gbhub: make GB drops attributable, not spooky

Link-drop investigation fallout - every drop looked identical because
the hub discarded all the evidence:

- gb_reader logs *how* a GB left (clean eof vs the exception) with
  timestamps on join/leave, for correlating against SSH/terminal events.
- Spawn mode keeps each emulator's stderr in /tmp/gbhub_<n>.err instead
  of devnull - postmortems get the emulator's side of the story.
- A second gbhub exits with a clear 'already running?' message instead
  of an ioctl traceback (and can't damage the live hub's NAT/socket).

1 files changed, 15 insertions, 5 deletions

diff --git a/tools/gbhub b/tools/gbhub
index 3823463..85a4843 100755
--- a/tools/gbhub
+++ b/tools/gbhub
@@ -93,13 +93,16 @@ def route(gb, fr):
 def gb_reader(gb):
     buf, inf, esc = bytearray(), False, False
     while True:
+        why = "eof"
         try: chunk = gb.conn.recv(2048)
-        except Exception: chunk = None
+        except Exception as e:
+            chunk = None; why = "%s: %s" % (type(e).__name__, e)
         if not chunk:
             with lock:                              # GB left: free its address
                 conns.pop(gb.ip, None)
                 if gb.octet not in pool: pool.append(gb.octet); pool.sort()
-            sys.stderr.write("[gbhub] GB%d (10.0.0.%d) left\n" % (gb.idx, gb.octet))
+            sys.stderr.write("[gbhub] %s GB%d (10.0.0.%d) left (%s)\n"
+                             % (time.strftime("%H:%M:%S"), gb.idx, gb.octet, why))
             return
         for c in chunk:
             if c == END:
@@ -125,7 +128,11 @@ def tun_reader():
 def net_setup():
     global tun
     tun = os.open("/dev/net/tun", os.O_RDWR)
-    fcntl.ioctl(tun, 0x400454ca, struct.pack("16sH", IFACE.encode(), 0x0001|0x1000))
+    try:
+        fcntl.ioctl(tun, 0x400454ca, struct.pack("16sH", IFACE.encode(), 0x0001|0x1000))
+    except OSError as e:
+        sys.exit("gbhub: cannot claim %s (%s) - another gbhub is already running?\n"
+                 "       (sudo pkill -f tools/gbhub  to stop it)" % (IFACE, e.strerror))
     wan = subprocess.check_output("ip route show default|awk '{print $5;exit}'", shell=True).decode().strip() or "eth0"
     sh("ip addr add 10.0.0.1/24 dev %s" % IFACE); sh("ip link set %s mtu %d up" % (IFACE, MTU))
     sh("sysctl -w net.ipv4.ip_forward=1")
@@ -146,7 +153,8 @@ def register(conn):
         octet = pool.pop(0)
     gb = GB(octet, conn); conns[gb.ip] = gb
     threading.Thread(target=gb_reader, args=(gb,), daemon=True).start()
-    sys.stderr.write("[gbhub] GB%d joined -> 10.0.0.%d\n" % (gb.idx, octet))
+    sys.stderr.write("[gbhub] %s GB%d joined -> 10.0.0.%d\n"
+                     % (time.strftime("%H:%M:%S"), gb.idx, octet))
     return gb
 
 def main():
@@ -176,9 +184,11 @@ def main():
     for i in range(n):
         rom = "/tmp/gbhub_%d.gb" % i                     # per-GB ROM so saves don't clash
         shutil.copyfile(ROM, rom)
+        err = open("/tmp/gbhub_%d.err" % i, "ab", buffering=0)   # keep emulator stderr for postmortems
         emus.append(subprocess.Popen([EMU, "--headless", "--uncapped", "--serial-sock", SOCK, rom],
-                    stdin=subprocess.DEVNULL, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
+                    stdin=subprocess.DEVNULL, stdout=subprocess.DEVNULL, stderr=err,
                     start_new_session=True))
+        err.close()
     sys.stderr.write("[gbhub] %d Game Boys on 10.0.0.0/24 (10.0.0.2..%d), NAT via %s\n" % (n, 1+n, wan))
     srv.settimeout(12)
     for i in range(n):