Test four README settings pixa does not have yet (closes #61)

Failing tests for access_control_allow_origin, upstream_fetch_timeout,
upstream_max_response_size and downstream_timeout: their defaults,
valid values from the file and the environment, invalid values aborting
startup naming the key or variable and the value, the CORS middleware
answering with the configured origin, and the server's write timeout
coming from downstream_timeout. They do not compile until the settings
exist.

Model: opus-5-5

internal/middleware/middleware_internal_test.go

@@ -9,6 +9,53 @@ import (
 	"sneak.berlin/go/pixa/internal/config"
 )
 
+// TestCORSAnswersWithConfiguredOrigin checks that the CORS middleware
+// uses access_control_allow_origin: "*" lets any origin read responses,
+// and a single origin lets that origin read them and no other.
+func TestCORSAnswersWithConfiguredOrigin(t *testing.T) {
+	t.Parallel()
+
+	const appOrigin = "https://app.example.com"
+
+	cases := []struct {
+		configured    string
+		requestOrigin string
+		want          string
+	}{
+		{"*", "https://any.example.com", "*"},
+		{appOrigin, appOrigin, appOrigin},
+		{appOrigin, "https://other.example.com", ""},
+	}
+
+	testHandler := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
+	for _, tc := range cases {
+		mw := &Middleware{
+			log:    slog.Default(),
+			config: &config.Config{AccessControlAllowOrigin: tc.configured},
+		}
+
+		handler := mw.CORS()(testHandler)
+
+		req := httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, "/v1/image/example.com/a.jpg/1x1.png", nil)
+		req.Header.Set("Origin", tc.requestOrigin)
+
+		rec := httptest.NewRecorder()
+
+		handler.ServeHTTP(rec, req)
+
+		got := rec.Header().Get("Access-Control-Allow-Origin")
+		if got != tc.want {
+			t.Errorf("configured %q, request from %q: "+
+				"Access-Control-Allow-Origin = %q, want %q",
+				tc.configured, tc.requestOrigin, got, tc.want)
+		}
+	}
+}
+
 func TestSecurityHeaders(t *testing.T) {
 	t.Parallel()