tac: Flush BufWriter

src/uu/tac/src/tac.rs

@@ -164,6 +164,7 @@ fn buffer_tac_regex(
     // After the loop terminates, write whatever bytes are remaining at
     // the beginning of the buffer.
     out.write_all(&data[0..following_line_start])?;
+    out.flush()?;
     Ok(())
 }
 
@@ -215,6 +216,7 @@ fn buffer_tac(data: &[u8], before: bool, separator: &str) -> std::io::Result<()>
     // After the loop terminates, write whatever bytes are remaining at
     // the beginning of the buffer.
     out.write_all(&data[0..following_line_start])?;
+    out.flush()?;
     Ok(())
 }
 

tests/by-util/test_tac.rs

@@ -309,3 +309,13 @@ fn test_regex_before() {
         //          |--------||----||---|
         .stdout_is("+---+c+d-e+--++b+-+a+");
 }
+
+#[cfg(target_os = "linux")]
+#[test]
+fn test_failed_write_is_reported() {
+    new_ucmd!()
+        .pipe_in("hello")
+        .set_stdout(std::fs::File::create("/dev/full").unwrap())
+        .fails()
+        .stderr_is("tac: failed to write to stdout: No space left on device (os error 28)\n");
+}