fix: concurrent cache save uses unique temp files (#17)

Replace deterministic .index.tmp path with per-save unique temp files
via tempfile::Builder. Eliminates race condition where background
indexer thread and TUI main thread could collide on the same temp path,
causing data truncation or corrupt cache writes.

Changes:
- Add write_cache_atomically() helper using tempfile::Builder
- Refactor save_with_hash() and save() to use the helper
- Extract encode_cache() to deduplicate serialization logic
- Move tempfile from [dev-dependencies] to [dependencies]
- Add 2 concurrent tests validating no corruption under parallel writes

Fixes #17

crates/core/Cargo.toml

@@ -18,7 +18,7 @@ directories.workspace = true
 xxhash-rust.workspace = true
 bincode.workspace = true
 unicode-width.workspace = true
+tempfile.workspace = true
 
 [dev-dependencies]
 insta.workspace = true
-tempfile.workspace = true

crates/core/src/io/index_cache.rs

@@ -6,8 +6,44 @@ use crate::io::line_index::LineIndex;
 
 pub struct IndexCache;
 
+/// Write `buf` to `dest` atomically using a unique temporary file in the same directory.
+///
+/// Each call creates its own temp file via `tempfile::Builder`, eliminating collisions
+/// when multiple threads (or processes) save to the same cache path concurrently.
+/// The temp file is created in `dest.parent()` so the final `rename` stays on the
+/// same filesystem and remains atomic.
+fn write_cache_atomically(dest: &Path, buf: &[u8]) -> std::io::Result<()> {
+    let dir = dest.parent().ok_or_else(|| {
+        std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            "cache destination has no parent directory",
+        )
+    })?;
+
+    let mut tmp = tempfile::Builder::new()
+        .prefix("index-cache-")
+        .suffix(".tmp")
+        .tempfile_in(dir)?;
+
+    tmp.as_file_mut().write_all(buf)?;
+    tmp.as_file_mut().sync_all()?;
+
+    tmp.persist(dest).map(|_| ()).map_err(|e| e.error)
+}
+
+fn encode_cache(file_hash: u64, index: &LineIndex) -> std::io::Result<Vec<u8>> {
+    let index_bytes = bincode::serialize(index)
+        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e.to_string()))?;
+
+    let mut buf = Vec::with_capacity(1 + 8 + index_bytes.len());
+    buf.push(CACHE_VERSION);
+    buf.extend_from_slice(&file_hash.to_le_bytes());
+    buf.extend_from_slice(&index_bytes);
+    Ok(buf)
+}
+
 impl IndexCache {
-    /// Save a `LineIndex` to disk using atomic write (write to .tmp, then rename).
+    /// Save a `LineIndex` to disk using atomic write (unique temp file, then rename).
     ///
     /// The file hash is derived from `data` (the same byte slice used to build the index),
     /// avoiding TOCTOU issues from re-reading the file from disk.
@@ -21,23 +57,8 @@ impl IndexCache {
         })?;
 
         let file_hash = compute_data_hash(data);
-        let index_bytes = bincode::serialize(index)
-            .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e.to_string()))?;
-
-        let mut buf = Vec::with_capacity(1 + 8 + index_bytes.len());
-        buf.push(CACHE_VERSION);
-        buf.extend_from_slice(&file_hash.to_le_bytes());
-        buf.extend_from_slice(&index_bytes);
-
-        let tmp_path = dest.with_extension("index.tmp");
-        {
-            let mut f = std::fs::File::create(&tmp_path)?;
-            f.write_all(&buf)?;
-            f.sync_all()?;
-        }
-        std::fs::rename(&tmp_path, &dest)?;
-
-        Ok(())
+        let buf = encode_cache(file_hash, index)?;
+        write_cache_atomically(&dest, &buf)
     }
 
     /// Save a `LineIndex` to disk, computing the hash by re-reading the file.
@@ -50,23 +71,8 @@ impl IndexCache {
         })?;
 
         let file_hash = compute_file_hash(file_path)?;
-        let index_bytes = bincode::serialize(index)
-            .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e.to_string()))?;
-
-        let mut buf = Vec::with_capacity(1 + 8 + index_bytes.len());
-        buf.push(CACHE_VERSION);
-        buf.extend_from_slice(&file_hash.to_le_bytes());
-        buf.extend_from_slice(&index_bytes);
-
-        let tmp_path = dest.with_extension("index.tmp");
-        {
-            let mut f = std::fs::File::create(&tmp_path)?;
-            f.write_all(&buf)?;
-            f.sync_all()?;
-        }
-        std::fs::rename(&tmp_path, &dest)?;
-
-        Ok(())
+        let buf = encode_cache(file_hash, index)?;
+        write_cache_atomically(&dest, &buf)
     }
 
     /// Load a cached `LineIndex` from disk.
@@ -305,4 +311,77 @@ mod tests {
 
         assert_ne!(h1, h2, "hash should change when content changes");
     }
+
+    #[test]
+    fn test_concurrent_save_with_hash_no_corruption() {
+        use std::sync::{Arc, Barrier};
+
+        let file = make_test_file(300);
+        let data = std::fs::read(file.path()).unwrap();
+
+        let num_threads = 8;
+        let iterations = 50;
+        let barrier = Arc::new(Barrier::new(num_threads));
+        let path = file.path().to_path_buf();
+
+        let handles: Vec<_> = (0..num_threads)
+            .map(|_| {
+                let barrier = Arc::clone(&barrier);
+                let path = path.clone();
+                let data = data.clone();
+                std::thread::spawn(move || {
+                    let index = LineIndex::from_bytes(&data);
+                    barrier.wait();
+                    for _ in 0..iterations {
+                        IndexCache::save_with_hash(&path, &index, &data)
+                            .expect("concurrent save_with_hash should succeed");
+                    }
+                })
+            })
+            .collect();
+
+        for h in handles {
+            h.join().expect("thread should not panic");
+        }
+
+        let loaded = IndexCache::load(file.path()).expect("final cache should load successfully");
+        let expected = LineIndex::from_bytes(&data);
+        assert_eq!(loaded.line_count(), expected.line_count());
+        assert_eq!(
+            loaded.sampled_offsets(),
+            expected.sampled_offsets(),
+            "concurrent writes must not produce interleaved data"
+        );
+    }
+
+    #[test]
+    fn test_concurrent_save_same_dest_all_succeed() {
+        use std::sync::{Arc, Barrier};
+
+        let dir = tempfile::tempdir().unwrap();
+        let dest = dir.path().join("test.index");
+
+        let payloads: Vec<Vec<u8>> = (0..8).map(|i| vec![i; 64 * 1024]).collect();
+        let barrier = Arc::new(Barrier::new(8));
+
+        let handles: Vec<_> = payloads
+            .into_iter()
+            .map(|payload| {
+                let barrier = Arc::clone(&barrier);
+                let dest = dest.clone();
+                std::thread::spawn(move || {
+                    barrier.wait();
+                    write_cache_atomically(&dest, &payload)
+                        .expect("concurrent atomic write should succeed");
+                })
+            })
+            .collect();
+
+        for h in handles {
+            h.join().expect("thread should not panic");
+        }
+
+        let final_data = std::fs::read(&dest).expect("dest file should exist");
+        assert_eq!(final_data.len(), 64 * 1024, "final file must be exactly one payload");
+    }
 }