send SIGUSR1 signals only to ueberzug processes

ueberzug/__main__.py

@@ -1,10 +1,14 @@
 #!/usr/bin/env python3
 """Usage:
-    ueberzug MODULE [options]
+    ueberzug layer [options]
+    ueberzug library
+    ueberzug query_windows PIDS ...
 
 Routines:
     layer                   Display images
     library                 Prints the path to the bash library
+    query_windows           Orders ueberzug to search for windows.
+                            Only for internal use.
 
 Layer options:
     -p, --parser <parser>  one of json, simple, bash
@@ -26,28 +30,22 @@ License:
     This is free software, and you are welcome to redistribute it
     under certain conditions.
 """
-import sys
-
 import docopt
 
 
 def main():
     options = docopt.docopt(__doc__)
-    module_name = options['MODULE']
     module = None
 
-    if module_name == 'layer':
+    if options['layer']:
         import ueberzug.layer as layer
         module = layer
-    elif module_name == 'library':
+    elif options['library']:
         import ueberzug.library as library
         module = library
-
-    if module is None:
-        print("Unknown module '{}'"
-              .format(module_name),
-              file=sys.stderr)
-        return
+    elif options['query_windows']:
+        import ueberzug.query_windows as query_windows
+        module = query_windows
 
     module.main(options)
 

ueberzug/layer.py

@@ -116,7 +116,7 @@ def setup_tmux_hooks():
     lock_directory_path = pathlib.PosixPath(tempfile.gettempdir()) / 'ueberzug'
     lock_file_path = lock_directory_path / tmux_util.get_session_id()
     own_pid = str(os.getpid())
-    command_template = 'kill -USR1 '
+    command_template = 'ueberzug query_windows '
 
     try:
         lock_directory_path.mkdir()

ueberzug/query_windows.py

@@ -0,0 +1,99 @@
+import os
+import signal
+import errno
+
+
+def get_command(pid):
+    """Figures out the associated command name
+    of a process with the given pid.
+
+    Args:
+        pid (int): the pid of the process of interest
+
+    Returns:
+        str: the associated command name
+    """
+    with open('/proc/{}/comm'.format(pid), 'r') as commfile:
+        return '\n'.join(commfile.readlines())
+
+
+def is_same_command(pid0, pid1):
+    """Checks whether the associated command name
+    of the processes of the given pids equals to each other.
+
+    Args:
+        pid0 (int): the pid of the process of interest
+        pid1 (int): the pid of another process of interest
+
+    Returns:
+        bool: True if both processes have
+              the same associated command name
+    """
+    return get_command(pid0) == get_command(pid1)
+
+
+def send_signal_safe(own_pid, target_pid):
+    """Sends SIGUSR1 to a process if both
+    processes have the same associated command name.
+    (Race condition free)
+
+    Requires:
+        - Python 3.9+
+        - Linux 5.1+
+
+    Args:
+        own_pid (int): the pid of this process
+        target_pid (int):
+            the pid of the process to send the signal to
+    """
+    pidfile = None
+    try:
+        pidfile = os.pidfile_open(target_pid)
+        if is_same_command(own_pid, target_pid):
+            signal.pidfd_send_signal(pidfile, signal.SIGUSR1)
+    except FileNotFoundError:
+        pass
+    except OSError as error:
+        # not sure if errno is really set..
+        # at least the documentation of the used functions says so..
+        # see e.g.: https://github.com/python/cpython/commit/7483451577916e693af6d20cf520b2cc7e2174d2#diff-99fb04b208835118fdca0d54b76a00c450da3eaff09d2b53e8a03d63bbe88e30R1279-R1281
+        # and https://docs.python.org/3/c-api/exceptions.html#c.PyErr_SetFromErrno
+
+        # caused by either pidfile_open or pidfd_send_signal
+        if error.errno != errno.ESRCH:
+            raise
+        # else: the process is death
+    finally:
+        if pidfile is not None:
+            os.close(pidfile)
+
+
+def send_signal_unsafe(own_pid, target_pid):
+    """Sends SIGUSR1 to a process if both
+    processes have the same associated command name.
+    (Race condition if process dies)
+
+    Args:
+        own_pid (int): the pid of this process
+        target_pid (int):
+            the pid of the process to send the signal to
+    """
+    try:
+        if is_same_command(own_pid, target_pid):
+            os.kill(target_pid, signal.SIGUSR1)
+    except (FileNotFoundError, ProcessLookupError):
+        pass
+
+
+def main(options):
+    # assumption:
+    # started by calling the programs name
+    # ueberzug layer and
+    # ueberzug query_windows
+    own_pid = os.getpid()
+
+    for pid in options['PIDS']:
+        try:
+            send_signal_safe(own_pid, int(pid))
+        except AttributeError:
+            send_signal_unsafe(own_pid, int(pid))