Compression across the board: directory answers, uploads, metrics, traces, the page (#253)

Measured 2026-10-16, uncompressed until now: the directory's answers (the feed list ipx reads,
603 KB of JSON, ~134 ms of a 159 ms Directory request), ipx's snapshot upload (~100 KB each half
hour), both /metrics (4.7 and 0.9 KB every 15 s) and every trace export to Tempo. Already
compressed: ipx-web's answers (zstd, #239), the home page and the directory's public pages
through Cloudflare (brotli, zstd).

- The directory compresses with brotli, zstd or gzip as asked, and unpacks a gzipped request.
- ipx sends its snapshot gzipped; the directory is deployed first.
- Both /metrics answer gzipped, as Prometheus asks.
- opentelemetry-otlp's gzip-http: OTEL_EXPORTER_OTLP_COMPRESSION=gzip compresses the export, set
  for all three in production's compose file; Caddy's exporter reads the same variable.
- web/build.mjs writes .br (quality 11) and .gz (level 9) beside each text file, and Caddy serves
  them in place (file_server precompressed): app.js 30 KB where zstd on the fly gave 37, app.css
  11 where 14. A .br or .gz asked for by name is refused, or /admin.js.br would be the admin
  page's script without the check that keeps it to admins; in a handle of its own, since a bare
  respond runs after the handles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/directory/src/lib.rs

@@ -268,6 +268,12 @@ pub fn router(dir: Shared) -> Router {
         .merge(v1)
         // Anyone may browse, no token, read only (#215).
         .merge(site::router())
+        // Brotli, zstd or gzip as the caller takes them: the feed list went to ipx as 603 KB of
+        // JSON, most of a Directory page's time (#253). Inside the access log, which records what
+        // was sent.
+        .layer(tower_http::compression::CompressionLayer::new().br(true).zstd(true).gzip(true))
+        // And takes a request's body gzipped: ipx sends its snapshot so, about 100 KB as JSON.
+        .layer(tower_http::decompression::RequestDecompressionLayer::new().gzip(true))
         .layer(axum::middleware::from_fn(access_log))
         .with_state(dir)
 }
@@ -477,6 +483,45 @@ mod tests {
         (status, serde_json::from_slice(&bytes).unwrap_or(serde_json::Value::Null))
     }
 
+    #[tokio::test]
+    async fn a_snapshot_may_come_gzipped() {
+        use std::io::Write as _;
+        let (app, _) = app().await;
+        let json = serde_json::json!([{"url": "https://example.com/feed.xml", "subscribers": 2}]).to_string();
+        let mut gz = flate2::write::GzEncoder::new(Vec::new(), flate2::Compression::default());
+        gz.write_all(json.as_bytes()).unwrap();
+        // As ipx sends it (#253).
+        let req = axum::http::Request::builder()
+            .method("POST")
+            .uri("/v1/snapshot")
+            .header("authorization", "Bearer secret-rw")
+            .header("content-type", "application/json")
+            .header("content-encoding", "gzip")
+            .body(Body::from(gz.finish().unwrap()))
+            .unwrap();
+        let resp = app.clone().oneshot(req).await.unwrap();
+        assert_eq!(resp.status(), 200);
+        let got: serde_json::Value = serde_json::from_slice(&axum::body::to_bytes(resp.into_body(), 1 << 20).await.unwrap()).unwrap();
+        assert_eq!(got["accepted"], 1);
+    }
+
+    #[tokio::test]
+    async fn answers_come_compressed_as_the_caller_asks() {
+        let (app, _) = app().await;
+        for (asked, sent) in [("br", Some("br")), ("zstd", Some("zstd")), ("gzip", Some("gzip")), ("identity", None)] {
+            // A page, not an empty feed list: under 32 bytes is sent as it is.
+            let req = axum::http::Request::builder()
+                .uri("/")
+                .header("accept-encoding", asked)
+                .body(Body::empty())
+                .unwrap();
+            let resp = app.clone().oneshot(req).await.unwrap();
+            assert_eq!(resp.status(), 200);
+            let got = resp.headers().get("content-encoding").map(|v| v.to_str().unwrap().to_owned());
+            assert_eq!(got.as_deref(), sent, "asked for {asked} (#253)");
+        }
+    }
+
     async fn get_page(app: &Router, path: &str) -> (u16, String, Option<String>) {
         let req = axum::http::Request::builder().uri(path).body(Body::empty()).unwrap();
         let resp = app.clone().oneshot(req).await.unwrap();