mermaid_cli/utils/
atomic.rs

1//! Atomic file writes.
2//!
3//! Plain `fs::write` truncates the target to zero length and then writes the
4//! new contents in place. A crash / kill / disk-full between those two steps
5//! leaves the file empty or half-written — catastrophic for session and
6//! checkpoint state that is rewritten in full on every save. [`write_atomic`]
7//! writes to a temp sibling, fsyncs it, then renames over the target, so a
8//! reader always sees either the old complete file or the new complete file.
9
10use std::fs::{self, File};
11use std::io::Write;
12use std::path::Path;
13use std::sync::atomic::{AtomicU64, Ordering};
14
15static COUNTER: AtomicU64 = AtomicU64::new(0);
16
17/// Write `bytes` to `path` atomically: temp file in the same directory →
18/// `sync_all` → rename over the destination. The rename is atomic on the same
19/// filesystem (and replaces an existing target on both Unix and Windows).
20pub fn write_atomic(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
21    let parent = path.parent().unwrap_or_else(|| Path::new("."));
22    fs::create_dir_all(parent)?;
23
24    let stem = path.file_name().and_then(|n| n.to_str()).unwrap_or("tmp");
25    let n = COUNTER.fetch_add(1, Ordering::Relaxed);
26    let tmp = parent.join(format!(".{}.{}.{}.tmp", stem, std::process::id(), n));
27
28    {
29        let mut f = File::create(&tmp)?;
30        f.write_all(bytes)?;
31        f.sync_all()?;
32    }
33
34    if let Err(e) = fs::rename(&tmp, path) {
35        let _ = fs::remove_file(&tmp);
36        return Err(e);
37    }
38
39    // Best-effort durability of the rename itself. Opening a directory as a
40    // File is not supported on Windows, so this is a silent no-op there.
41    if let Ok(dir) = File::open(parent) {
42        let _ = dir.sync_all();
43    }
44    Ok(())
45}
46
47#[cfg(test)]
48mod tests {
49    use super::*;
50
51    #[test]
52    fn atomic_write_replaces_existing_and_no_temp_left() {
53        let dir = std::env::temp_dir().join(format!("mermaid_atomic_{}", std::process::id()));
54        let _ = fs::create_dir_all(&dir);
55        let target = dir.join("conv.json");
56        write_atomic(&target, b"first").unwrap();
57        assert_eq!(fs::read_to_string(&target).unwrap(), "first");
58        write_atomic(&target, b"second").unwrap();
59        assert_eq!(fs::read_to_string(&target).unwrap(), "second");
60        // No leftover temp files.
61        let leftovers = fs::read_dir(&dir)
62            .unwrap()
63            .flatten()
64            .filter(|e| e.file_name().to_string_lossy().ends_with(".tmp"))
65            .count();
66        assert_eq!(leftovers, 0);
67        let _ = fs::remove_dir_all(&dir);
68    }
69}
mermaid_cli/utils/atomic.rs

mermaid_cli/utils/
atomic.rs
