Fix memoisation of lazy parser & bump version

Reported by @Rafiot: the lazy parser is not memoised, this has limited
effect on the basic / pure Python parser as its initialisation is
trivial, but it *significantly* impact the re2 and regex parsers as
they need to process regexes into a filter tree.

The memoization was mistakenly removed in #230: while refactoring
initialisation I removed the setting of the `parser` global.

- add a test to ensure the parser is correctly memoized, not
  re-instantiated every time
- reinstate setting the global
- add a mutex on `__getattr__`, it should only be used on first access
  and avoids two threads creating an expensive parser at the same
  time (which is a waste of CPU)

Fixes #253

pyproject.toml

@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "ua-parser"
 description = "Python port of Browserscope's user agent parser"
-version = "1.0.0"
+version = "1.0.1"
 readme = "README.rst"
 requires-python = ">=3.9"
 dependencies = ["ua-parser-builtins"]

src/ua_parser/__init__.py

@@ -41,7 +41,8 @@
 ]
 
 import importlib.util
-from typing import Callable, Optional
+import threading
+from typing import Callable, Optional, cast
 
 from .basic import Resolver as BasicResolver
 from .caching import CachingResolver, S3Fifo as Cache
@@ -78,7 +79,7 @@
 )
 
 
-VERSION = (1, 0, 0)
+VERSION = (1, 0, 1)
 
 
 class Parser:
@@ -135,15 +136,27 @@ def parse_device(self: Resolver, ua: str) -> Optional[Device]:
 initialisation, rather than pay for it at first call.
 """
 
+_lazy_globals_lock = threading.Lock()
+
 
 def __getattr__(name: str) -> Parser:
     global parser
-    if name == "parser":
-        if RegexResolver or Re2Resolver or IS_GRAAL:
-            matchers = load_lazy_builtins()
-        else:
-            matchers = load_builtins()
-        return Parser.from_matchers(matchers)
+    with _lazy_globals_lock:
+        if name == "parser":
+            # if two threads access `ua_parser.parser` before it's
+            # initialised, the second one will wait until the first
+            # one's finished by which time the parser global should be
+            # set and can be returned with no extra work
+            if p := globals().get("parser"):
+                return cast(Parser, p)
+
+            if RegexResolver or Re2Resolver or IS_GRAAL:
+                matchers = load_lazy_builtins()
+            else:
+                matchers = load_builtins()
+            parser = Parser.from_matchers(matchers)
+            return parser
+
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
 
 

tests/test_convenience_parser.py

@@ -1,6 +1,23 @@
+import ua_parser
 from ua_parser import Domain, Parser, PartialResult, Result
 
 
+def test_parser_memoized() -> None:
+    """The global parser should be lazily instantiated but memoized"""
+    # ensure there is no global parser
+    vars(ua_parser).pop("parser", None)
+
+    p1 = ua_parser.parser
+    p2 = ua_parser.parser
+
+    assert p1 is p2
+
+    # force the creation of a clean parser
+    del ua_parser.parser
+    p3 = ua_parser.parser
+    assert p3 is not p1
+
+
 def resolver(s: str, d: Domain) -> PartialResult:
     return PartialResult(d, None, None, None, s)