fix: rewrite posthog requests to pass in full header context (#1431)

packages/ui/docs-bundle/next.config.js

@@ -50,6 +50,7 @@ const nextConfig = {
         optimizePackageImports: ["@fern-ui/ui"],
     },
     trailingSlash: isTruthy(process.env.TRAILING_SLASH),
+    skipTrailingSlashRedirect: true,
     /**
      * Customers who opt-in for subpath routing must use rewrite rules from their hosting provider. Because of the
      * multi-tenant nature of this app, we cannot set a global basepath in the next.config.js. As a result, the `_next`

packages/ui/docs-bundle/src/middleware.ts

@@ -0,0 +1,13 @@
+import { NextResponse, type NextRequest } from "next/server";
+import { rewritePosthog } from "./utils/rewritePosthog";
+
+export function middleware(request: NextRequest): NextResponse {
+    if (request.nextUrl.pathname.includes("/api/fern-docs/analytics/posthog")) {
+        return rewritePosthog(request);
+    }
+    return NextResponse.next();
+}
+
+export const config = {
+    matcher: ["/api/fern-docs/analytics/posthog/:path*", "/:prefix*/api/fern-docs/analytics/posthog/:path*"],
+};

packages/ui/docs-bundle/src/utils/rewritePosthog.ts

@@ -0,0 +1,31 @@
+import { withDefaultProtocol } from "@fern-ui/core-utils";
+import { NextResponse, type NextRequest } from "next/server";
+
+const DEFAULT_POSTHOG_INSTANCE = "https://us.i.posthog.com";
+
+export function rewritePosthog(request: NextRequest): NextResponse {
+    const url = request.nextUrl.clone();
+
+    const destination = new URL(
+        withDefaultProtocol(request.headers.get("x-fern-posthog-host") ?? DEFAULT_POSTHOG_INSTANCE),
+    );
+
+    const [, intendedPathname = "/"] = request.nextUrl.pathname.split("/api/fern-docs/analytics/posthog");
+
+    const hostname =
+        intendedPathname.startsWith("/static/") && destination.hostname.endsWith(".i.posthog.com")
+            ? destination.hostname.replace(".i.posthog.com", "-assets.i.posthog.com") // i.e. us.i.posthog.com -> us-assets.i.posthog.com
+            : destination.hostname;
+
+    const requestHeaders = new Headers(request.headers);
+    requestHeaders.set("host", hostname);
+
+    url.pathname = intendedPathname;
+    url.protocol = "https";
+    url.hostname = hostname;
+    url.port = "443";
+
+    return NextResponse.rewrite(url, {
+        headers: requestHeaders,
+    });
+}