sort: make merging stable

When merging files we need to prioritize files that occur earlier in the
command line arguments with -m.

This also makes the extsort merge step (and thus extsort itself) stable again.

src/uu/sort/src/sort.rs

@@ -686,6 +686,7 @@ struct MergeableFile<'a> {
     lines: Box<dyn Iterator<Item = Line> + 'a>,
     current_line: Line,
     settings: &'a GlobalSettings,
+    file_index: usize,
 }
 
 // BinaryHeap depends on `Ord`. Note that we want to pop smallest items
@@ -693,7 +694,14 @@ struct MergeableFile<'a> {
 // trick it into the right order by calling reverse() here.
 impl<'a> Ord for MergeableFile<'a> {
     fn cmp(&self, other: &MergeableFile) -> Ordering {
-        compare_by(&self.current_line, &other.current_line, self.settings).reverse()
+        let comparison = compare_by(&self.current_line, &other.current_line, self.settings);
+        if comparison == Ordering::Equal {
+            // If lines are equal, the earlier file takes precedence.
+            self.file_index.cmp(&other.file_index)
+        } else {
+            comparison
+        }
+        .reverse()
     }
 }
 
@@ -729,6 +737,7 @@ impl<'a> FileMerger<'a> {
                 lines,
                 current_line: next_line,
                 settings: &self.settings,
+                file_index: self.heap.len(),
             };
             self.heap.push(mergeable_file);
         }

tests/by-util/test_sort.rs

@@ -51,6 +51,18 @@ fn test_smaller_than_specified_segment() {
         .stdout_is_fixture("ext_sort.expected");
 }
 
+#[test]
+fn test_ext_sort_stable() {
+    new_ucmd!()
+        .arg("-n")
+        .arg("--stable")
+        .arg("-S")
+        .arg("0M")
+        .arg("ext_stable.txt")
+        .succeeds()
+        .stdout_only_fixture("ext_stable.expected");
+}
+
 #[test]
 fn test_extsort_zero_terminated() {
     new_ucmd!()
@@ -566,6 +578,18 @@ fn test_merge_unique() {
         .stdout_only_fixture("merge_ints_interleaved.expected");
 }
 
+#[test]
+fn test_merge_stable() {
+    new_ucmd!()
+        .arg("-m")
+        .arg("--stable")
+        .arg("-n")
+        .arg("merge_stable_1.txt")
+        .arg("merge_stable_2.txt")
+        .succeeds()
+        .stdout_only_fixture("merge_stable.expected");
+}
+
 #[test]
 fn test_merge_reversed() {
     new_ucmd!()

tests/fixtures/sort/ext_stable.expected

@@ -0,0 +1,4 @@
+0a
+0a
+0b
+0b

tests/fixtures/sort/ext_stable.txt

@@ -0,0 +1,4 @@
+0a
+0a
+0b
+0b

tests/fixtures/sort/merge_stable.expected

@@ -0,0 +1,3 @@
+0a
+0c
+0b

tests/fixtures/sort/merge_stable_1.txt

@@ -0,0 +1,2 @@
+0a
+0c

tests/fixtures/sort/merge_stable_2.txt

@@ -0,0 +1 @@
+0b