fold: insert fold newline at read-buffer boundary (#12682)

follow up of #11314

src/uu/fold/src/fold.rs

@@ -194,42 +194,59 @@ fn fold_file_bytewise<T: Read, W: Write>(
     let mut line = Vec::new();
 
     loop {
-        // Ensures that our line always has enough bytes to either
-        // come across a newline, a whitespace or just wrap around.
-        while width > line.len() {
+        // Pull bytes from the reader until we have strictly more than `width`
+        // buffered (enough to know whether content follows a width-driven fold)
+        // or we reach EOF. Reading at most `width` bytes ahead keeps memory
+        // bounded even on endless streams like /dev/zero, where the old
+        // read_until(NL, ..) would buffer forever waiting for a newline.
+        while line.len() <= width {
             let buf = file
                 .fill_buf()
                 .map_err_context(|| translate!("fold-error-readline"))?;
             if buf.is_empty() {
                 break;
             }
-            line.extend_from_slice(buf);
             let len = buf.len();
+            line.extend_from_slice(buf);
             file.consume(len);
         }
 
-        // The width exceeds the line read so far if we have
-        // reached EOF.
-        if width > line.len() {
+        // EOF with a tail shorter than (or equal to) `width`: no width/space
+        // fold can apply, so emit it verbatim (newlines inside are preserved).
+        if line.len() <= width {
+            if line.is_empty() {
+                break;
+            }
             output.write_all(&line)?;
             break;
         }
 
+        // We have a full `width`-byte chunk plus at least one lookahead byte.
         let chunk = &line[..width];
-        let newline_end = chunk.iter().position(|c| NL.eq(c)).map(|v| v + 1);
 
-        let space_end = chunk
-            .iter()
-            .rposition(|c| spaces && c.is_ascii_whitespace() && !CR.eq(c))
-            .map(|v| v + 1);
+        // An existing newline within the chunk ends the line naturally; the
+        // newline is part of the slice, so no extra newline is emitted.
+        if let Some(end) = chunk.iter().position(|c| *c == NL).map(|i| i + 1) {
+            output.write_all(&line[..end])?;
+            line.drain(..end);
+            continue;
+        }
 
-        let end = newline_end.or(space_end).unwrap_or(width);
-        let slice = &line[..end];
+        // No newline: with -s, break after the last whitespace (excluding CR);
+        // otherwise hard-wrap at `width`.
+        let end = if spaces {
+            chunk
+                .iter()
+                .rposition(|c| c.is_ascii_whitespace() && *c != CR)
+                .map_or(width, |i| i + 1)
+        } else {
+            width
+        };
 
-        output.write_all(slice)?;
-        let slice_ends_without_newline = line[end - 1] != NL;
-        let no_newline_follows_slice = line.get(end).is_some_and(|c| *c != NL);
-        if slice_ends_without_newline && no_newline_follows_slice {
+        output.write_all(&line[..end])?;
+        // Width/space-driven fold: insert a newline unless the next byte is
+        // already a newline (it is emitted on the next pass).
+        if line[end] != NL {
             output.write_all(&[NL])?;
         }
         line.drain(..end);

tests/by-util/test_fold.rs

@@ -2,7 +2,7 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
-// spell-checker:ignore fullwidth refgh tefgh nefgh
+// spell-checker:ignore fullwidth refgh tefgh nefgh unflushed
 
 use bytecount::count;
 use unicode_width::UnicodeWidthChar;
@@ -891,6 +891,10 @@ fn test_bytewise_carriage_return_is_not_word_boundary() {
 }
 
 #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "netbsd"))]
+#[cfg_attr(
+    wasi_runner,
+    ignore = "WASI: killing the wasmtime process discards the unflushed output buffer, so the streamed bytes never reach stdout"
+)]
 #[test]
 fn test_bytewise_read_from_pseudo_device() {
     let mut child = new_ucmd!().arg("-b").arg("/dev/zero").run_no_wait();
@@ -905,6 +909,27 @@ fn test_bytewise_read_from_pseudo_device() {
         .no_stderr();
 }
 
+/// A fold boundary that lands exactly on the read-buffer boundary must still
+/// insert the fold newline. The streaming reader fills at most `BufReader`
+/// capacity per call, so folding at `width == capacity` is the case where the
+/// byte following the fold point is not yet buffered.
+#[cfg(not(target_arch = "wasm32"))]
+#[test]
+fn test_bytewise_fold_at_read_buffer_boundary() {
+    let width = buf_reader_capacity();
+    let input = vec![b'a'; width * 2];
+
+    let mut expected = vec![b'a'; width];
+    expected.push(b'\n');
+    expected.extend(std::iter::repeat_n(b'a', width));
+
+    new_ucmd!()
+        .args(&["-b", &format!("-w{width}")])
+        .pipe_in(input)
+        .succeeds()
+        .stdout_is_bytes(expected);
+}
+
 #[test]
 fn test_obsolete_syntax() {
     new_ucmd!()