net: wget - real HTTP over the link port via the gateway

Grow the link-port demo from echo to actual network access. The Game Boy
still only does SLIP framing + display; the host gateway does DNS/TCP/HTTP.

- c/netlib.h: SLIP framing factored out (header-only, per-program copy).
  necho.c now uses it too.
- c/wget.c: `wget URL` frames the URL, then prints the reply body. The
  gateway streams the body back as typed frames: 'D'<chunk> ... 'E'.
- tools/gateway.py: add --mode http (urlopen the frame as a URL, cap the
  body, chunk it) alongside --mode echo; --cmd runs any gbos command.

Verified: `wget example.com` streams back the full Example Domain HTML onto
the terminal; `wget sl0p.foo` fetches the real page. A Game Boy on the web,
over the link cable.

1 files changed, 45 insertions, 21 deletions

diff --git a/tools/gateway.py b/tools/gateway.py
index 81a70af..3129773 100644
--- a/tools/gateway.py
+++ b/tools/gateway.py
@@ -1,18 +1,22 @@
 #!/usr/bin/env python3
-"""gateway.py - a host-side "link cable adapter" for gbos networking.
+"""gateway.py - host-side "link cable adapter" for gbos networking.
 
-Wraps the emulator, owns its link-port serial (stdio), and speaks SLIP. This
-milestone just ECHOES every frame back (loopback), so `necho` on gbos can prove
-a framed round-trip. Console (ASCII) bytes on the same channel are printed.
+Wraps the emulator, owns its link-port serial (stdio), speaks SLIP, and does
+the real network work the Game Boy can't (DNS/TCP/HTTP). Modes:
 
-usage: tools/gateway.py [rom]
+  echo  - bounce every frame back (loopback test)
+  http  - treat each frame as a URL, HTTP GET it, stream the body back
+
+Reply frames are typed: b'D'+chunk for data, b'E' to end the stream.
+
+usage: tools/gateway.py [--mode echo|http] [--cmd "wget example.com"] [--rom PATH]
 """
-import os, sys, subprocess, threading, time
+import os, sys, subprocess, threading, time, argparse, urllib.request
 
 EMU = os.path.expanduser("~/dev/gbc/build/sl0pboy")
-ROM = sys.argv[1] if len(sys.argv) > 1 else os.path.join(os.path.dirname(__file__), "..", "gbos.gb")
-
 END, ESC, ESC_END, ESC_ESC = 0xC0, 0xDB, 0xDC, 0xDD
+CHUNK = 200
+LIMIT = 800                       # cap the fetched body (link is ~1-32 KB/s)
 
 def slip_encode(payload):
     out = bytearray([END])
@@ -23,13 +27,28 @@ def slip_encode(payload):
     out.append(END)
     return bytes(out)
 
-def handle_frame(payload):
-    """The 'network'. For now: echo. (Later: DNS/TCP/HTTP/IRC.)"""
-    sys.stderr.write("\n[gw] frame in : %r\n" % bytes(payload))
-    sys.stderr.write("[gw] frame out: %r (echo)\n" % bytes(payload))
-    return bytes(payload)
+def http_fetch(payload):
+    url = bytes(payload).decode("latin1", "replace").strip()
+    if not url.startswith("http"):
+        url = "http://" + url
+    sys.stderr.write("\n[gw] GET %s\n" % url)
+    try:
+        req = urllib.request.Request(url, headers={"User-Agent": "gbos/0.1"})
+        with urllib.request.urlopen(req, timeout=8) as r:
+            body = r.read(LIMIT)
+    except Exception as e:
+        body = ("ERR: " + str(e)).encode()
+    sys.stderr.write("[gw] %d bytes\n" % len(body))
+    frames = [b"D" + body[i:i+CHUNK] for i in range(0, len(body), CHUNK)]
+    frames.append(b"E")
+    return frames
+
+def handle(payload, mode):
+    if mode == "http":
+        return http_fetch(payload)
+    return [bytes(payload)]        # echo
 
-def reader(out, inp):
+def reader(out, inp, mode):
     buf, in_frame, esc = bytearray(), False, False
     while True:
         b = out.read(1)
@@ -38,8 +57,8 @@ def reader(out, inp):
         c = b[0]
         if c == END:
             if in_frame and buf:
-                reply = handle_frame(buf)
-                inp.write(slip_encode(reply)); inp.flush()
+                for f in handle(buf, mode):
+                    inp.write(slip_encode(f)); inp.flush()
                 buf, in_frame, esc = bytearray(), False, False
             else:
                 buf, in_frame, esc = bytearray(), True, False
@@ -55,12 +74,17 @@ def reader(out, inp):
             sys.stderr.flush()
 
 def main():
-    p = subprocess.Popen([EMU, "--headless", "--uncapped", ROM],
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--mode", default="echo", choices=["echo", "http"])
+    ap.add_argument("--cmd", default="necho")
+    ap.add_argument("--rom", default=os.path.join(os.path.dirname(__file__), "..", "gbos.gb"))
+    a = ap.parse_args()
+    p = subprocess.Popen([EMU, "--headless", "--uncapped", a.rom],
                          stdin=subprocess.PIPE, stdout=subprocess.PIPE, bufsize=0)
-    threading.Thread(target=reader, args=(p.stdout, p.stdin), daemon=True).start()
-    time.sleep(1.0)                       # boot to the shell
-    p.stdin.write(b"necho\n"); p.stdin.flush()
-    time.sleep(2.0)                       # let the framed round-trip happen
+    threading.Thread(target=reader, args=(p.stdout, p.stdin, a.mode), daemon=True).start()
+    time.sleep(1.0)
+    p.stdin.write((a.cmd + "\n").encode()); p.stdin.flush()
+    time.sleep(4.0)
     p.stdin.write(b"exit\n"); p.stdin.flush()
     try: p.wait(timeout=3)
     except Exception: p.kill()