csplit: include the file name in output-file errors

src/uu/csplit/src/csplit.rs

@@ -268,10 +268,12 @@ impl SplitWriter<'_> {
     ///
     /// # Errors
     ///
-    /// The creation of the split file may fail with some [`io::Error`].
-    fn new_writer(&mut self) -> io::Result<()> {
+    /// Returns an error if creating the split file fails.
+    fn new_writer(&mut self) -> Result<(), CsplitError> {
         let file_name = self.options.split_name.get(self.counter);
-        let file = File::create(file_name)?;
+        let file = File::create(&file_name)
+            .map_err_context(|| file_name.clone())
+            .map_err(CsplitError::from)?;
         self.current_writer = Some(BufWriter::new(file));
         self.counter += 1;
         self.size = 0;

src/uu/csplit/src/csplit_error.rs

@@ -6,13 +6,13 @@
 use std::io;
 use thiserror::Error;
 use uucore::display::Quotable;
-use uucore::error::UError;
+use uucore::error::{UError, strip_errno};
 use uucore::translate;
 
 /// Errors thrown by the csplit command
 #[derive(Debug, Error)]
 pub enum CsplitError {
-    #[error("IO error: {}", _0)]
+    #[error("{}", strip_errno(_0))]
     IoError(#[from] io::Error),
     #[error("{}", translate!("csplit-error-line-out-of-range", "pattern" => _0.quote()))]
     LineOutOfRange(String),
@@ -54,3 +54,19 @@ impl UError for CsplitError {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::CsplitError;
+
+    #[cfg(unix)]
+    #[test]
+    fn io_error_display_is_clean() {
+        // GNU does not print "IO error:" nor the raw "(os error N)" suffix.
+        let err = CsplitError::IoError(std::io::Error::from_raw_os_error(13));
+        let msg = err.to_string();
+        assert_eq!(msg, "Permission denied");
+        assert!(!msg.contains("IO error:"));
+        assert!(!msg.contains("os error"));
+    }
+}

tests/by-util/test_csplit.rs

@@ -1582,3 +1582,22 @@ fn test_write_error_dev_full_keep_files() {
     assert!(at.file_exists("xx00"));
     assert_eq!(at.read("xx00"), "1\n");
 }
+
+/// Test that a failed split-file creation reports the filename.
+#[test]
+#[cfg(unix)]
+fn test_create_error_reports_filename() {
+    // Root can open a mode-000 file for writing, so File::create would not fail.
+    if rustix::process::geteuid().is_root() {
+        return;
+    }
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.write("input", "a\nb\nc\n");
+    // Pre-create the first split with no write permission so File::create fails.
+    at.touch("xx00");
+    at.set_mode("xx00", 0o000);
+
+    ucmd.args(&["input", "2"])
+        .fails()
+        .stderr_is("csplit: xx00: Permission denied\n");
+}