sort: write empty file when input is empty (#11958)

* adds implementations for this behavior in both threaded and wasi

* adds test for this behavior

Author: hideyosh1

src/uu/sort/src/ext_sort/threaded.rs

@@ -175,7 +175,8 @@ fn reader_writer<
             }
         }
         ReadResult::EmptyInput => {
-            // don't output anything
+            // output empty too, as coreutils does
+            print_sorted(vec![].iter(), settings, output)?;
         }
     }
     Ok(())

src/uu/sort/src/ext_sort/wasi.rs

@@ -34,6 +34,8 @@ pub fn ext_sort(
         file?.read_to_end(&mut input)?;
     }
     if input.is_empty() {
+        // empty files are sorted to empty like in coreutils
+        print_sorted(chunk.lines().iter(), settings, output)?;
         return Ok(());
     }
     let mut chunk = Chunk::try_new(input, |buffer| {

tests/by-util/test_sort.rs

@@ -2960,4 +2960,17 @@ e f 5436 down data path1 path2 path3 path4 path5\n";
         .stdout_is(input);
 }
 
+#[test]
+fn test_empty_input_empty_output() {
+    // check for inconsistency #11958
+    let input = "test test test";
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    at.touch("file");
+    at.append("file", input);
+
+    ucmd.args(&["-o", "file"]).pipe_in("").succeeds();
+    assert_eq!(at.read("file"), "");
+}
+
 /* spell-checker: enable */