TCP mode - forward request to different backends based on substring (without delay)

What I’d like:

• one frontend section (e.g. bound to port 80)
• two backends
  • the first one is “static files” served over HTTP
  • the second one is a TCP response generation service that let’s users generate responses based on HTTP request, e.g. ANYTHING /response/{base64-encoded response} - mostly used for testing HTTP responses, but can respond with any arbitrary bytes not conforming to HTTP (so a raw TCP stream) to test out clients handling faulty “HTTP” responses

Problem:

• how to proxy requests based on the magic string /response/ - if it is encountered in e.g. the first 50 bytes of the TCP stream, instantly use the response generation backend; otherwise, serve static files from the first backend
• the overall request (especially for the service) may be shorter than 50 bytes
• it should be as fast as possible, but also deal with somewhat slower clients or lost packets (e.g. TCP handshake made, but some time to get the first data packets flowing in)

Current test set-up - I can get the needed condition working with either an arbitrary delay (not wanted! do it as fast as possible, but have a timeout) or “reject/accept” instantly, but then no choice can be made between backends.

haproxy.cfg:

global
    log stdout format raw local0

defaults
    mode tcp
    timeout connect 28s
    timeout client 29s
    timeout server 30s

frontend fe_main
    log global
    option tcplog
    
    bind :80

    tcp-request inspect-delay 3s

    # OPTION 1: it works, but delay is necessary (otherwise acl not enforced)
    tcp-request content accept if WAIT_END
    acl contains_bbb req.payload(0,0) -m sub BBB
    use_backend be_data if contains_bbb

    # (Almost an) OPTION 2: it works instantly, but can't choose backends this way
    # tcp-request content reject if { req.payload(0,0) -m sub AAA }

    default_backend be_static

backend be_data
    server server1 data_backend:9001

backend be_static
    mode http
    server server1 static_backend:9002

docker-compose.yml:

services:
  haproxy:
    image: haproxy:3.0
    volumes:
      - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro
    ports:
      - "80:80"
    depends_on:
      - data_backend
      - static_backend

  data_backend:
    image: alpine/socat
    command: "tcp-l:9001,fork,reuseaddr exec:/bin/cat"

  static_backend:
    image: alpine/socat
    command: "tcp-l:9002,fork,reuseaddr exec:'/bin/echo -e HTTP/1.1 200 OK\\r\\n\\r\\nbody'"

Build:

$ docker compose up --build --force-recreate

Test:

$ printf "BBB" | nc localhost 80
$ printf "BBB" | nc localhost 80  # when have chosen the 2nd option

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