chore: import upstream snapshot with attribution

apps/webapp/app/routes/ph.$.ts

@@ -0,0 +1,99 @@
+import { type ActionFunctionArgs, type LoaderFunctionArgs } from "@remix-run/server-runtime";
+import { env } from "~/env.server";
+import { getRequestAbortSignal } from "~/services/httpAsyncStorage.server";
+import { logger } from "~/services/logger.server";
+
+// Same-origin reverse proxy for PostHog. posthog-js sets `api_host: "/ph"`, so
+// analytics is served first-party and forwarded to PostHog Cloud server-side.
+// Asset requests (recorder script, array bundles) go to the assets host and
+// everything else (ingest, feature flags, session recording) to the ingest
+// host, as PostHog's reverse-proxy docs require. Streaming and header handling
+// mirror the Electric proxy in longPollingFetch.ts.
+
+const PH_PREFIX = "/ph";
+
+function isAssetPath(upstreamPath: string): boolean {
+  return upstreamPath.startsWith("/static/") || upstreamPath.startsWith("/array/");
+}
+
+async function proxyToPostHog(request: Request): Promise<Response> {
+  const url = new URL(request.url);
+
+  const upstreamPath = url.pathname.slice(PH_PREFIX.length) || "/";
+  const hostname = isAssetPath(upstreamPath) ? env.POSTHOG_ASSETS_HOST : env.POSTHOG_INGEST_HOST;
+  const upstreamUrl = `https://${hostname}${upstreamPath}${url.search}`;
+
+  const headers = new Headers(request.headers);
+  // PostHog routes on Host, so point it at the upstream. accept-encoding is
+  // dropped so we don't get a compressed body we'd have to re-describe.
+  headers.set("host", hostname);
+  headers.delete("accept-encoding");
+
+  // /ph is same-origin, so the browser sends every first-party cookie. Forward
+  // only PostHog's own so we never leak the app session cookie to a third party.
+  const cookie = headers.get("cookie");
+  if (cookie) {
+    const forwarded = cookie
+      .split(";")
+      .filter((c) => {
+        const name = c.trimStart().split("=", 1)[0];
+        return name.startsWith("ph_") || name.startsWith("__ph");
+      })
+      .join(";");
+
+    if (forwarded) {
+      headers.set("cookie", forwarded);
+    } else {
+      headers.delete("cookie");
+    }
+  }
+
+  const hasBody = request.method !== "GET" && request.method !== "HEAD";
+
+  // `duplex` isn't in the DOM RequestInit lib types but undici needs it to
+  // stream a request body; widen the type rather than suppress.
+  const init: RequestInit & { duplex?: "half" } = {
+    method: request.method,
+    headers,
+    body: hasBody ? request.body : undefined,
+    signal: getRequestAbortSignal(),
+    duplex: hasBody ? "half" : undefined,
+  };
+
+  let upstream: Response | undefined;
+  try {
+    upstream = await fetch(upstreamUrl, init);
+
+    // Strip encoding headers that can misdescribe the proxied body (see longPollingFetch).
+    const responseHeaders = new Headers(upstream.headers);
+    responseHeaders.delete("content-encoding");
+    responseHeaders.delete("content-length");
+
+    return new Response(upstream.body, {
+      status: upstream.status,
+      statusText: upstream.statusText,
+      headers: responseHeaders,
+    });
+  } catch (error) {
+    try {
+      await upstream?.body?.cancel();
+    } catch {}
+
+    if (error instanceof Error && error.name === "AbortError") {
+      throw new Response(null, { status: 499 });
+    }
+
+    logger.error("[posthog-proxy] fetch error", {
+      error: error instanceof Error ? error.message : String(error),
+    });
+    throw new Response("PostHog proxy error", { status: 502 });
+  }
+}
+
+export async function loader({ request }: LoaderFunctionArgs) {
+  return proxyToPostHog(request);
+}
+
+export async function action({ request }: ActionFunctionArgs) {
+  return proxyToPostHog(request);
+}