Suppress "Compile called before Add" in re2.Filter

When compiling an empty set, ``FilteredRE2::Compile`` logs a warning
to stderr which can not be suppressed (google/re2#485).

Replace `re2.Filter` by a null object if the corresponding matchers
list is empty: not only do we need to skip `Filter.Compile` to
suppress the warning message, we need to skip `Filter.Match` or the
program will segfault (google/re2#484). Using a null object seems
safer and more reliable than adding conditionals, even if it requires
more code and reindenting half the file.

Doing this also seems safer than my first instinct of trying to use
low-level fd redirection: fd redirection suffers from race
conditions[^thread] and could suffer from other cross-platform
compatibility issues (e.g. does every python-supported OS have stderr
on fd 2 and correctly supports dup, dup2, and close?)

[^thread]: AFAIK CPython does not provide a python-level GIL-pin
    feature (even less so with the GILectomy plans), so we have no way
    to prevent context-switching and any message sent to stderr by
    sibling threads would be lost

pyproject.toml

@@ -51,7 +51,8 @@ ignore = [
 ]
 
 [tool.ruff.lint.isort]
-classes = ["LRU", "OS"]
+classes = ["OS"]
+known-first-party = ["ua_parser"]
 combine-as-imports = true
 
 [tool.mypy]

src/ua_parser/re2.py

@@ -16,6 +16,11 @@
 )
 
 
+class DummyFilter:
+    def Match(self) -> None:
+        pass
+
+
 class Resolver:
     ua: re2.Filter
     user_agent_matchers: List[Matcher[UserAgent]]
@@ -30,26 +35,35 @@ def __init__(
     ) -> None:
         self.user_agent_matchers, self.os_matchers, self.device_matchers = matchers
 
-        self.ua = re2.Filter()
-        for u in self.user_agent_matchers:
-            self.ua.Add(u.pattern)
-        self.ua.Compile()
+        if self.user_agent_matchers:
+            self.ua = re2.Filter()
+            for u in self.user_agent_matchers:
+                self.ua.Add(u.pattern)
+            self.ua.Compile()
+        else:
+            self.ua = DummyFilter()
 
-        self.os = re2.Filter()
-        for o in self.os_matchers:
-            self.os.Add(o.pattern)
-        self.os.Compile()
+        if self.os_matchers:
+            self.os = re2.Filter()
+            for o in self.os_matchers:
+                self.os.Add(o.pattern)
+            self.os.Compile()
+        else:
+            self.os = DummyFilter()
 
-        self.devices = re2.Filter()
-        for d in self.device_matchers:
-            # Prepend the i global flag if IGNORECASE is set. Assumes
-            # 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.pattern)
-            else:
-                self.devices.Add(d.pattern)
-        self.devices.Compile()
+        if self.device_matchers:
+            self.devices = re2.Filter()
+            for d in self.device_matchers:
+                # Prepend the i global flag if IGNORECASE is set. Assumes
+                # 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.pattern)
+                else:
+                    self.devices.Add(d.pattern)
+            self.devices.Compile()
+        else:
+            self.devices = DummyFilter()
 
     def __call__(self, ua: str, domains: Domain, /) -> PartialParseResult:
         user_agent = os = device = None

tests/test_re2.py

@@ -0,0 +1,13 @@
+import pytest  # type: ignore
+
+from ua_parser import Domain, PartialParseResult
+
+re2 = pytest.importorskip("ua_parser.re2")
+
+
+def test_empty(capfd: pytest.CaptureFixture[str]) -> None:
+    r = re2.Resolver(([], [], []))
+    assert r("", Domain.ALL) == PartialParseResult(Domain.ALL, None, None, None, "")
+    out, err = capfd.readouterr()
+    assert out == ""
+    assert err == ""