1 files changed, 74 insertions, 1 deletions

diff --git a/src/api/mod.rs b/src/api/mod.rs
index 9bda84a..35b1a06 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -77,13 +77,17 @@ pub(crate) fn router(state: AppState, auth_state: AuthState, debug_api: bool) ->
     api.layer(middleware::from_fn_with_state(auth_state, basic_auth))
         // The hand-written spec (`docs/openapi.yaml`), and the Swagger UI showing it.
         .merge(
-            SwaggerUi::new("/swagger-ui").config(utoipa_swagger_ui::Config::from("/openapi.yaml")),
+            // Relative to the UI's page (`/swagger-ui/`), so it is found behind a proxy that
+            // serves the API under a path of its own too.
+            SwaggerUi::new("/swagger-ui")
+                .config(utoipa_swagger_ui::Config::from("../openapi.yaml")),
         )
         .route("/openapi.yaml", get(openapi_spec))
         .route("/health", get(health::node_health))
         .route("/ready", get(health::ready))
         // Outermost, so authentication failures get the error body too.
         .layer(middleware::from_fn(json_errors))
+        .layer(middleware::from_fn(relative_redirects))
         .with_state(state)
 }
 
@@ -107,6 +111,7 @@ pub(crate) fn relay_only_router(
         .layer(middleware::from_fn_with_state(auth_state, basic_auth))
         .route("/health", get(health::health))
         .layer(middleware::from_fn(json_errors))
+        .layer(middleware::from_fn(relative_redirects))
         .with_state(relay)
 }
 
@@ -131,6 +136,56 @@ pub(crate) fn error_response(
     (status, Json(body)).into_response()
 }
 
+/// Redirects to a path of this API (`Location: /store/…`) made relative to the request's
+/// (`3`, `swagger-ui/`): they then lead to the same place behind a reverse proxy
+/// that serves the API under a path of its own (`/continuum/store/…`).
+async fn relative_redirects(request: Request, next: Next) -> Response {
+    let from = request.uri().path().to_owned();
+    let mut response = next.run(request).await;
+    let relative = response
+        .headers()
+        .get(axum::http::header::LOCATION)
+        .and_then(|location| location.to_str().ok())
+        .filter(|location| location.starts_with('/') && !location.starts_with("//"))
+        .map(|location| relative_to(&from, location));
+    if let Some(location) = relative.and_then(|location| HeaderValue::from_str(&location).ok()) {
+        response
+            .headers_mut()
+            .insert(axum::http::header::LOCATION, location);
+    }
+    response
+}
+
+/// `to` (a path, with its query if any) as a reference relative to the page at `from`
+/// (a path).
+fn relative_to(from: &str, to: &str) -> String {
+    let (path, query) = match to.split_once('?') {
+        Some((path, query)) => (path, Some(query)),
+        None => (to, None),
+    };
+    // The page's directory, and the target's segments (the last its name).
+    let from: Vec<&str> = from.split('/').collect();
+    let directory = &from[..from.len() - 1];
+    let target: Vec<&str> = path.split('/').collect();
+    let common = directory
+        .iter()
+        .zip(&target[..target.len() - 1])
+        .take_while(|(a, b)| a == b)
+        .count();
+    let mut relative = "../".repeat(directory.len() - common);
+    relative.push_str(&target[common..].join("/"));
+    // Not empty, and not taken for a scheme (`a:b`).
+    let first = relative.split('/').next().unwrap_or("");
+    if relative.is_empty() || first.contains(':') {
+        relative.insert_str(0, "./");
+    }
+    if let Some(query) = query {
+        relative.push('?');
+        relative.push_str(query);
+    }
+    relative
+}
+
 /// Gives every error answer the same body (`error_response`). Handlers that fail with
 /// a bare status or a text message, and axum's own rejections (a malformed query or
 /// body), get it here: the text becomes the message, or the status's reason if there
@@ -236,3 +291,21 @@ pub(crate) fn store_status(error: StoreError, context: &str) -> ApiError {
 pub(crate) fn media_type(value: &str) -> &str {
     value.split(';').next().unwrap_or("").trim()
 }
+
+#[cfg(test)]
+mod redirect_tests {
+    use super::relative_to;
+
+    #[test]
+    fn redirects_are_relative_to_the_request() {
+        assert_eq!(relative_to("/store/queue/-1", "/store/queue/3"), "3");
+        assert_eq!(
+            relative_to("/store/a/-1/b", "/store/a/0/b?x=1"),
+            "../0/b?x=1"
+        );
+        assert_eq!(relative_to("/store/-1", "/store/2"), "2");
+        assert_eq!(relative_to("/swagger-ui", "/swagger-ui/"), "swagger-ui/");
+        assert_eq!(relative_to("/store/a/b", "/store/a/"), "./");
+        assert_eq!(relative_to("/store/x", "/store/a:b"), "./a:b");
+    }
+}