Add finite-automaton simplifier, for re2 and graal

As I've discovered a while ago, finite automaton engines are not very
fond of large bounded repetitions.

In re2 and regex, that mostly translates to increased memory
consumption (e.g. in their default modes, converting `.*` to
`.{0,500}` increases the pattern's size by 115x in re2 and 84x in
regex, if a capture is added on top then regex balloons to 219x),
there is a performance impact but it's high single digit to low
double, in regex at least (didn't test re2).

However as it turns out Graal uses a JIT-ed DFA, and it *really*
doesn't like these patterns, it spends a lot of time JIT-compiling
(this is apparently the source of the extra 300% CPU use I could
observe on what are purely single-threaded workloads, the JIT
desperately trying to optimise regexes) them with no gain in
performance: down-converting the regex back to the sensible increases
performances by ~25%, though it doesn't seem to impact memory use...

So... do that: `fa_simplifier` is the same idea as
ua-parser/uap-rust@29b9195 but from
the Python side, and applied to graal and re2 (not regex because it
does that internally as linked above).

Also switch Graal over to the lazy builtins, it kinda spreads the cost
but it seems stupid to compile the regexes only to immediately swap
(fa_simplifier) and recompile them... so don't do that, especially as
I couldn't be arsed to make the replacement conditional (so every
eager regex is recompiled, even though only those which actually got
modified by `fa_simplifier` need it...).

Fixes ua-parser#228

pyproject.toml

@@ -110,6 +110,7 @@ module = [
        "test_core",
        "test_caches",
        "test_parsers_basics",
+       "test_fa_simplifier",
 ]
 
 #check_untyped_defs = false

src/ua_parser/__init__.py

@@ -57,6 +57,7 @@
     UserAgent,
 )
 from .loaders import load_builtins, load_lazy_builtins
+from .utils import IS_GRAAL
 
 Re2Resolver: Optional[Callable[[Matchers], Resolver]] = None
 if importlib.util.find_spec("re2"):
@@ -132,10 +133,11 @@ def parse_device(self: Resolver, ua: str) -> Optional[Device]:
 def __getattr__(name: str) -> Parser:
     global parser
     if name == "parser":
-        parser = Parser.from_matchers(
-            load_builtins() if Re2Resolver is None else load_lazy_builtins()
-        )
-        return parser
+        if Re2Resolver or IS_GRAAL:
+            matchers = load_lazy_builtins()
+        else:
+            matchers = load_builtins()
+        return Parser.from_matchers(matchers)
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
 
 

src/ua_parser/basic.py

@@ -1,7 +1,9 @@
 __all__ = ["Resolver"]
 
+import re
+from itertools import chain
 from operator import methodcaller
-from typing import List
+from typing import Any, List
 
 from .core import (
     Device,
@@ -12,6 +14,7 @@
     PartialResult,
     UserAgent,
 )
+from .utils import IS_GRAAL, fa_simplifier
 
 
 class Resolver:
@@ -30,6 +33,24 @@ def __init__(
         matchers: Matchers,
     ) -> None:
         self.user_agent_matchers, self.os_matchers, self.device_matchers = matchers
+        if IS_GRAAL:
+            matcher: Any
+            kind = next(
+                (
+                    "eager" if hasattr(type(m), "regex") else "lazy"
+                    for m in chain.from_iterable(matchers)
+                ),
+                None,
+            )
+            if kind == "eager":
+                for matcher in chain.from_iterable(matchers):
+                    matcher.pattern = re.compile(
+                        fa_simplifier(matcher.pattern.pattern),
+                        flags=matcher.pattern.flags,
+                    )
+            elif kind == "lazy":
+                for matcher in chain.from_iterable(matchers):
+                    matcher.regex = fa_simplifier(matcher.pattern.pattern)
 
     def __call__(self, ua: str, domains: Domain, /) -> PartialResult:
         parse = methodcaller("__call__", ua)

src/ua_parser/re2.py

@@ -14,6 +14,7 @@
     PartialResult,
     UserAgent,
 )
+from .utils import fa_simplifier
 
 
 class DummyFilter:
@@ -38,15 +39,15 @@ def __init__(
         if self.user_agent_matchers:
             self.ua = re2.Filter()
             for u in self.user_agent_matchers:
-                self.ua.Add(u.regex)
+                self.ua.Add(fa_simplifier(u.regex))
             self.ua.Compile()
         else:
             self.ua = DummyFilter()
 
         if self.os_matchers:
             self.os = re2.Filter()
             for o in self.os_matchers:
-                self.os.Add(o.regex)
+                self.os.Add(fa_simplifier(o.regex))
             self.os.Compile()
         else:
             self.os = DummyFilter()
@@ -58,9 +59,9 @@ def __init__(
                 # no pattern uses global flags, but since they're not
                 # supported in JS that seems safe.
                 if d.flags & re.IGNORECASE:
-                    self.devices.Add("(?i)" + d.regex)
+                    self.devices.Add("(?i)" + fa_simplifier(d.regex))
                 else:
-                    self.devices.Add(d.regex)
+                    self.devices.Add(fa_simplifier(d.regex))
             self.devices.Compile()
         else:
             self.devices = DummyFilter()

src/ua_parser/utils.py

@@ -1,6 +1,9 @@
+import platform
 import re
 from typing import Match, Optional
 
+IS_GRAAL: bool = platform.python_implementation() == "GraalVM"
+
 
 def get(m: Match[str], idx: int) -> Optional[str]:
     return (m[idx] or None) if 0 < idx <= m.re.groups else None
@@ -28,3 +31,33 @@ def replacer(repl: str, m: Match[str]) -> Optional[str]:
         return None
 
     return re.sub(r"\$(\d)", lambda n: get(m, int(n[1])) or "", repl).strip() or None
+
+
+REPETITION_PATTERN = re.compile(r"\{(0|1)\s*,\s*\d{3,}\}")
+CLASS_PATTERN = re.compile(
+    r"""
+\[[^]]*\\(d|w)[^]]*\]
+|
+\\(d|w)
+""",
+    re.VERBOSE,
+)
+
+
+def class_replacer(m: re.Match[str]) -> str:
+    d, w = ("0-9", "A-Za-z0-9_") if m[1] else ("[0-9]", "[A-Za-z0-9_]")
+    return m[0].replace(r"\d", d).replace(r"\w", w)
+
+
+def fa_simplifier(pattern: str) -> str:
+    """uap-core makes significant use of large bounded repetitions, to
+    mitigate catastrophic backtracking.
+
+    However this explodes the number of states (and thus graph size)
+    for finite automaton engines, which significantly increases their
+    memory use, and for those which use JITs it can exceed the JIT
+    threshold and force fallback to a slower engine (seems to be the
+    case for graal's TRegex).
+    """
+    pattern = REPETITION_PATTERN.sub(lambda m: "*" if m[1] == "0" else "+", pattern)
+    return CLASS_PATTERN.sub(class_replacer, pattern)

tests/test_fa_simplifier.py

@@ -0,0 +1,15 @@
+import pytest  # type: ignore
+
+from ua_parser.utils import fa_simplifier
+
+
+@pytest.mark.parametrize(
+    ("from_", "to"),
+    [
+        (r"\d", "[0-9]"),
+        (r"[\d]", "[0-9]"),
+        (r"[\d\.]", r"[0-9\.]"),
+    ],
+)
+def test_classes(from_, to):
+    assert fa_simplifier(from_) == to