uutils · cakebaker · May 4, 2025 · May 3, 2025
diff --git a/src/uu/split/src/split.rs b/src/uu/split/src/split.rs
@@ -834,13 +834,7 @@ struct LineChunkWriter<'a> {
 impl<'a> LineChunkWriter<'a> {
     fn new(chunk_size: u64, settings: &'a Settings) -> UResult<Self> {
         let mut filename_iterator = FilenameIterator::new(&settings.prefix, &settings.suffix)?;
-        let filename = filename_iterator
-            .next()
-            .ok_or_else(|| USimpleError::new(1, "output file suffixes exhausted"))?;
-        if settings.verbose {
-            println!("creating file {}", filename.quote());
-        }
-        let inner = settings.instantiate_current_writer(&filename, true)?;
+        let inner = Self::start_new_chunk(settings, &mut filename_iterator)?;
         Ok(LineChunkWriter {
             settings,
             chunk_size,
@@ -850,6 +844,19 @@ impl<'a> LineChunkWriter<'a> {
             filename_iterator,
         })
     }
+
+    fn start_new_chunk(
+        settings: &Settings,
+        filename_iterator: &mut FilenameIterator,
+    ) -> io::Result<BufWriter<Box<dyn Write>>> {
+        let filename = filename_iterator
+            .next()
+            .ok_or_else(|| io::Error::other("output file suffixes exhausted"))?;
+        if settings.verbose {
+            println!("creating file {}", filename.quote());
+        }
+        settings.instantiate_current_writer(&filename, true)
+    }
 }
 
 impl Write for LineChunkWriter<'_> {
@@ -869,14 +876,7 @@ impl Write for LineChunkWriter<'_> {
             // corresponding writer.
             if self.num_lines_remaining_in_current_chunk == 0 {
                 self.num_chunks_written += 1;
-                let filename = self
-                    .filename_iterator
-                    .next()
-                    .ok_or_else(|| io::Error::other("output file suffixes exhausted"))?;
-                if self.settings.verbose {
-                    println!("creating file {}", filename.quote());
-                }
-                self.inner = self.settings.instantiate_current_writer(&filename, true)?;
+                self.inner = Self::start_new_chunk(self.settings, &mut self.filename_iterator)?;
                 self.num_lines_remaining_in_current_chunk = self.chunk_size;
             }
 
@@ -889,9 +889,19 @@ impl Write for LineChunkWriter<'_> {
             self.num_lines_remaining_in_current_chunk -= 1;
         }
 
-        let num_bytes_written =
-            custom_write(&buf[prev..buf.len()], &mut self.inner, self.settings)?;
-        total_bytes_written += num_bytes_written;
+        // There might be bytes remaining in the buffer, and we write
+        // them to the current chunk. But first, we may need to rotate
+        // the current chunk in case it has already reached its line
+        // limit.
+        if prev < buf.len() {
+            if self.num_lines_remaining_in_current_chunk == 0 {
+                self.inner = Self::start_new_chunk(self.settings, &mut self.filename_iterator)?;
+                self.num_lines_remaining_in_current_chunk = self.chunk_size;
+            }
+            let num_bytes_written =
+                custom_write(&buf[prev..buf.len()], &mut self.inner, self.settings)?;
+            total_bytes_written += num_bytes_written;
+        }
         Ok(total_bytes_written)
     }
 

diff --git a/tests/by-util/test_split.rs b/tests/by-util/test_split.rs
@@ -120,6 +120,15 @@ impl RandomFile {
             n -= 1;
         }
     }
+
+    /// Add n lines each of the given size.
+    fn add_lines_with_line_size(&mut self, lines: usize, line_size: usize) {
+        let mut n = lines;
+        while n > 0 {
+            writeln!(self.inner, "{}", random_chars(line_size)).unwrap();
+            n -= 1;
+        }
+    }
 }
 
 #[test]
@@ -430,6 +439,28 @@ fn test_split_lines_number() {
         .stderr_only("split: invalid number of lines: 'file'\n");
 }
 
+/// Test interference between split line size and IO buffer capacity.
+/// See issue #7869.
+#[test]
+fn test_split_lines_interfere_with_io_buf_capacity() {
+    let buf_capacity = BufWriter::new(Vec::new()).capacity();
+    // We intentionally set the line size to be less than the IO write buffer
+    // capacity. This is to trigger the condition where after the first split
+    // file is written, there are still bytes left in the buffer. We then
+    // test that those bytes are written to the next split file.
+    let line_size = buf_capacity - 2;
+
+    let (at, mut ucmd) = at_and_ucmd!();
+    let name = "split_lines_interfere_with_io_buf_capacity";
+    RandomFile::new(&at, name).add_lines_with_line_size(2, line_size);
+    ucmd.args(&["-l", "1", name]).succeeds();
+
+    // Note that `lines_size` doesn't take the trailing newline into account,
+    // we add 1 for adjustment.
+    assert_eq!(at.read("xaa").len(), line_size + 1);
+    assert_eq!(at.read("xab").len(), line_size + 1);
+}
+
 /// Test short lines option with value concatenated
 #[test]
 fn test_split_lines_short_concatenated_with_value() {