qemu: rs485 silence|restore takes a simulated device off the bus mid-run

ui-drive.sh --rs485-devices fixed the roster for the whole boot, so no
script could show the guest noticing a device go quiet. mbsim.py now
serves a control socket (flare-edge --control); ui-drive.sh opens it next
to the pty and hands its path to qmp.py drive (--rs485-control), whose new
verb `rs485 silence|restore ADDR` sends one command and judges the reply.
A run without a simulated bus records the step as fatal rather than a
silent pass. Rig: unit 5 silenced reads online=false after 65 s in the
status json, restored reads online=true after 64 s (flare-edge #184).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3G6m9Aw5RyVY4ZowtKzEj

qemu/tests/qmp.py

@@ -37,6 +37,10 @@ a real panel, so these verbs mean the same thing on the rig and on hardware:
                            eats the touch that ends either); every compiled
                            flow script opens with it
     page | stats | hit X Y print the reply, judge nothing
+    rs485 silence|restore ADDR
+                           take a simulated bus device off the bus (holds its
+                           address, answers nothing) or put it back; needs
+                           --rs485-control (ui-drive.sh --rs485-devices)
     ctl WORDS...           raw passthrough for anything the channel grows
     assert_page MENU/TAB   the active page is exactly this
     assert_hit X Y CLASS [TEXT...]
@@ -486,7 +490,18 @@ def write_png(img, path):
     _PILImage.frombytes("RGB", (w, h), data).save(path)
 
 
-def drive(s, f, script_path, outdir, size, ctl_path=None, console_path=None, refs_path=None):
+def rs485_send(control_path, line):
+    """One command to mbsim.py's control socket (flare-edge tools/modbus-sim,
+    --control): -> its one-line reply, `ok ...` or `error ...`."""
+    with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as c:
+        c.settimeout(5)
+        c.connect(control_path)
+        c.sendall((line + "\n").encode())
+        return c.makefile("r").readline().strip()
+
+
+def drive(s, f, script_path, outdir, size, ctl_path=None, console_path=None, refs_path=None,
+          rs485_control=None):
     os.makedirs(outdir, exist_ok=True)
     with open(script_path) as fh:
         lines = fh.readlines()
@@ -594,6 +609,26 @@ def drive(s, f, script_path, outdir, size, ctl_path=None, console_path=None, ref
             need_ctl(lineno, cmd)
             reply = ctl.send(line if cmd != "ctl" else " ".join(args))
             record(lineno, line, "ok", reply)
+        elif cmd == "rs485":
+            # rs485 silence|restore ADDR: take a simulated device off the bus
+            # (it holds its address and answers nothing, exactly an absent
+            # unit) or put it back, while the guest keeps polling. This is
+            # the only runtime lever on the roster ui-drive.sh --rs485-devices
+            # fixed at boot, and it exists so a flow can prove a screen
+            # noticing an adopted device go quiet (flare-edge #184). No
+            # simulated bus in this run is fatal: the step could not check
+            # anything, and a silent skip would read as a pass.
+            if rs485_control is None:
+                record(lineno, line, "fatal", "no simulated RS485 bus in this run "
+                       "(ui-drive.sh --rs485-devices)")
+            elif len(args) != 2 or args[0] not in ("silence", "restore"):
+                record(lineno, line, "fatal", "expected: rs485 silence|restore ADDR")
+            else:
+                try:
+                    reply = rs485_send(rs485_control, f"{args[0]} {args[1]}")
+                except OSError as e:
+                    reply = f"error control socket: {e}"
+                record(lineno, line, "ok" if reply.startswith("ok") else "fail", reply)
         elif cmd == "assert_page":
             need_ctl(lineno, cmd)
             want = " ".join(args)
@@ -815,6 +850,8 @@ def main():
     ctl_path = sys.argv[sys.argv.index("--ctl") + 1] if "--ctl" in sys.argv else None
     console_path = sys.argv[sys.argv.index("--console") + 1] if "--console" in sys.argv else None
     refs_path = sys.argv[sys.argv.index("--refs") + 1] if "--refs" in sys.argv else None
+    rs485_control = (sys.argv[sys.argv.index("--rs485-control") + 1]
+                     if "--rs485-control" in sys.argv else None)
 
     s = socket.socket(socket.AF_UNIX)
     s.connect(path)
@@ -827,7 +864,8 @@ def main():
     elif cmd == "tap":
         do_tap(s, f, int(sys.argv[3]), int(sys.argv[4]))
     elif cmd == "drive":
-        drive(s, f, sys.argv[3], sys.argv[4], size, ctl_path, console_path, refs_path)
+        drive(s, f, sys.argv[3], sys.argv[4], size, ctl_path, console_path, refs_path,
+              rs485_control)
     elif cmd == "quit":
         s.sendall(b'{"execute":"quit"}\n')