Disable debugcachestats on python < 3.9

Since I added weakrefs into the cache stats debug registry thing it no longer
works on python version 3.5 to 3.8, since lru_cache doesn't support weak refs
for those versions. It should be possible to do some
conditional stuff and get some functionality working on all supported versions
but I don't think there is any great need to for a debug command.

Links:
https://bugs.python.org/issue14373
https://bugs.python.org/issue40504
https://github.com/qutebrowser/qutebrowser/pull/7079#issuecomment-1079539046

2 files changed, 20 insertions, 5 deletions

diff --git a/qutebrowser/misc/debugcachestats.py b/qutebrowser/misc/debugcachestats.py
index 1e4139e43..4d15a6fa3 100644
--- a/qutebrowser/misc/debugcachestats.py
+++ b/qutebrowser/misc/debugcachestats.py
@@ -24,8 +24,11 @@ dependencies as possible to avoid cyclic dependencies.
 """
 
 import weakref
+import sys
 from typing import Any, Callable, List, Optional, Tuple, TypeVar
 
+from qutebrowser.utils import log
+
 
 # The second element of each tuple should be a lru_cache wrapped function
 _CACHE_FUNCTIONS: List[Tuple[str, Any]] = []
@@ -37,15 +40,23 @@ _T = TypeVar('_T', bound=Callable[..., Any])
 def register(name: Optional[str] = None) -> Callable[[_T], _T]:
     """Register a lru_cache wrapped function for debug_cache_stats."""
     def wrapper(fn: _T) -> _T:
-        fn_ref = weakref.ref(fn)
-        _CACHE_FUNCTIONS.append((fn.__name__ if name is None else name, fn_ref))
-        return fn
+        fn_name = fn.__name__ if name is None else name
+        if sys.version_info < (3, 9):
+            log.misc.vdebug(  # type: ignore[attr-defined]
+                'debugcachestats not supported on python < 3.9, not adding \'%s\'',
+                fn_name,
+            )
+            return fn
+
+        else:
+            fn_ref = weakref.ref(fn)
+            _CACHE_FUNCTIONS.append((fn_name, fn_ref))
+            return fn
     return wrapper
 
 
 def debug_cache_stats() -> None:
     """Print LRU cache stats."""
-    from qutebrowser.utils import log
     for idx, (name, fn_ref) in reversed(list(enumerate(_CACHE_FUNCTIONS))):
         fn = fn_ref()
         if not fn:
diff --git a/qutebrowser/misc/utilcmds.py b/qutebrowser/misc/utilcmds.py
index 7ba45bdc3..da774379f 100644
--- a/qutebrowser/misc/utilcmds.py
+++ b/qutebrowser/misc/utilcmds.py
@@ -23,6 +23,7 @@
 
 import functools
 import os
+import sys
 import traceback
 from typing import Optional
 
@@ -125,7 +126,10 @@ def debug_all_objects() -> None:
 @cmdutils.register(debug=True)
 def debug_cache_stats() -> None:
     """Print LRU cache stats."""
-    debugcachestats.debug_cache_stats()
+    if sys.version_info < (3, 9):
+        message.error('debugcachestats not supported on python < 3.9')
+    else:
+        debugcachestats.debug_cache_stats()
 
 
 @cmdutils.register(debug=True)