fix(bench): eliminate SIGBUS handler static mut UB with Once + raw atomics (closes #33)

Replace `static mut OLD_SIGBUS_HANDLER` with AtomicU8 + AtomicPtr to
remove data race UB when concurrent benchmarks call open() from multiple
threads.

Key changes:
- Use `Once::call_once` to guarantee single handler installation
- Publish old handler to atomics BEFORE installing new handler (closes
  the handler-active-but-state-unpublished race window)
- Read atomics with Acquire in signal handler (async-signal-safe)
- Align si_addr to page boundary before mmap(MAP_FIXED)
- Add concurrent test: 8 threads open all 5 variants simultaneously

crates/bench/src/data_gen.rs

@@ -0,0 +1,219 @@
+use std::fs;
+use std::io::{BufRead, BufReader, Write};
+use std::path::{Path, PathBuf};
+
+pub struct TestFileInfo {
+    pub path: PathBuf,
+    pub size_bytes: u64,
+    pub line_count: u64,
+    pub avg_line_length: f64,
+}
+
+/// Check if test file exists and return its info, or generate it
+pub fn ensure_test_file(path: &Path) -> std::io::Result<TestFileInfo> {
+    if path.exists() {
+        return get_file_info(path);
+    }
+    generate_test_file(path)
+}
+
+/// Get info about an existing test file
+fn get_file_info(path: &Path) -> std::io::Result<TestFileInfo> {
+    let metadata = fs::metadata(path)?;
+    let size_bytes = metadata.len();
+
+    let file = fs::File::open(path)?;
+    let mut reader = BufReader::new(file);
+    let mut line_count: u64 = 0;
+    let mut buf = Vec::new();
+    while reader.read_until(b'\n', &mut buf)? > 0 {
+        line_count += 1;
+        buf.clear();
+    }
+
+    let avg_line_length = if line_count > 0 {
+        size_bytes as f64 / line_count as f64
+    } else {
+        0.0
+    };
+
+    Ok(TestFileInfo {
+        path: path.to_path_buf(),
+        size_bytes,
+        line_count,
+        avg_line_length,
+    })
+}
+
+/// Generate a large test file (~5GB / ~74M lines) if it doesn't exist
+fn generate_test_file(path: &Path) -> std::io::Result<TestFileInfo> {
+    if let Some(parent) = path.parent() {
+        fs::create_dir_all(parent)?;
+    }
+
+    let mut file = fs::File::create(path)?;
+    let target_lines: u64 = 74_000_000;
+    for i in 0..target_lines {
+        writeln!(
+            file,
+            "2024-01-15 10:30:{:02} INFO [thread-{}] Application processing request id={} user_id={}",
+            i % 60,
+            i % 16,
+            i,
+            i * 7
+        )?;
+    }
+
+    get_file_info(path)
+}
+
+/// Generate a smaller file (~10MB / ~150K lines) for growth/rotation tests
+pub fn generate_growable_file(dir: &Path) -> std::io::Result<PathBuf> {
+    fs::create_dir_all(dir)?;
+    let path = dir.join("growable.log");
+
+    let mut file = fs::File::create(&path)?;
+    for i in 0..150_000u64 {
+        writeln!(
+            file,
+            "2024-01-15 10:30:{:02} INFO [thread-{}] Appending test line {}",
+            i % 60,
+            i % 16,
+            i
+        )?;
+    }
+
+    Ok(path)
+}
+
+/// Append `count` lines to the file
+pub fn append_lines(path: &Path, count: usize) -> std::io::Result<()> {
+    let mut file = fs::OpenOptions::new().append(true).open(path)?;
+    let existing_lines = count_existing_lines(path).unwrap_or(0);
+    for i in 0..count {
+        writeln!(
+            file,
+            "2024-01-15 10:30:00 INFO Appended line {}",
+            existing_lines + i as u64
+        )?;
+    }
+    Ok(())
+}
+
+/// Truncate file to specified size
+pub fn truncate_file(path: &Path, size: u64) -> std::io::Result<()> {
+    let file = fs::OpenOptions::new().write(true).open(path)?;
+    file.set_len(size)
+}
+
+/// Rotate file: rename existing file, create new empty file
+pub fn rotate_file(path: &Path) -> std::io::Result<PathBuf> {
+    let rotated = path.with_extension("log.1");
+    fs::rename(path, &rotated)?;
+    fs::File::create(path)?;
+    Ok(rotated)
+}
+
+/// Count lines in a file (helper)
+fn count_existing_lines(path: &Path) -> std::io::Result<u64> {
+    let file = fs::File::open(path)?;
+    let reader = BufReader::new(file);
+    Ok(reader.lines().count() as u64)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_generate_growable_file_creates_approximately_correct_size() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = generate_growable_file(dir.path()).unwrap();
+
+        assert!(path.exists());
+        let metadata = fs::metadata(&path).unwrap();
+        let size_mb = metadata.len() as f64 / (1024.0 * 1024.0);
+        // ~150K lines × ~67 bytes ≈ ~10MB; allow 5MB–15MB range
+        assert!(
+            (5.0..=15.0).contains(&size_mb),
+            "Expected ~10MB, got {size_mb:.1}MB"
+        );
+    }
+
+    #[test]
+    fn test_append_lines_increases_line_count() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = {
+            let mut f = fs::File::create(dir.path().join("test.log")).unwrap();
+            for i in 0..10u64 {
+                writeln!(f, "line {i}").unwrap();
+            }
+            dir.path().join("test.log")
+        };
+
+        let before = count_existing_lines(&path).unwrap();
+        append_lines(&path, 5).unwrap();
+        let after = count_existing_lines(&path).unwrap();
+        assert_eq!(after, before + 5);
+    }
+
+    #[test]
+    fn test_truncate_file_reduces_size() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("trunc.log");
+        {
+            let mut f = fs::File::create(&path).unwrap();
+            write!(f, "{}", "A".repeat(1024)).unwrap();
+        }
+
+        let before = fs::metadata(&path).unwrap().len();
+        assert_eq!(before, 1024);
+
+        truncate_file(&path, 512).unwrap();
+        let after = fs::metadata(&path).unwrap().len();
+        assert_eq!(after, 512);
+    }
+
+    #[test]
+    fn test_rotate_file_renames_and_creates_empty() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("rotate.log");
+        {
+            let mut f = fs::File::create(&path).unwrap();
+            write!(f, "original content").unwrap();
+        }
+
+        let rotated = rotate_file(&path).unwrap();
+
+        // Rotated file has the old content
+        assert!(rotated.exists());
+        assert_eq!(fs::read_to_string(&rotated).unwrap(), "original content");
+
+        // New file is empty
+        assert!(path.exists());
+        assert_eq!(fs::metadata(&path).unwrap().len(), 0);
+    }
+
+    #[test]
+    fn test_ensure_test_file_generates_when_missing() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("fresh.log");
+        assert!(!path.exists());
+
+        // Override the generator to use a small file for test speed:
+        // We'll test ensure_test_file indirectly by checking it calls generate_test_file.
+        // Since generate_test_file creates 74M lines (too slow for tests), test the logic
+        // by directly creating a small file and checking get_file_info works.
+        {
+            let mut f = fs::File::create(&path).unwrap();
+            for i in 0..100u64 {
+                writeln!(f, "2024-01-15 10:30:00 INFO line {i}").unwrap();
+            }
+        }
+
+        let info = ensure_test_file(&path).unwrap();
+        assert_eq!(info.line_count, 100);
+        assert!(info.size_bytes > 0);
+        assert!(info.avg_line_length > 0.0);
+    }
+}