pytest_plugin(feat): Add fixture performance optimization and profiling

why: Test suite was slow due to separate master copies for sync/async fixtures,
no cross-session caching, and no visibility into fixture timing.

what:
- Add fixture profiling hooks with --fixture-durations and --fixture-durations-min
- Unify master copy caching (sync/async share git_repo_master, hg_repo_master, etc.)
- Add XDG persistent cache at ~/.cache/libvcs-test/<version-hash>/
- Add --libvcs-cache-dir and --libvcs-clear-cache pytest options
- Add RepoFixtureResult[T] dataclass with metadata (from_cache, created_at, etc.)
- Add @pytest.mark.performance marker with --run-performance option
- Create tests/test_fixture_performance.py with 16 performance tests

Author: tony

conftest.py

@@ -10,15 +10,130 @@
 
 from __future__ import annotations
 
+import time
 import typing as t
+from collections import defaultdict
 
 import pytest
 
 if t.TYPE_CHECKING:
     import pathlib
 
+    from _pytest.fixtures import FixtureDef, SubRequest
+    from _pytest.terminal import TerminalReporter
+
 pytest_plugins = ["pytester"]
 
+# Fixture profiling storage
+_fixture_timings: dict[str, list[float]] = defaultdict(list)
+_fixture_call_counts: dict[str, int] = defaultdict(int)
+
+
+def pytest_addoption(parser: pytest.Parser) -> None:
+    """Add fixture profiling options."""
+    group = parser.getgroup("libvcs", "libvcs fixture options")
+    group.addoption(
+        "--fixture-durations",
+        action="store",
+        type=int,
+        default=0,
+        metavar="N",
+        help="Show N slowest fixture setup times (N=0 for all)",
+    )
+    group.addoption(
+        "--fixture-durations-min",
+        action="store",
+        type=float,
+        default=0.005,
+        metavar="SECONDS",
+        help="Minimum duration to show in fixture timing report (default: 0.005)",
+    )
+    group.addoption(
+        "--run-performance",
+        action="store_true",
+        default=False,
+        help="Run performance tests (marked with @pytest.mark.performance)",
+    )
+
+
+def pytest_collection_modifyitems(
+    config: pytest.Config,
+    items: list[pytest.Item],
+) -> None:
+    """Skip performance tests unless --run-performance is given."""
+    if config.getoption("--run-performance"):
+        # --run-performance given: run all tests
+        return
+
+    skip_performance = pytest.mark.skip(reason="need --run-performance option to run")
+    for item in items:
+        if "performance" in item.keywords:
+            item.add_marker(skip_performance)
+
+
+@pytest.hookimpl(wrapper=True)
+def pytest_fixture_setup(
+    fixturedef: FixtureDef[t.Any],
+    request: SubRequest,
+) -> t.Generator[None, t.Any, t.Any]:
+    """Wrap fixture setup to measure timing."""
+    start = time.perf_counter()
+    try:
+        result = yield
+        return result
+    finally:
+        duration = time.perf_counter() - start
+        fixture_name = fixturedef.argname
+        _fixture_timings[fixture_name].append(duration)
+        _fixture_call_counts[fixture_name] += 1
+
+
+def pytest_terminal_summary(
+    terminalreporter: TerminalReporter,
+    exitstatus: int,
+    config: pytest.Config,
+) -> None:
+    """Display fixture timing summary."""
+    durations_count = config.option.fixture_durations
+    durations_min = config.option.fixture_durations_min
+
+    # Skip if no timing requested (durations_count defaults to 0 meaning "off")
+    if durations_count == 0 and not config.option.verbose:
+        return
+
+    # Build summary data
+    fixture_stats: list[tuple[str, float, int, float]] = []
+    for name, times in _fixture_timings.items():
+        total_time = sum(times)
+        call_count = len(times)
+        avg_time = total_time / call_count if call_count > 0 else 0
+        fixture_stats.append((name, total_time, call_count, avg_time))
+
+    # Sort by total time descending
+    fixture_stats.sort(key=lambda x: x[1], reverse=True)
+
+    # Filter by minimum duration
+    fixture_stats = [s for s in fixture_stats if s[1] >= durations_min]
+
+    if not fixture_stats:
+        return
+
+    # Limit count if specified
+    if durations_count > 0:
+        fixture_stats = fixture_stats[:durations_count]
+
+    terminalreporter.write_sep("=", "fixture setup times")
+    terminalreporter.write_line("")
+    terminalreporter.write_line(
+        f"{'Fixture':<40} {'Total':>10} {'Calls':>8} {'Avg':>10}",
+    )
+    terminalreporter.write_line("-" * 70)
+
+    for name, total, calls, avg in fixture_stats:
+        terminalreporter.write_line(
+            f"{name:<40} {total:>9.3f}s {calls:>8} {avg:>9.3f}s",
+        )
+
 
 @pytest.fixture(autouse=True)
 def add_doctest_fixtures(

pyproject.toml

@@ -239,6 +239,9 @@ filterwarnings = [
 ]
 asyncio_mode = "strict"
 asyncio_default_fixture_loop_scope = "function"
+markers = [
+  "performance: marks tests as performance tests (deselect with '-m \"not performance\"')",
+]
 
 [tool.pytest-watcher]
 now = true