perf(config): mtime-signature fast path for load_config (#410)

load_config re-read and re-parsed config.json, the secrets file, AND the
template (running the recursive migration diff) on every call — with
~30 call sites in web request handlers, some hit 2-3x per request.

Fast path: stat all three files (mtime_ns + size); when unchanged since
the last successful load, return the already-parsed self.config (same
aliasing semantics as before). The signature is taken AFTER load +
migration so a migration write-back doesn't retrigger, and both save
paths refresh it. Cross-process freshness is preserved by construction:
a save from the other process bumps the file mtime, so the next load
here re-reads — verified by a dedicated test. Same-second edits are
caught by mtime_ns plus a size check.


Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

test/test_config_load_cache.py

@@ -0,0 +1,129 @@
+"""Tests for load_config's mtime fast path (src/config_manager.py).
+
+load_config used to re-read + re-parse config.json, the template (with a
+recursive migration diff) and secrets on EVERY call — ~30 web request
+handlers call it, some 2-3x per request. The fast path skips all of it
+when the three files' (mtime_ns, size) signatures are unchanged.
+
+The invariant that matters most: cross-process freshness — a save from
+the web process must be picked up by the display process's next load.
+That's guaranteed because the signature is re-stat'd on every call.
+"""
+
+import json
+import os
+import sys
+
+import pytest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+from src.config_manager import ConfigManager  # noqa: E402
+
+
+@pytest.fixture
+def mgr(tmp_path):
+    config = tmp_path / "config.json"
+    secrets = tmp_path / "secrets.json"
+    template = tmp_path / "template.json"
+    config.write_text(json.dumps({"display": {"brightness": 90}, "timezone": "UTC"}))
+    secrets.write_text(json.dumps({"weather": {"api_key": "sek"}}))
+    template.write_text(json.dumps({"display": {"brightness": 90}, "timezone": "UTC"}))
+    m = ConfigManager(config_path=str(config), secrets_path=str(secrets))
+    m.template_path = str(template)
+    return m, config, secrets, template
+
+
+def _count_opens(monkeypatch, mgr_paths):
+    """Count open() calls hitting the config files."""
+    counts = {"n": 0}
+    real_open = open
+
+    def counting_open(file, *args, **kwargs):
+        if str(file) in mgr_paths:
+            counts["n"] += 1
+        return real_open(file, *args, **kwargs)
+
+    import builtins
+    monkeypatch.setattr(builtins, "open", counting_open)
+    return counts
+
+
+class TestFastPath:
+    def test_unchanged_files_are_not_reread(self, mgr, monkeypatch):
+        m, config, secrets, template = mgr
+        first = m.load_config()
+        assert first["weather"]["api_key"] == "sek"  # secrets merged
+        counts = _count_opens(monkeypatch, {str(config), str(secrets), str(template)})
+        for _ in range(10):
+            again = m.load_config()
+        assert counts["n"] == 0, "fast path must not re-open any config file"
+        assert again is first  # same aliasing semantics as the full path
+
+    def test_config_change_triggers_reload(self, mgr):
+        m, config, secrets, template = mgr
+        m.load_config()
+        data = json.loads(config.read_text())
+        data["display"]["brightness"] = 55
+        config.write_text(json.dumps(data))
+        os.utime(config, (os.stat(config).st_atime, os.stat(config).st_mtime + 2))
+        assert m.load_config()["display"]["brightness"] == 55
+
+    def test_secrets_change_triggers_reload(self, mgr):
+        m, config, secrets, template = mgr
+        m.load_config()
+        secrets.write_text(json.dumps({"weather": {"api_key": "NEW"}}))
+        os.utime(secrets, (os.stat(secrets).st_atime, os.stat(secrets).st_mtime + 2))
+        assert m.load_config()["weather"]["api_key"] == "NEW"
+
+    def test_template_change_triggers_reload_and_migration(self, mgr):
+        m, config, secrets, template = mgr
+        m.load_config()
+        template.write_text(json.dumps({
+            "display": {"brightness": 90}, "timezone": "UTC",
+            "brand_new_key": {"added": True}}))
+        os.utime(template, (os.stat(template).st_atime, os.stat(template).st_mtime + 2))
+        reloaded = m.load_config()
+        assert reloaded.get("brand_new_key") == {"added": True}
+
+    def test_same_second_edit_detected_via_mtime_ns_or_size(self, mgr):
+        """Coarse-mtime same-second edits: size difference still busts it."""
+        m, config, secrets, template = mgr
+        m.load_config()
+        st = os.stat(config)
+        data = json.loads(config.read_text())
+        data["timezone"] = "America/New_York"  # different byte length
+        config.write_text(json.dumps(data))
+        os.utime(config, (st.st_atime, st.st_mtime))  # force same mtime
+        assert m.load_config()["timezone"] == "America/New_York"
+
+
+class TestSaveCoherence:
+    def test_save_config_then_load_returns_saved_data(self, mgr, monkeypatch):
+        m, config, secrets, template = mgr
+        m.load_config()
+        new = {"display": {"brightness": 42}, "timezone": "UTC",
+               "weather": {"api_key": "sek"}}
+        m.save_config(new)
+        counts = _count_opens(monkeypatch, {str(config), str(secrets), str(template)})
+        loaded = m.load_config()
+        assert loaded["display"]["brightness"] == 42
+        assert loaded["weather"]["api_key"] == "sek"  # secrets survive in memory
+        assert counts["n"] == 0  # signature refreshed by save; no re-read
+
+    def test_cross_process_save_is_picked_up(self, mgr):
+        """Another process writing config.json (different mtime) must bust
+        this process's fast path — the core cross-process guarantee."""
+        m, config, secrets, template = mgr
+        m.load_config()
+        other = ConfigManager(config_path=str(config), secrets_path=str(secrets))
+        other.template_path = str(template)
+        other.load_config()
+        other.save_config({"display": {"brightness": 11}, "timezone": "UTC",
+                           "weather": {"api_key": "sek"}})
+        os.utime(config, (os.stat(config).st_atime, os.stat(config).st_mtime + 2))
+        assert m.load_config()["display"]["brightness"] == 11
+
+
+if __name__ == "__main__":
+    sys.exit(pytest.main([__file__, "-v"]))