util: add jsonl_load/jsonl_append helpers, convert graph.rs

Shared JSONL read/write in util.rs replaces hand-rolled patterns.
graph.rs metrics load/save converted as first consumer.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

poc-memory/src/graph.rs

@@ -524,27 +524,12 @@ fn metrics_log_path() -> std::path::PathBuf {
 
 /// Load previous metrics snapshots
 pub fn load_metrics_history() -> Vec<MetricsSnapshot> {
-    let path = metrics_log_path();
+    crate::util::jsonl_load(&metrics_log_path())
-    let content = match std::fs::read_to_string(&path) {
-        Ok(c) => c,
-        Err(_) => return Vec::new(),
-    };
-    content.lines()
-        .filter_map(|line| serde_json::from_str(line).ok())
-        .collect()
 }
 
 /// Append a metrics snapshot to the log
 pub fn save_metrics_snapshot(snap: &MetricsSnapshot) {
-    let path = metrics_log_path();
+    let _ = crate::util::jsonl_append(&metrics_log_path(), snap);
-    if let Ok(json) = serde_json::to_string(snap) {
-        use std::io::Write;
-        if let Ok(mut f) = std::fs::OpenOptions::new()
-            .create(true).append(true).open(&path)
-        {
-            let _ = writeln!(f, "{}", json);
-        }
-    }
 }
 
 /// Compute current graph metrics as a snapshot (no side effects).

poc-memory/src/util.rs

@@ -2,8 +2,12 @@
 
 use crate::store;
 
+use serde::de::DeserializeOwned;
+use serde::Serialize;
+
 use std::fs;
-use std::path::PathBuf;
+use std::io::Write;
+use std::path::{Path, PathBuf};
 
 /// Ensure a subdirectory of the memory dir exists and return its path.
 pub fn memory_subdir(name: &str) -> Result<PathBuf, String> {
@@ -29,6 +33,30 @@ pub fn first_n_chars(s: &str, n: usize) -> String {
     s.chars().take(n).collect()
 }
 
+// ── JSONL helpers ───────────────────────────────────────────────────
+
+/// Read a JSONL file, deserializing each line. Silently skips bad lines.
+pub fn jsonl_load<T: DeserializeOwned>(path: &Path) -> Vec<T> {
+    let content = match fs::read_to_string(path) {
+        Ok(c) => c,
+        Err(_) => return Vec::new(),
+    };
+    content.lines()
+        .filter_map(|line| serde_json::from_str(line).ok())
+        .collect()
+}
+
+/// Append one record as a JSON line to a file (create if missing).
+pub fn jsonl_append<T: Serialize>(path: &Path, item: &T) -> Result<(), String> {
+    let json = serde_json::to_string(item)
+        .map_err(|e| format!("serialize: {}", e))?;
+    let mut f = fs::OpenOptions::new()
+        .create(true).append(true).open(path)
+        .map_err(|e| format!("open {}: {}", path.display(), e))?;
+    writeln!(f, "{}", json)
+        .map_err(|e| format!("write {}: {}", path.display(), e))
+}
+
 /// Parse a timestamp string to unix epoch seconds.
 /// Handles: "2026-03-05T19:56:00", "2026-03-05T19:56", "2026-03-05 19:56:00", "2026-03-05 19:56"
 pub fn parse_timestamp_to_epoch(ts: &str) -> Option<i64> {