From c2896d8896235df027dc38d7c93ffe4761d4b02d Mon Sep 17 00:00:00 2001
From: "Danilo M." <danix@danix.xyz>
Date: Wed, 12 Aug 2026 11:30:06 +0200
Subject: 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.
---
 mailrules.py | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

(limited to 'mailrules.py')

diff --git a/mailrules.py b/mailrules.py
index 7df1b42..8c65708 100755
--- a/mailrules.py
+++ b/mailrules.py
@@ -31,6 +31,7 @@ runs on every sync, and mailctl has no dependencies to inherit.
 import json
 import os
 import re
+import tempfile
 from dataclasses import dataclass, field
 from pathlib import Path
 
@@ -125,6 +126,59 @@ def load(path=None):
     return store
 
 
+def save(store, path=None):
+    """Write the store atomically: a temp file in the same directory, then
+    rename. Rename within a filesystem is atomic, so a concurrent reader sees
+    either the old file or the new one and never a partial write.
+
+    There is no locking. Last writer wins on a true collision, which is
+    accepted for a single-user setup; the failure that would actually hurt is
+    a truncated read by the hook, and rename eliminates it.
+    """
+    path = Path(path) if path else default_path()
+    path.parent.mkdir(parents=True, exist_ok=True)
+
+    payload = dict(store.unknown)
+    payload["version"] = FORMAT_VERSION
+    payload["rules"] = [_rule_to_dict(r) for r in store.rules]
+
+    # delete=False plus an explicit replace: NamedTemporaryFile would unlink
+    # the file on close, and the rename is the whole point.
+    handle = tempfile.NamedTemporaryFile(
+        mode="w", dir=path.parent, prefix=".rules-", suffix=".tmp",
+        delete=False)
+    try:
+        with handle:
+            json.dump(payload, handle, indent=2, ensure_ascii=False)
+            handle.write("\n")
+            handle.flush()
+            os.fsync(handle.fileno())
+        os.replace(handle.name, path)
+    except BaseException:
+        # A failed write must not leave the temp file beside the real one.
+        try:
+            os.unlink(handle.name)
+        except OSError:
+            pass
+        raise
+
+
+def _rule_to_dict(rule):
+    """Known fields first in a stable order, then anything this version did
+    not understand. Stable ordering keeps a diff of this file readable."""
+    out = {
+        "id": rule.id,
+        "stage": rule.stage,
+        "enabled": rule.enabled,
+        "add": list(rule.add),
+        "remove": list(rule.remove),
+        "query": rule.query,
+        "note": rule.note,
+    }
+    out.update(rule.unknown)
+    return out
+
+
 def ordered(rules):
     """Enabled rules in execution order: by stage ascending, ties by position.
 
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