aspect ratio and keep awake

photobooth.py

@@ -6,10 +6,17 @@ CAPTURING (countdowns, flashes, 3 photos per shoot). Every 4th shoot the
 12 photos are composed onto a contact sheet and printed. Five minutes of
 inactivity brings up the fish tank screensaver.
 
+While running, the booth re-execs itself under systemd-inhibit so the
+machine will not sleep or suspend (and, when the lock is available, ignores
+the laptop lid) - it can run headless with the lid closed.
+
 Run with --windowed for testing without taking over the display.
 Esc or Q quits.
 """
 
+import os
+import shutil
+import subprocess
 import sys
 import time
 import tomllib
@@ -26,6 +33,51 @@ ROOT = Path(__file__).resolve().parent
 
 IDLE, PREVIEW, COUNTDOWN, FLASH, SCREENSAVER = range(5)
 
+KEEP_AWAKE_ENV = "PHOTOBOOTH_INHIBITED"
+KEEP_AWAKE_WHY = "Photobooth is running (keep awake, ignore lid)"
+
+
+def _probe_inhibit_locks():
+    """Largest lock set systemd-inhibit can hold on this machine.
+
+    Returns a --what value like "sleep:idle:handle-lid-switch", or None when
+    systemd-inhibit is missing or no lock can be taken. handle-lid-switch
+    allows only one block holder, so it is skipped when a desktop
+    environment already holds it. Each probe runs `true` under the lock, so
+    locks are taken and released instantly.
+    """
+    inhibit = shutil.which("systemd-inhibit")
+    if not inhibit:
+        return None
+    for what in ("sleep:idle:handle-lid-switch", "sleep:idle", "sleep"):
+        probe = subprocess.run([inhibit, "--mode=block", f"--what={what}",
+                                "--", "true"], capture_output=True)
+        if probe.returncode == 0:
+            return what
+    return None
+
+
+def reexec_keep_awake():
+    """Re-exec this process under systemd-inhibit (once) so the machine
+    stays awake - and ignores the lid when that lock is free - while the
+    booth runs. Locks are released when the program exits. Best-effort:
+    hosts without systemd-inhibit simply run without locks."""
+    if os.environ.get(KEEP_AWAKE_ENV):
+        return
+    what = _probe_inhibit_locks()
+    if not what:
+        return
+    os.environ[KEEP_AWAKE_ENV] = "1"
+    argv = [shutil.which("systemd-inhibit"), "--mode=block",
+            f"--what={what}", f"--why={KEEP_AWAKE_WHY}", "--",
+            sys.executable, str(ROOT / "photobooth.py"), *sys.argv[1:]]
+    # flush: execvp replaces the process without draining stdout's buffer.
+    print(f"Keep-awake: re-running under systemd-inhibit ({what})", flush=True)
+    try:
+        os.execvp(argv[0], argv)
+    except OSError as e:
+        print(f"Keep-awake: re-exec failed ({e}); running without locks")
+
 
 def load_config():
     with open(ROOT / "config.toml", "rb") as f:
@@ -290,6 +342,7 @@ class Booth:
 
 
 def main():
+    reexec_keep_awake()
     windowed = "--windowed" in sys.argv
     booth = Booth(windowed=windowed)
     try: