fix: prevent CORS header duplication on middleware early-return responses

The 0.82.0 performance commit changed the response finalization path so
that global response headers are unconditionally `extend`ed onto every
response, including early-return responses from before-middleware. When
ALLOW_CORS is used, this causes Access-Control-Allow-Origin (and other
CORS headers) to appear twice — once from the middleware and once from
the global defaults. Per the Fetch spec, browsers reject responses with
duplicate Access-Control-Allow-Origin values, breaking preflight.

Fix: replace `extend` (append) with `set_missing` (set-if-absent) when
merging global response headers. This gives middleware-set headers
precedence over global defaults, preventing duplication.

Closes #1346

Made-with: Cursor

Author: sansyrox

integration_tests/cors_app.py

@@ -0,0 +1,39 @@
+"""
+Standalone Robyn app with ALLOW_CORS enabled, used by CORS integration tests.
+Runs on a separate port (8083) so it doesn't conflict with base_routes.py.
+"""
+
+import os
+
+from robyn import ALLOW_CORS, Robyn
+
+app = Robyn(__file__)
+
+ALLOWED_ORIGINS = ["http://localhost:3000", "https://frontend.example.com"]
+ALLOW_CORS(app, origins=ALLOWED_ORIGINS)
+
+
+@app.get("/")
+def index():
+    return "OK"
+
+
+@app.post("/data")
+def post_data(request):
+    return "created"
+
+
+@app.get("/custom-header")
+def custom_header(request):
+    from robyn import Response
+
+    return Response(
+        status_code=200,
+        headers={"x-custom": "hello"},
+        description="custom",
+    )
+
+
+if __name__ == "__main__":
+    port = int(os.getenv("ROBYN_PORT", "8083"))
+    app.start(port=port, _check_port=False)

