Use an HTTP redirect for same-origin requests

The meta refresh page renders as a broken image when the request came
from an <img> on one of our own pages. Such requests are already past
the redirect-loop hazard the refresh works around, so send them a real
redirect instead.

2 files changed, 29 insertions, 1 deletions

diff --git a/cgithub/src/app.test.ts b/cgithub/src/app.test.ts
index 5474f8d..59ed352 100644
--- a/cgithub/src/app.test.ts
+++ b/cgithub/src/app.test.ts
@@ -110,6 +110,26 @@ describe("redirects to GitHub", () => {
     assert.doesNotMatch(body, /<script/);
   });
 
+  // Same-origin requests are mostly subresources, which render an HTML page as
+  // a broken image rather than following its refresh.
+  it("should redirect over HTTP when the referer is one of our own pages", async () => {
+    const res = await app.request("http://cgithub.example/a/b/c?x=1", {
+      headers: { Referer: "http://cgithub.example/a/b" },
+    });
+
+    assert.strictEqual(res.status, 302);
+    assert.strictEqual(res.headers.get("location"), "https://github.com/a/b/c?x=1");
+  });
+
+  it("should use a meta refresh when the referer is another site", async () => {
+    const res = await app.request("http://cgithub.example/a/b/c?x=1", {
+      headers: { Referer: "https://github.example/a/b" },
+    });
+
+    assert.strictEqual(res.status, 200);
+    assert.strictEqual(res.headers.get("location"), null);
+  });
+
   it("should redirect unhandled search types", async () => {
     const res = await app.request(
       "http://cgithub.example/actions/deploy-pages/search?q=x&type=code",
diff --git a/cgithub/src/app.ts b/cgithub/src/app.ts
index e27db59..9781a98 100644
--- a/cgithub/src/app.ts
+++ b/cgithub/src/app.ts
@@ -29,8 +29,16 @@ export function createApp(eta: Eta) {
   const app = new Hono();
 
   // Use a meta refresh to avoid redirect loops in certain situations; we become
-  // the initiator origin even if we are the target of a redirection.
+  // the initiator origin even if we are the target of a redirection. Requests
+  // coming from one of our own pages are already past that hazard, and are
+  // often subresources (an <img> in a rendered README) that can't do anything
+  // with an HTML page, so those get a real HTTP redirect.
   function redirectToGitHub(c: Context, location: string) {
+    const referer = c.req.header("Referer");
+    if (referer && URL.parse(referer)?.origin === new URL(c.req.url).origin) {
+      return c.redirect(location);
+    }
+
     c.header("Referrer-Policy", "no-referrer");
     return c.html(eta.render("redirect.eta", { location }));
   }