expand, unexpand: flush output after each line

src/uu/expand/Cargo.toml

@@ -21,7 +21,7 @@ fluent = { workspace = true }
 thiserror = { workspace = true }
 uucore = { workspace = true }
 
-[target.'cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd"))'.dependencies]
+[target.'cfg(unix)'.dependencies]
 rustix = { workspace = true, features = ["fs"] }
 
 [dev-dependencies]

src/uu/expand/src/expand.rs

@@ -439,12 +439,29 @@ fn expand_buf(
     Ok(())
 }
 
+/// Whether stdin is a regular file, i.e. reads from it never wait for a writer.
+fn stdin_is_regular_file() -> bool {
+    #[cfg(unix)]
+    {
+        rustix::fs::fstat(stdin())
+            .is_ok_and(|st| rustix::fs::FileType::from_raw_mode(st.st_mode).is_file())
+    }
+    #[cfg(not(unix))]
+    {
+        false
+    }
+}
+
 fn expand_file(
     file: &OsString,
     output: &mut BufWriter<std::io::Stdout>,
     options: &Options,
 ) -> UResult<()> {
     let mut input = open(file)?;
+    // Only input that can block between lines (a pipe or terminal on stdin)
+    // needs output flushed before the next read. Regular files keep the fully
+    // buffered path.
+    let flush_lines = file == "-" && !stdin_is_regular_file();
     let ts = options.tabstops.as_ref();
     let mut col = 0;
     loop {
@@ -455,6 +472,11 @@ fn expand_file(
                     .map_err_context(|| translate!("expand-error-failed-to-write-output"))?;
                 let len = buf.len();
                 input.consume(len);
+                if flush_lines {
+                    output
+                        .flush()
+                        .map_err_context(|| translate!("expand-error-failed-to-write-output"))?;
+                }
             }
             Err(e) => return Err(e.map_err_context(|| file.maybe_quote().to_string())),
         }

src/uu/unexpand/Cargo.toml

@@ -21,7 +21,7 @@ fluent = { workspace = true }
 thiserror = { workspace = true }
 uucore = { workspace = true }
 
-[target.'cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd"))'.dependencies]
+[target.'cfg(unix)'.dependencies]
 rustix = { workspace = true, features = ["fs"] }
 
 [dev-dependencies]

src/uu/unexpand/src/unexpand.rs

@@ -637,6 +637,19 @@ fn unexpand_buf(
     Ok(())
 }
 
+/// Whether stdin is a regular file, i.e. reads from it never wait for a writer.
+fn stdin_is_regular_file() -> bool {
+    #[cfg(unix)]
+    {
+        rustix::fs::fstat(stdin())
+            .is_ok_and(|st| rustix::fs::FileType::from_raw_mode(st.st_mode).is_file())
+    }
+    #[cfg(not(unix))]
+    {
+        false
+    }
+}
+
 fn unexpand_file(
     file: &OsString,
     output: &mut BufWriter<Stdout>,
@@ -645,6 +658,8 @@ fn unexpand_file(
     tab_config: &TabConfig,
 ) -> UResult<()> {
     let mut input = open(file)?;
+    // See expand: flush before the next read only when stdin can block.
+    let flush_lines = file == "-" && !stdin_is_regular_file();
     let mut print_state = PrintState {
         col: 0,
         scol: 0,
@@ -665,6 +680,9 @@ fn unexpand_file(
         }
         let len = buf.len();
         input.consume(len);
+        if flush_lines {
+            output.flush()?;
+        }
     }
     // write out anything remaining
     write_tabs(output, tab_config, &mut print_state, options.aflag)

tests/by-util/test_expand.rs

@@ -5,6 +5,8 @@
 
 // spell-checker:ignore (ToDO) taaaa tbbbb tcccc
 
+#[cfg(unix)]
+use std::process::Stdio;
 use uucore::display::Quotable;
 use uutests::new_ucmd;
 
@@ -496,3 +498,23 @@ fn test_buffered_reads_new_line_no_tabs_in_first_chunk() {
         .succeeds()
         .stdout_is_fixture("new_line_in_chunk_expected.txt");
 }
+
+/// Interactive / line-at-a-time stdin must show each expanded line before the
+/// next input arrives. BufWriter alone only flushes on exit.
+#[cfg(unix)]
+#[test]
+fn test_stdin_tty_flushes_each_line() {
+    let mut child = new_ucmd!()
+        .args(&["-t3"])
+        .set_stdin(Stdio::piped())
+        .set_stdout(Stdio::piped())
+        .run_no_wait();
+
+    child.write_in(b"\ta\n");
+    assert_eq!(&child.stdout_exact_bytes(5), b"   a\n");
+
+    child.write_in(b"b\tc\n");
+    assert_eq!(&child.stdout_exact_bytes(5), b"b  c\n");
+
+    child.wait().unwrap().success();
+}

tests/by-util/test_unexpand.rs

@@ -5,6 +5,8 @@
 
 // spell-checker:ignore contenta edgecase behaviour tcaf
 
+#[cfg(unix)]
+use std::process::Stdio;
 use uutests::{at_and_ucmd, new_ucmd};
 
 #[test]
@@ -544,3 +546,21 @@ fn test_buffered_read_edgecase_behaviour() {
             .stdout_only(String::from_utf8(expected).unwrap());
     }
 }
+
+/// Interactive / line-at-a-time stdin must show each unexpanded line before the
+/// next input arrives.
+#[cfg(unix)]
+#[test]
+fn test_stdin_tty_flushes_each_line() {
+    let mut child = new_ucmd!()
+        .args(&["-t3"])
+        .set_stdin(Stdio::piped())
+        .set_stdout(Stdio::piped())
+        .run_no_wait();
+
+    // three leading spaces -> one tab at tabstop 3
+    child.write_in(b"   a\n");
+    assert_eq!(&child.stdout_exact_bytes(3), b"\ta\n");
+
+    child.wait().unwrap().success();
+}