fetch can't connect to HTTP/2-only servers

[andreubotella]

This came up when working on #12472, since WPT's HTTP/2 server on port 9000 is HTTP/2-only. It also seems to be the cause of #12208.

Here's a test case – run it on a checkout of the Deno repo.

import { runWithTestUtil } from "./tools/wpt/runner.ts";

// `runWithTestUtil` starts the WPT server
await runWithTestUtil(false, async () => {
  const client = Deno.createHttpClient({
    caCerts: [
      await Deno.readTextFile("./tools/wpt/certs/cacert.pem"),
    ]
  });
  const res = await fetch("https://web-platform.test:9000", { client });
  console.log(await res.text());
});

error: Uncaught (in promise) TypeError: error sending request for url (https://web-platform.test:9000/): invalid HTTP version parsed
    at async mainFetch (deno:ext/fetch/26_fetch.js:280:14)

---

reqwest with rustls does support connecting to HTTP/2-only servers, as shown by the following Rust sample:

#[tokio::main]
async fn main() -> Result<()> {
  let cert = fs::read("tools/wpt/certs/cacert.pem")?;
  let client = ClientBuilder::new()
    .add_root_certificate(Certificate::from_pem(&cert)?)
    .build()?;
  let response = client.get("https://web-platform.test:9000").send().await?;

  println!("HTTP version: {:?}", response.version());
  println!("{}", response.text().await?);
  Ok(())
}

The configuration of reqwest that Deno uses for fetching is more complicated than this, however, and I'm not sure if this is a bug with Deno's configuration, or a bug with reqwest.

[lucacasonato]

@andreubotella What happens if you set http2_prior_knowledge on the client builder?

[andreubotella]

@andreubotella What happens if you set http2_prior_knowledge on the client builder?

The fetch does succeed if you do that. But of course, that's not something that can be enabled by default (although maybe it could be an option in Deno.createHttpClient(), alongside http1_only).

[lucacasonato]

The fetch does succeed if you do that. But of course, that's not something that can be enabled by default (although maybe it could be an option in Deno.createHttpClient(), alongside http1_only).

Sure. That suggests the issue stems from ALPN.

[andreubotella]

This also affects servers which support both HTTP/1.1 and HTTP/2, since fetch() will connect only to HTTP/1.1 even if h2 is earlier in the server's ALPN list.

It seems like this regressed in #11491.