         let up = upstream_for(

crates/fakecloud-e2e/tests/cloudfront_dataplane.rs

@@ -69,6 +69,33 @@ pub async fn make_spa_distribution(
         .clone()
 }
 
+/// Create a SPA distribution that also sets `DefaultRootObject`.
+pub async fn make_spa_distribution_with_root_object(
+    cf: &aws_sdk_cloudfront::Client,
+    default_origin_domain: &str,
+    root_object: &str,
+) -> aws_sdk_cloudfront::types::Distribution {
+    let mut config = spa_config(
+        default_origin_domain,
+        None,
+        &format!("spa-{}", uuid_like()),
+        true,
+        true,
+        &[],
+    );
+    config.default_root_object = Some(root_object.to_string());
+    let create = cf
+        .create_distribution()
+        .distribution_config(config)
+        .send()
+        .await
+        .expect("create_distribution");
+    create
+        .distribution()
+        .expect("distribution returned")
+        .clone()
+}
+
 /// Build the SPA distribution config. Shared by create (enabled=true) and the
 /// disable-via-update path (enabled=false) so both use an identical shape; the
 /// `caller_reference` must be preserved across an UpdateDistribution.
@@ -644,3 +671,41 @@ async fn api_traffic_is_not_intercepted() {
         .await
         .expect("s3 list_buckets must pass through the viewer middleware");
 }
+
+/// Regression: `DefaultRootObject` was stored on the model but never applied, so a
+/// viewer request for the distribution root reached the bucket root and returned
+/// S3's `ListBucketResult` XML instead of the SPA shell.
+#[tokio::test]
+async fn serves_default_root_object_at_the_distribution_root() {
+    let server = TestServer::start().await;
+    let s3 = server.s3_client().await;
+    s3.create_bucket()
+        .bucket("rootsite")
+        .send()
+        .await
+        .expect("create_bucket");
+    put_object(&s3, "rootsite", "index.html", "text/html", b"SHELL").await;
+    put_object(&s3, "rootsite", "nested/index.html", "text/html", b"NESTED").await;
+
+    let cf = server.cloudfront_client().await;
+    let dist = make_spa_distribution_with_root_object(
+        &cf,
+        "rootsite.s3-website-us-east-1.amazonaws.com",
+        "index.html",
+    )
+    .await;
+    assert!(wait_for_served(&server, dist.id(), Duration::from_secs(10)).await);
+    let host = dist.domain_name();
+
+    // The root serves the default root object, not a bucket listing.
+    let r = viewer_get(&server, host, "/").await;
+    assert_eq!(r.status(), 200);
+    assert_eq!(r.text().await.unwrap(), "SHELL");
+
+    // A subdirectory is NOT rewritten to `nested/index.html`: AWS applies the
+    // default root object to the distribution root only. Here the miss falls
+    // through to the SPA CustomErrorResponse rule instead.
+    let r = viewer_get(&server, host, "/nested/").await;
+    assert_eq!(r.status(), 200);
+    assert_eq!(r.text().await.unwrap(), "SHELL");
+}