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).

Use low-level fd redirection to suppress that message as it does not
seem to hurt, and not compiling is a lot worse (google/re2#484).

To add to the fun, when running under pytest not only is stderr
captured `sys.stderr` is redirected to an object with a different fd.
Since the C stdlib does not even know about that it obviously keeps
writing to the "normal" stderr, so use the raw fd number (2) instead
of use the symbolic `sys.stderr.fileno()`.

Thankfully this mess seems to be working on windows

Less thankfully, since fds are process global and there's no real lock
I know of this introduces a race, anyone could be trying to write to
stderr while we've redirected the fd and I don't know how to fix
this...

src/ua_parser/re2.py

@@ -1,7 +1,10 @@
 __all__ = ["Resolver"]
 
+import contextlib
+import os
 import re
-from typing import List
+import sys
+from typing import Iterator, List
 
 import re2  # type: ignore
 
@@ -15,6 +18,21 @@
     UserAgent,
 )
 
+devnull = os.open(os.devnull, os.O_WRONLY | os.O_APPEND)
+@contextlib.contextmanager
+def suppress_stderr() -> Iterator[None]:
+    """Temporarily redirect stderr to devnull.
+
+    """
+    with contextlib.ExitStack() as defer:
+        old = os.dup(2)
+        defer.callback(os.close, old)
+
+        os.dup2(devnull, 2)
+        defer.callback(os.dup2, old, 2)
+
+        yield
+
 
 class Resolver:
     ua: re2.Filter

tests/test_re2.py

@@ -0,0 +1,29 @@
+import pytest
+
+from ua_parser import Domain, PartialParseResult
+from ua_parser.loaders import load_data
+re2 = pytest.importorskip("ua_parser.re2")
+from re2 import Filter
+
+
+def test_suppress_stderr(capfd) -> None:
+    f = Filter()
+    f.Compile()
+    _, err = capfd.readouterr()
+    assert err == "re2/filtered_re2.cc:74: Compile called before Add.\n"
+
+    f = Filter()
+    with re2.suppress_stderr():
+        f.Compile()
+    out, err = capfd.readouterr()
+    assert out == ""
+    assert err == ""
+
+
+def test_empty(capfd) -> None:
+    r = re2.Resolver(([], [], []))
+    assert r("", Domain.ALL) == PartialParseResult(Domain.ALL, None, None, None, "")
+    out, err = capfd.readouterr()
+    assert out == ""
+    assert err == ""
+