integration_tests/test_cors_preflight.py

@@ -0,0 +1,169 @@
+"""
+Integration tests for CORS preflight (OPTIONS) handling.
+
+Regression test for https://github.com/sparckles/robyn/issues/1346
+These tests spin up a real Robyn server with ALLOW_CORS enabled and make
+actual HTTP requests — no TestClient.
+"""
+
+import os
+import pathlib
+import socket
+import subprocess
+import time
+
+import pytest
+import requests
+
+CORS_PORT = 8083
+CORS_HOST = "127.0.0.1"
+CORS_BASE_URL = f"http://{CORS_HOST}:{CORS_PORT}"
+
+ALLOWED_ORIGIN = "http://localhost:3000"
+DISALLOWED_ORIGIN = "http://evil.example.com"
+
+
+def _start_cors_server():
+    app_path = os.path.join(pathlib.Path(__file__).parent.resolve(), "cors_app.py")
+    env = os.environ.copy()
+    env["ROBYN_HOST"] = CORS_HOST
+    env["ROBYN_PORT"] = str(CORS_PORT)
+
+    process = subprocess.Popen(
+        ["python3", app_path],
+        env=env,
+        preexec_fn=os.setsid,
+    )
+
+    timeout = 15
+    start = time.time()
+    while True:
+        if time.time() - start > timeout:
+            os.killpg(os.getpgid(process.pid), 9)
+            raise ConnectionError(f"CORS server didn't start on {CORS_HOST}:{CORS_PORT}")
+        try:
+            sock = socket.create_connection((CORS_HOST, CORS_PORT), timeout=2)
+            sock.close()
+            break
+        except Exception:
+            time.sleep(0.5)
+
+    time.sleep(1)
+    return process
+
+
+@pytest.fixture(scope="module")
+def cors_server():
+    process = _start_cors_server()
+    yield
+    import signal
+
+    try:
+        os.killpg(os.getpgid(process.pid), signal.SIGKILL)
+    except ProcessLookupError:
+        pass
+
+
+# ---------------------------------------------------------------------------
+# Preflight (OPTIONS) tests
+# ---------------------------------------------------------------------------
+
+
+def test_options_preflight_returns_204_with_cors_headers(cors_server):
+    """Browser sends OPTIONS preflight; server must return 204 with all CORS headers."""
+    resp = requests.options(
+        f"{CORS_BASE_URL}/data",
+        headers={
+            "Origin": ALLOWED_ORIGIN,
+            "Access-Control-Request-Method": "POST",
+            "Access-Control-Request-Headers": "Content-Type, Authorization",
+        },
+    )
+    assert resp.status_code == 204
+    assert resp.headers["Access-Control-Allow-Origin"] == ALLOWED_ORIGIN
+    assert "POST" in resp.headers["Access-Control-Allow-Methods"]
+    assert resp.headers["Access-Control-Allow-Credentials"] == "true"
+
+
+def test_options_preflight_no_duplicate_allow_origin(cors_server):
+    """
+    Regression: ensure Access-Control-Allow-Origin appears exactly once.
+    Duplicate values cause browsers to reject the preflight (Fetch spec).
+    """
+    resp = requests.options(
+        f"{CORS_BASE_URL}/",
+        headers={
+            "Origin": ALLOWED_ORIGIN,
+            "Access-Control-Request-Method": "GET",
+        },
+    )
+    raw_headers = resp.raw.headers if hasattr(resp.raw, "headers") else resp.headers
+
+    origin_values = raw_headers.getall("Access-Control-Allow-Origin", None) if hasattr(raw_headers, "getall") else None
+
+    if origin_values is not None:
+        assert len(origin_values) == 1, (
+            f"Access-Control-Allow-Origin appeared {len(origin_values)} times: {origin_values}"
+        )
+
+    assert resp.headers["Access-Control-Allow-Origin"] == ALLOWED_ORIGIN
+
+
+def test_options_preflight_disallowed_origin_returns_403(cors_server):
+    """Origins not in the allowed list should be rejected."""
+    resp = requests.options(
+        f"{CORS_BASE_URL}/data",
+        headers={
+            "Origin": DISALLOWED_ORIGIN,
+            "Access-Control-Request-Method": "POST",
+        },
+    )
+    assert resp.status_code == 403
+
+
+# ---------------------------------------------------------------------------
+# Normal request tests (non-preflight)
+# ---------------------------------------------------------------------------
+
+
+def test_get_with_allowed_origin_has_cors_headers(cors_server):
+    """Normal GET from an allowed origin should carry CORS response headers."""
+    resp = requests.get(
+        f"{CORS_BASE_URL}/",
+        headers={"Origin": ALLOWED_ORIGIN},
+    )
+    assert resp.status_code == 200
+    allow_origin = resp.headers.get("Access-Control-Allow-Origin")
+    assert allow_origin is not None
+    assert "Access-Control-Allow-Methods" in resp.headers
+
+
+def test_get_without_origin_still_has_global_cors_headers(cors_server):
+    """Requests without Origin (e.g. curl, Postman) should still get global CORS headers."""
+    resp = requests.get(f"{CORS_BASE_URL}/")
+    assert resp.status_code == 200
+    assert "Access-Control-Allow-Methods" in resp.headers
+
+
+def test_post_with_allowed_origin(cors_server):
+    """POST from allowed origin should succeed with CORS headers."""
+    resp = requests.post(
+        f"{CORS_BASE_URL}/data",
+        headers={
+            "Origin": ALLOWED_ORIGIN,
+            "Content-Type": "application/json",
+        },
+        data="{}",
+    )
+    assert resp.status_code == 200
+
+
+def test_custom_response_headers_not_clobbered_by_globals(cors_server):
+    """Route-level headers set by the handler should not be overwritten by globals."""
+    resp = requests.get(
+        f"{CORS_BASE_URL}/custom-header",
+        headers={"Origin": ALLOWED_ORIGIN},
+    )
+    assert resp.status_code == 200
+    assert resp.headers.get("x-custom") == "hello"
+    assert "Access-Control-Allow-Methods" in resp.headers

src/server.rs

@@ -575,7 +575,7 @@ async fn index(
         .is_some_and(|paths| paths.contains(&request.url.path));
 
     if !is_excluded {
-        response.headers_mut().extend(&global_response_headers);
+        response.headers_mut().set_missing(&global_response_headers);
     }
 
     // After middleware

src/types/headers.rs

@@ -177,6 +177,16 @@ impl Headers {
         }
     }
 
+    /// Merge headers from `headers` into `self`, but only for keys not already present.
+    /// This gives middleware-set headers precedence over global defaults,
+    /// preventing duplicate `Access-Control-Allow-Origin` (and similar) violations.
+    pub fn set_missing(&mut self, headers: &Headers) {
+        for iter in headers.headers.iter() {
+            let (key, values) = iter.pair();
+            self.headers.entry(key.clone()).or_insert_with(|| values.clone());
+        }
+    }
+
     pub fn from_actix_headers(req_headers: &HeaderMap) -> Self {
         let headers = Headers::default();