i3-companion: fix dampening

Cancellation is asynchronous. So, there was a race condition where we
were throwing away the task we just scheduled. Don't really on
cancellation for synchronization. We also want to have only one
instance running. So, use locks to ensure only one instance is running
and only cancel running functions while in sleeping phase, otherwise,
let them run.

Currently, this OK, however, it is assumed the function has somehow
the same effect whatever the arguments we provide. This is true for
the two callbacks we use `@dampen` on.

bin/i3-companion

@@ -99,19 +99,20 @@ def on(*events):
 
 
 def dampen(sleep, *, unless=None, retry=0):
-    """Dampen a function call."""
+    """Dampen a function call. Optional retry on failure. Ensure only one
+    instance is executed. It is assumed the arguments provided to the
+    dampened function have no effect on its execution.
+    """
 
     def decorator(fn):
-        async def fn_now(retry, *args, **kwargs):
-            try:
-                if unless is not None and unless(*args, **kwargs):
-                    # see https://github.com/ldo/dbussy/issues/15
-                    await asyncio.sleep(0.1)
-                else:
+        async def fn_now(me, retry, *args, **kwargs):
+            if unless is None or not unless(*args, **kwargs):
                 await asyncio.sleep(sleep)
-            except asyncio.CancelledError:
-                return
-            fn.running = None
+            me["sleeping"] = False
+
+            # From here, we do not expect to be cancelled. Ensure only
+            # one of us is running.
+            async with fn.lock:
                 try:
                     return await fn(*args, **kwargs)
                 except Exception as e:
@@ -124,22 +125,32 @@ def dampen(sleep, *, unless=None, retry=0):
                         retry,
                         str(e),
                     )
-                if fn.running is not None:
+                    # Run again, unless we have something already scheduled
+                    if fn.last_task["sleeping"]:
                         return
-                fn.running = asyncio.create_task(
-                    fn_now(retry, *args, **kwargs)
+                    fn.last_task = dict(sleeping=True)
+                    fn.last_task["task"] = asyncio.create_task(
+                        fn_now(fn.last_task, retry, *args, **kwargs)
                     )
 
         @functools.wraps(fn)
         async def wrapper(*args, **kwargs):
-            if fn.running is not None:
-                logger.debug(f"cancel call to previous {fn}")
-                fn.running.cancel()
-                fn.running = None
-            logger.debug(f"dampening call to {fn}")
-            fn.running = asyncio.create_task(fn_now(retry, *args, **kwargs))
+            # Initialize a lock (we need an active loop for that)
+            if fn.lock is None:
+                fn.lock = asyncio.Lock()
 
-        fn.running = None
+            # If possible, cancel last task if it's sleeping
+            if fn.last_task is not None and fn.last_task["sleeping"]:
+                logger.debug(f"cancel call to {fn}")
+                fn.last_task["task"].cancel()
+            logger.debug(f"dampening call to {fn}")
+            fn.last_task = dict(sleeping=True)
+            fn.last_task["task"] = asyncio.create_task(
+                fn_now(fn.last_task, retry, *args, **kwargs)
+            )
+
+        fn.last_task = None
+        fn.lock = None
         return wrapper
 
     return decorator
@@ -254,7 +265,7 @@ async def worksplace_exclusive(i3, event):
 
     # Can the new window just intrude?
     if can_intrude(w):
-        logger.debug("window {w.name} can intrude")
+        logger.debug(f"window {w.name} can intrude")
         return
 
     # Does the current workspace contains an exclusive app?