cli (base): Add a failsafe on guess_background:

In some cases (e.g. inherited env values in a wrapping tool), the background detection
code can be validated but hangs. This patch avoids that by adding a timeout with a
`SIGALRM`.

libervia/cli/base.py

@@ -26,6 +26,7 @@ import logging as log
 import os
 import os.path
 from pathlib import Path
+import signal
 import sys
 import termios
 import tty
@@ -168,7 +169,15 @@ class LiberviaCli:
                 or vte_version >= 3502
             )
         ):
-            # ANSI escape sequence
+            # ANSI escape sequence with alarm timeout to avoid hanging on unresponsive ptys
+            _bg_timeout = 0.5
+
+            def _alarm_handler(signum, frame):
+                raise TimeoutError("ANSI query timed out")
+
+            old_handler = signal.signal(signal.SIGALRM, _alarm_handler)
+            signal.setitimer(signal.ITIMER_REAL, _bg_timeout)
+
             stdin_fd = sys.stdin.fileno()
             old_settings = termios.tcgetattr(stdin_fd)
             try:
@@ -188,7 +197,11 @@ class LiberviaCli:
                 ]
                 # '\a' is the last character
                 sys.stdin.read(1)
+            except TimeoutError:
+                return "dark"
             finally:
+                signal.setitimer(signal.ITIMER_REAL, 0)
+                signal.signal(signal.SIGALRM, old_handler)
                 termios.tcsetattr(stdin_fd, termios.TCSADRAIN, old_settings)
 
             lum = utils.per_luminance(red, green, blue)