feat(rules): save atomically, preserving unknown fields

Write to a temp file and rename, so the hook can never read a partial
file. Fields this version does not understand round-trip untouched,
which is what makes the format belong to neither tool.

1 files changed, 47 insertions, 0 deletions

diff --git a/test_mailrules.py b/test_mailrules.py
index e08f490..a3faa5e 100755
--- a/test_mailrules.py
+++ b/test_mailrules.py
@@ -190,6 +190,53 @@ def test_ordered_skips_disabled_rules():
         assert [r.id for r in store.rules] == ["on", "off"]
 
 
+def test_save_round_trips_unknown_fields():
+    """The neutrality guarantee. If this tool strips a field qtmaildir
+    added, the file is this tool's file that qtmaildir may read."""
+    with tempfile.TemporaryDirectory() as tmp:
+        path = write_rules(tmp, {
+            "version": 1,
+            "future_top_level": {"set_by": "another tool"},
+            "rules": [{
+                "id": "keeper",
+                "add": ["x"],
+                "query": "from:a@example.com",
+                "future_field": [1, 2, 3],
+            }],
+        })
+        store = mailrules.load(path)
+        assert store.rules[0].unknown == {"future_field": [1, 2, 3]}
+
+        mailrules.save(store, path)
+
+        raw = json.loads(path.read_text())
+        assert raw["future_top_level"] == {"set_by": "another tool"}
+        assert raw["rules"][0]["future_field"] == [1, 2, 3]
+        assert raw["rules"][0]["id"] == "keeper"
+        assert raw["version"] == 1
+
+
+def test_save_is_atomic():
+    """A reader must never see a half-written file: the hook runs every ten
+    minutes and a truncated read would be a failed sync."""
+    with tempfile.TemporaryDirectory() as tmp:
+        path = Path(tmp) / "rules.json"
+        store = mailrules.Store(rules=[
+            mailrules.Rule(id="a", query="from:a@example.com", add=["x"])])
+        mailrules.save(store, path)
+        # The temp file the write went through must not be left behind.
+        assert [p.name for p in Path(tmp).iterdir()] == ["rules.json"]
+        assert json.loads(path.read_text())["rules"][0]["id"] == "a"
+
+
+def test_save_creates_the_directory():
+    with tempfile.TemporaryDirectory() as tmp:
+        path = Path(tmp) / "nested" / "rules.json"
+        mailrules.save(mailrules.Store(), path)
+        assert path.exists()
+        assert json.loads(path.read_text()) == {"version": 1, "rules": []}
+
+
 def run_all():
     for name, fn in sorted(globals().items()):
         if name.startswith("test_") and callable(fn):