Improve feedback when hook script fails; Add hook script signal handling

README.md

@@ -160,7 +160,6 @@ default values in the form `option-name=option-argument`.
 A common and effective use of this is to specify default `skip-options`, for
 instance skipping the `gamma` setting if using
 [`redshift`](https://github.com/jonls/redshift) as a daemon.  To implement
-the equivalent of `--skip-options gamma`, your `settings.ini` file should look
 like this:
 
 ```
@@ -238,6 +237,57 @@ notify-send -i display "Display profile" "$AUTORANDR_CURRENT_PROFILE"
 The one kink is that during `preswitch`, `AUTORANDR_CURRENT_PROFILE` is
 reporting the *upcoming* profile rather than the *current* one.
 
+#### Exit status
+
+If a hook script exits with a non-zero exit status, a warning will be output, but the
+operation continue.
+
+#### Canceling operation from within a hook script
+
+A hook script can send a `SIGUSR1` signal to its parent process to indicate that the
+current operation should halt. The script is allowed to finish in any case.
+Only scripts with names starting with `pre` support this feature. 
+
+Bash example:
+
+```bash
+#!/usr/bin/env bash
+
+should_continue() {
+  # No
+  return 1
+}
+
+if ! should_continue; then
+  # Send SIGUSR1 signal to parent process to cancel operation
+  kill -s USR1 "$PPID"
+  exit
+fi
+
+# Continue ...
+```
+
+Python example:
+
+```python
+#!/usr/bin/env python
+import os
+import sys
+import signal
+
+def should_continue():
+  # No
+  return False
+
+if __name__ == "__main__":
+  if not should_continue():
+    # Send SIGUSR1 signal to parent process to cancel operation
+    os.kill(os.getppid(), signal.SIGUSR1)
+    sys.exit()
+
+  # Continue ...
+```
+
 ### Wildcard EDID matching
 
 The EDID strings in the `~/.config/autorandr/*/setup` files may contain an

autorandr.py

@@ -24,6 +24,8 @@
 
 from __future__ import print_function
 
+from signal import signal, SIGUSR1, SIG_DFL
+
 import binascii
 import copy
 import getopt
@@ -61,6 +63,9 @@
 except NameError:
     pass
 
+
+cancel_signal = SIGUSR1
+
 virtual_profiles = [
     # (name, description, callback)
     ("off", "Disable all outputs", None),
@@ -1179,10 +1184,7 @@ def exec_scripts(profile_path, script_name, meta_information=None):
         if script_name not in ran_scripts:
             script = os.path.join(folder, script_name)
             if os.access(script, os.X_OK | os.F_OK):
-                try:
-                    all_ok &= subprocess.call(script, env=env) != 0
-                except:
-                    raise AutorandrException("Failed to execute user command: %s" % (script,))
+                all_ok &= exec_one_script(script, script_name, env) != 0
                 ran_scripts.add(script_name)
 
         script_folder = os.path.join(folder, "%s.d" % script_name)
@@ -1192,15 +1194,44 @@ def exec_scripts(profile_path, script_name, meta_information=None):
                 if check_name not in ran_scripts:
                     script = os.path.join(script_folder, file_name)
                     if os.access(script, os.X_OK | os.F_OK):
-                        try:
-                            all_ok &= subprocess.call(script, env=env) != 0
-                        except:
-                            raise AutorandrException("Failed to execute user command: %s" % (script,))
+                        all_ok &= exec_one_script(script, script_name, env) != 0
                         ran_scripts.add(check_name)
 
     return all_ok
 
 
+def exec_one_script(script, script_name, env):
+    """"Run a userscript and return the exit code.
+
+    If the script exits with a non-zero exit status, a warning is sent to stderr but the operation continues.
+
+    If the script sends a SIGUSR1 signal to the parent, the parent process will
+    wait for the child process to exit, then halt with a zero status.
+    """
+    def handle_cancel_signal(signum, frame):
+        if not script_name.startswith('pre'):
+            print("Script %s issued %s signal, but it is ignored for %s scripts." % (
+                script, cancel_signal.name, script_name), file=sys.stderr)
+            return
+
+        main_operation_name = script_name[3:]
+
+        # Clean up, inform, exit
+        print("Script %s issued %s signal. Cancelling %s operation." % (script, cancel_signal.name, main_operation_name))
+        sys.exit()
+
+    try:
+        signal(cancel_signal, handle_cancel_signal)
+        return subprocess.check_call(script, env=env)
+    except subprocess.CalledProcessError as e:
+        print("Warning: Script %s returned exit code %d." % (script, e.returncode), file=sys.stderr)
+    except Exception as e:
+        raise AutorandrException("Failed to execute user command: %s" % (script,), original_exception=e)
+    finally:
+        # Revert to default handler
+        signal(cancel_signal, SIG_DFL)
+
+
 def dispatch_call_to_sessions(argv):
     """Invoke autorandr for each open local X11 session with the given options.