1 files changed, 60 insertions, 4 deletions
diff --git a/src/api/mod.rs b/src/api/mod.rs
index bec7764..b92f421 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -10,15 +10,17 @@ mod store;
 
 use axum::{
     Json, Router,
-    extract::State,
+    body::Body,
+    extract::{Request, State},
     http::{
         HeaderValue, StatusCode,
-        header::{CONTENT_TYPE, ETAG},
+        header::{CONTENT_LENGTH, CONTENT_TYPE, ETAG},
     },
+    middleware::{self, Next},
     response::{IntoResponse, Response},
     routing::{delete, get, put},
 };
-use serde_json::Value as JsonValue;
+use serde_json::{Value as JsonValue, json};
 use sha2::{Digest, Sha256};
 use tracing::{error, warn};
 use utoipa::OpenApi;
@@ -61,11 +63,65 @@ pub(crate) fn router(state: AppState, auth_state: AuthState, debug_api: bool) ->
             .route("/debug/blacklist", put(debug::put_debug_blacklist));
     }
 
-    api.layer(axum::middleware::from_fn_with_state(auth_state, basic_auth))
+    api.layer(middleware::from_fn_with_state(auth_state, basic_auth))
         .merge(SwaggerUi::new("/swagger-ui").url("/openapi.json", ApiDoc::openapi()))
+        // Outermost, so authentication failures get the error body too.
+        .layer(middleware::from_fn(json_errors))
         .with_state(state)
 }
 
+/// An error answer: `{"error": message}` with `details`' fields added (`docs/openapi.yaml`,
+/// `Error`).
+pub(crate) fn error_response(
+    status: StatusCode,
+    message: impl Into<String>,
+    details: Option<JsonValue>,
+) -> Response {
+    let mut body = json!({ "error": message.into() });
+    if let (Some(JsonValue::Object(extra)), Some(object)) = (details, body.as_object_mut()) {
+        object.extend(extra);
+    }
+    (status, Json(body)).into_response()
+}
+
+/// 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
+/// is none. Errors that are JSON already pass unchanged, and so do their headers
+/// (`WWW-Authenticate` on a 401).
+async fn json_errors(request: Request, next: Next) -> Response {
+    let response = next.run(request).await;
+    let status = response.status();
+    if !(status.is_client_error() || status.is_server_error()) {
+        return response;
+    }
+    let is_json = response
+        .headers()
+        .get(CONTENT_TYPE)
+        .and_then(|value| value.to_str().ok())
+        .is_some_and(|value| value.starts_with("application/json"));
+    if is_json {
+        return response;
+    }
+    let (mut parts, body) = response.into_parts();
+    // Error texts are short; a larger one is cut to its reason.
+    let text = axum::body::to_bytes(body, 64 * 1024)
+        .await
+        .map(|bytes| String::from_utf8_lossy(&bytes).trim().to_owned())
+        .unwrap_or_default();
+    let message = if text.is_empty() {
+        status.canonical_reason().unwrap_or("error").to_lowercase()
+    } else {
+        text
+    };
+    let body = json!({ "error": message }).to_string();
+    parts.headers.remove(CONTENT_LENGTH);
+    parts
+        .headers
+        .insert(CONTENT_TYPE, HeaderValue::from_static("application/json"));
+    Response::from_parts(parts, Body::from(body))
+}
+
 /// `GET /admin/config`: the effective configuration with each value's source;
 /// imported values are left out.
 async fn get_config(State(state): State<AppState>) -> Json<JsonValue> {