📝 document runtime sink file policies

2026-07-24 00:42:18 +00:00 · 2026-06-13 23:16:37 +08:00
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+---
+name: runtime-sink-file-default-policy
+group: api
+category: runtime
+update-time: 20260613
+description: Read the initial default file policy associated with a RuntimeSink.
+key-word:
+    - runtime
+    - sink
+    - file
+    - public
+---
+
+## Runtime-sink-file-default-policy
+
+Read the initial default file policy associated with a `RuntimeSink`. This helper exposes the baseline file policy captured when the runtime sink was created.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::file_default_policy(self : RuntimeSink) -> FileSinkPolicy {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose default file policy should be inspected.
+
+#### output
+
+- `FileSinkPolicy` - Initial default file policy.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain `File` runtime variants return the default policy captured at creation time.
+- `QueuedFile` runtime variants forward the default policy from the wrapped inner `FileSink`.
+- Non-file runtime variants return the neutral fallback policy `FileSinkPolicy::new(append=false, auto_flush=false, rotation=None)`.
+- This helper is useful when callers need to compare runtime drift or restore defaults later.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Baseline Policy Visibility
+
+When diagnostics should show the original file policy separately from the live one:
+```moonbit
+let defaults = sink.file_default_policy()
+```
+
+In this example, the runtime sink exposes its original file policy snapshot.
+
+#### When Prepare For Policy Reset Logic
+
+When tooling should capture or compare default settings explicitly:
+```moonbit
+let original = sink.file_default_policy()
+```
+
+In this example, callers can reason about factory file policy separately from runtime changes.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not file-backed, the return value is a neutral fallback policy.
+
+- If callers only need to know whether runtime drift exists, `file_policy_matches_default()` is the simpler API.
+
+### Notes
+
+1. Use this helper when the original file policy matters operationally.
+
+2. It complements `file_policy()` and `file_reset_policy()`.