log_json: append the server-notice/log stream to a file as JSONL (log_json = <path>)

src/modules/log_json.rs

@@ -0,0 +1,46 @@
+//! log_json — append the server-notice / log stream to a file as JSON, one object
+//! per line (JSONL). Off unless `log_json = <path>` is set. Reuses the
+//! `draft/json-log` object builder. The file handle is cached on the core thread
+//! (snotice is single-threaded) and reopened if the path changes or a write fails —
+//! so an external logrotate that renames the file is picked up on the next line.
+
+use std::cell::RefCell;
+use std::fs::{File, OpenOptions};
+use std::io::Write;
+
+use crate::server::Server;
+
+thread_local! {
+    /// (configured path, open append handle) cached for reuse.
+    static SINK: RefCell<Option<(String, File)>> = const { RefCell::new(None) };
+}
+
+/// Append `msg` as a JSON line to the configured log file. Called at the tail of
+/// [`Server::snotice`].
+pub fn tee(s: &Server, msg: &str) {
+    let Some(path) = s.conf("log_json") else {
+        SINK.with(|c| *c.borrow_mut() = None); // disabled: drop any handle
+        return;
+    };
+    let line = crate::modules::jsonlog::json_line(s, msg);
+    SINK.with(|cell| {
+        let mut slot = cell.borrow_mut();
+        let need_open = match slot.as_ref() {
+            Some((p, _)) => p != path,
+            None => true,
+        };
+        if need_open {
+            *slot = OpenOptions::new()
+                .create(true)
+                .append(true)
+                .open(path)
+                .ok()
+                .map(|f| (path.to_string(), f));
+        }
+        if let Some((_, f)) = slot.as_mut() {
+            if writeln!(f, "{line}").is_err() {
+                *slot = None; // reopen next time
+            }
+        }
+    });
+}