fix(ext/http): skip auto-compression if content-encoding present (#14641)

Regression from #14552

cli/tests/unit/http_test.ts

@@ -1978,6 +1978,64 @@ Deno.test({
   },
 });
 
+Deno.test({
+  name: "http server custom content-encoding is left untouched",
+  permissions: { net: true, run: true },
+  async fn() {
+    const hostname = "localhost";
+    const port = 4501;
+    let contentLength: string;
+
+    async function server() {
+      const listener = Deno.listen({ hostname, port });
+      const tcpConn = await listener.accept();
+      const httpConn = Deno.serveHttp(tcpConn);
+      const e = await httpConn.nextRequest();
+      assert(e);
+      const { request, respondWith } = e;
+      assertEquals(request.headers.get("Accept-Encoding"), "deflate, gzip");
+      const body = new Uint8Array([3, 1, 4, 1]);
+      contentLength = String(body.length);
+      const response = new Response(
+        body,
+        {
+          headers: {
+            "content-length": contentLength,
+            "content-encoding": "arbitrary",
+          },
+        },
+      );
+      await respondWith(response);
+      httpConn.close();
+      listener.close();
+    }
+
+    async function client() {
+      const url = `http://${hostname}:${port}/`;
+      const cmd = [
+        "curl",
+        "-i",
+        "--request",
+        "GET",
+        "--url",
+        url,
+        // "--compressed", // Windows curl does not support --compressed
+        "--header",
+        "Accept-Encoding: deflate, gzip",
+      ];
+      const proc = Deno.run({ cmd, stdout: "piped", stderr: "null" });
+      const status = await proc.status();
+      assert(status.success);
+      const output = decoder.decode(await proc.output());
+      assert(output.includes("vary: Accept-Encoding\r\n"));
+      assert(output.includes("content-encoding: arbitrary\r\n"));
+      proc.close();
+    }
+
+    await Promise.all([server(), client()]);
+  },
+});
+
 Deno.test("upgradeHttp tcp", async () => {
   async function client() {
     const tcpConn = await Deno.connect({ port: 4501 });

ext/http/lib.rs

@@ -680,8 +680,11 @@ fn should_compress(headers: &hyper::HeaderMap) -> bool {
   // indicates the contents of the body were negotiated based directly
   // with the user code and we can't compress the response
   let content_range = headers.contains_key(hyper::header::CONTENT_RANGE);
+  // assume body is already compressed if Content-Encoding header present, thus avoid recompressing
+  let is_precompressed = headers.contains_key(hyper::header::CONTENT_ENCODING);
 
   !content_range
+    && !is_precompressed
     && !cache_control_no_transform(headers).unwrap_or_default()
     && headers
       .get(hyper::header::CONTENT_TYPE)