Improve io error handling and tests

Cargo.toml

@@ -17,6 +17,7 @@ unicode-width = ">=0.1, <0.2"
 rstest = "0.18"
 assert_cmd = "2"
 tempfile = "3"
+libc = ">=0.2, <0.3"
 
 [lints.clippy]
 pedantic = "warn"

src/io.rs

@@ -4,30 +4,51 @@ use std::{fs, path::Path};
 
 use crate::process::{process_stream, process_stream_no_wrap};
 
-/// Rewrite a file in place with wrapped tables.
+/// Read `path`, process the contents with `f`, and write the result back.
+///
+/// This helper encapsulates the common pattern used by [`rewrite`] and
+/// [`rewrite_no_wrap`].
 ///
 /// # Errors
 /// Returns an error if reading or writing the file fails.
-pub fn rewrite(path: &Path) -> std::io::Result<()> {
+fn rewrite_with<F>(path: &Path, f: F) -> std::io::Result<()>
+where
+    F: Fn(&[String]) -> Vec<String>,
+{
     let text = fs::read_to_string(path)?;
     let lines: Vec<String> = text.lines().map(str::to_string).collect();
-    let fixed = process_stream(&lines);
-    fs::write(path, fixed.join("\n") + "\n")
+    let fixed = f(&lines);
+    let output = if fixed.is_empty() {
+        String::new()
+    } else {
+        fixed.join("\n") + "\n"
+    };
+    fs::write(path, output)
 }
 
+/// Rewrite a file in place with wrapped tables.
+///
+/// # Errors
+/// Returns an error if reading or writing the file fails.
+pub fn rewrite(path: &Path) -> std::io::Result<()> { rewrite_with(path, process_stream) }
+
 /// Rewrite a file in place without wrapping text.
 ///
 /// # Errors
 /// Returns an error if reading or writing the file fails.
 pub fn rewrite_no_wrap(path: &Path) -> std::io::Result<()> {
-    let text = fs::read_to_string(path)?;
-    let lines: Vec<String> = text.lines().map(str::to_string).collect();
-    let fixed = process_stream_no_wrap(&lines);
-    fs::write(path, fixed.join("\n") + "\n")
+    rewrite_with(path, process_stream_no_wrap)
 }
 
 #[cfg(test)]
 mod tests {
+    #[cfg(unix)]
+    use std::os::unix::fs::PermissionsExt;
+    use std::{fs::Permissions, path::Path};
+
+    #[cfg(unix)]
+    use libc;
+    use rstest::rstest;
     use tempfile::tempdir;
 
     use super::*;
@@ -51,4 +72,58 @@ mod tests {
         let out = fs::read_to_string(&file).unwrap();
         assert_eq!(out, "| A | B |\n| 1 | 2 |\n");
     }
+
+    #[cfg(unix)]
+    fn can_write_as_root() -> bool {
+        // SAFETY: `geteuid()` has no side effects and is safe to call in tests.
+        let uid = unsafe { libc::geteuid() };
+        uid == 0
+    }
+
+    fn assert_permission_error_or_root_success(result: std::io::Result<()>) {
+        #[cfg(unix)]
+        if can_write_as_root() {
+            assert!(result.is_ok());
+        } else {
+            let err = result.expect_err("expected permission denied error");
+            assert_eq!(err.kind(), std::io::ErrorKind::PermissionDenied);
+        }
+        #[cfg(not(unix))]
+        {
+            let err = result.expect_err("expected permission denied error");
+            assert_eq!(err.kind(), std::io::ErrorKind::PermissionDenied);
+        }
+    }
+
+    #[rstest]
+    #[case(rewrite)]
+    #[case(rewrite_no_wrap)]
+    fn missing_file_error(#[case] rewrite_fn: fn(&Path) -> std::io::Result<()>) {
+        let dir = tempdir().unwrap();
+        let file = dir.path().join("missing.md");
+        let err = rewrite_fn(&file).expect_err("expected error for missing file");
+        assert_eq!(err.kind(), std::io::ErrorKind::NotFound);
+    }
+
+    #[rstest]
+    #[case(rewrite)]
+    #[case(rewrite_no_wrap)]
+    fn permission_denied_error(#[case] rewrite_fn: fn(&Path) -> std::io::Result<()>) {
+        let dir = tempdir().unwrap();
+        let file = dir.path().join("deny.md");
+        fs::write(&file, "data").unwrap();
+        fs::set_permissions(&file, Permissions::from_mode(0o444)).unwrap();
+        let result = rewrite_fn(&file);
+        assert_permission_error_or_root_success(result);
+    }
+
+    #[test]
+    fn rewrite_empty_file_no_extra_newline() {
+        let dir = tempdir().unwrap();
+        let file = dir.path().join("empty.md");
+        fs::write(&file, "").unwrap();
+        rewrite(&file).unwrap();
+        let contents = fs::read_to_string(&file).unwrap();
+        assert!(contents.is_empty());
+    }
 }