fix: align ESP32 WiFi SSID/channel with QEMU access_points[] array

Two fixes for ESP32 WiFi not connecting in production:

1. espidf_compiler.py: Change WiFi normalization from 'Velxio-GUEST' on
   channel 6 to 'Espressif' on channel 5. The lcgamboa QEMU binary
   downloaded from GitHub Releases only contains the original three APs:
   PICSimLabWifi (ch1), Espressif (ch5), MasseyWifi (ch10). Channel 6
   had no matching AP, so the beacon timer's channel-match condition never
   fired → firmware scanned forever and never connected.

2. esp32_worker.py: Redirect fd 1 to /dev/null before loading QEMU so
   raw UART bytes from QEMU's -nographic mux don't corrupt the JSON
   event pipe. The real pipe fd is saved and sys.stdout is rebound so
   _emit() continues to work. This also prevents stdout pipe back-pressure
   from stalling qemu_main_loop() (and thus REALTIME timers).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/esp32_worker.py

@@ -611,6 +611,25 @@ def main() -> None:  # noqa: C901  (complexity OK for inline worker)
     os.dup2(_nul, 0)
     os.close(_nul)
 
+    # Also redirect fd 1 (stdout) to /dev/null so QEMU's -nographic UART mux
+    # doesn't write raw UART bytes onto our JSON event pipe.  Without this:
+    #   1. Raw UART bytes prefix each JSON line, corrupting the protocol.
+    #   2. On a busy host the pipe fills up, causing _on_uart_tx (called
+    #      synchronously from qemu_main_loop) to block inside sys.stdout.flush(),
+    #      which stalls qemu_main_loop() and prevents QEMU_CLOCK_REALTIME timers
+    #      (including Esp32_WLAN_beacon_timer) from firing → WiFi never connects.
+    # Save the real pipe fd and rebind sys.stdout so _emit() keeps working.
+    import io as _io
+    _orig_stdout_fd = os.dup(1)
+    _nul_w = os.open(os.devnull, os.O_WRONLY)
+    os.dup2(_nul_w, 1)
+    os.close(_nul_w)
+    sys.stdout = _io.TextIOWrapper(
+        _io.FileIO(_orig_stdout_fd, mode='w', closefd=True),
+        line_buffering=True,
+        write_through=True,
+    )
+
     qemu_t = threading.Thread(target=_qemu_thread, daemon=True, name=f'qemu-{machine}')
     qemu_t.start()
 

backend/app/services/espidf_compiler.py

@@ -34,8 +34,11 @@ _STATIC_IP = '192.168.4.15'
 _GATEWAY_IP = '192.168.4.2'
 _NETMASK = '255.255.255.0'
 
-# SSID the QEMU WiFi AP broadcasts
-_QEMU_WIFI_SSID = 'Velxio-GUEST'
+# SSID the QEMU WiFi AP broadcasts.
+# Must match one of the access_point_info entries in esp32_wifi_ap.c
+# (the lcgamboa QEMU fork). "Espressif" is on channel 5 in that array.
+_QEMU_WIFI_SSID = 'Espressif'
+_QEMU_WIFI_CHANNEL = 5
 
 
 class ESPIDFCompiler:
@@ -109,11 +112,11 @@ class ESPIDFCompiler:
         """
         Normalize WiFi SSID/password/channel in Arduino sketches for QEMU.
 
-        QEMU's WiFi AP broadcasts "Velxio-GUEST" on channel 6 with open auth.
+        QEMU's WiFi AP broadcasts _QEMU_WIFI_SSID on _QEMU_WIFI_CHANNEL with open auth.
         This method rewrites the user's sketch so that:
-          - Any SSID string literal → "Velxio-GUEST"
+          - Any SSID string literal → _QEMU_WIFI_SSID
           - Password → "" (open auth)
-          - Channel → 6
+          - Channel → _QEMU_WIFI_CHANNEL
         The user's editor still shows their original code; only the compiled
         binary is modified.
         """
@@ -121,9 +124,9 @@ class ESPIDFCompiler:
             return code
 
         # 1) Replace SSID variable definitions:
-        #    const char* ssid = "anything" → "Velxio-GUEST"
-        #    char ssid[] = "anything"      → "Velxio-GUEST"
-        #    #define WIFI_SSID "anything"   → "Velxio-GUEST"
+        #    const char* ssid = "anything" → _QEMU_WIFI_SSID
+        #    char ssid[] = "anything"      → _QEMU_WIFI_SSID
+        #    #define WIFI_SSID "anything"   → _QEMU_WIFI_SSID
         code = re.sub(
             r'((?:const\s+)?char\s*\*?\s*ssid\s*\[?\]?\s*=\s*)"[^"]*"',
             rf'\1"{_QEMU_WIFI_SSID}"',
@@ -138,19 +141,19 @@ class ESPIDFCompiler:
         )
 
         # 2) Normalize WiFi.begin() calls:
-        #    WiFi.begin("X")           → WiFi.begin("Velxio-GUEST", "", 6)
-        #    WiFi.begin("X", "pass")   → WiFi.begin("Velxio-GUEST", "", 6)
-        #    WiFi.begin(ssid, pass, N) → WiFi.begin(ssid, "", 6)
-        #    WiFi.begin(ssid)          → WiFi.begin(ssid, "", 6)
+        #    WiFi.begin("X")           → WiFi.begin(_QEMU_WIFI_SSID, "", _QEMU_WIFI_CHANNEL)
+        #    WiFi.begin("X", "pass")   → WiFi.begin(_QEMU_WIFI_SSID, "", _QEMU_WIFI_CHANNEL)
+        #    WiFi.begin(ssid, pass, N) → WiFi.begin(ssid, "", _QEMU_WIFI_CHANNEL)
+        #    WiFi.begin(ssid)          → WiFi.begin(ssid, "", _QEMU_WIFI_CHANNEL)
 
         def _rewrite_wifi_begin(m: re.Match) -> str:
             args = m.group(1)
             parts = [a.strip() for a in args.split(',')]
             ssid_arg = parts[0]
-            # If SSID is a string literal, force to Velxio-GUEST
+            # If SSID is a string literal, force to _QEMU_WIFI_SSID
             if ssid_arg.startswith('"'):
                 ssid_arg = f'"{_QEMU_WIFI_SSID}"'
-            return f'WiFi.begin({ssid_arg}, "", 6)'
+            return f'WiFi.begin({ssid_arg}, "", {_QEMU_WIFI_CHANNEL})'
 
         code = re.sub(
             r'WiFi\.begin\s*\(([^)]+)\)',
@@ -158,7 +161,7 @@ class ESPIDFCompiler:
             code
         )
 
-        logger.info('[espidf] WiFi normalized: SSID→%s, channel→6, open auth', _QEMU_WIFI_SSID)
+        logger.info('[espidf] WiFi normalized: SSID→%s, channel→%d, open auth', _QEMU_WIFI_SSID, _QEMU_WIFI_CHANNEL)
         return code
 
     def _translate_sketch_to_espidf(self, sketch_code: str) -> str: