skip header rules on asset handler error

.changeset/stale-books-greet.md

@@ -0,0 +1,8 @@
+---
+"@cloudflare/pages-shared": patch
+---
+
+fix: omit headers rules on internal error
+
+The Pages asset handler will no longer apply headers rules on 500 responses caused by some internal error.
+This prevents transient errors from being cached when caching headers are being set by headers rules.

packages/pages-shared/__tests__/asset-server/handler.test.ts

@@ -774,6 +774,36 @@ describe("asset-server handler", () => {
 			expect(isPreservationCacheResponseExpiring(res)).toBe(true);
 		});
 	});
+
+	test("internal asset error doesn't set headers", async () => {
+		const metadata = createMetadataObject({
+			deploymentId: "mock-deployment-id",
+			headers: {
+				invalid: [],
+				rules: [
+					{
+						path: "/*",
+						headers: {"x-unwanted-header": "foo"},
+						unsetHeaders: [],
+					},
+				],
+			}
+		}) as Metadata;
+
+		const { response } = await getTestResponse({
+			request: "https://foo.com/",
+			metadata,
+			fetchAsset: async (...args) => {
+				throw "uh oh";
+			},
+			findAssetEntryForPath: async (path: string) => {
+				return "some page";
+			},
+		});
+
+		expect(response.status).toBe(500);
+		expect(Object.fromEntries(response.headers)).not.toHaveProperty("x-unwanted-header");
+	});
 });
 
 interface HandlerSpies {

packages/pages-shared/asset-server/handler.ts

@@ -446,7 +446,11 @@ export async function generateHandler<
 				};
 			}
 		);
-		const matches = headersMatcher({ request });
+
+		// If the response was an internal error from the asset handler, skip applying header rules.
+		// This avoids a case where we can cache 500s for a long time, depending on headers set by a user.
+		const assetError = response.status >= 400 && response.status <= 599
+		const matches = assetError ? [] : headersMatcher({ request });
 
 		// This keeps track of every header that we've set from _headers
 		// because we want to combine user declared headers but overwrite