sort: insert line separators after non-empty files

If files don't end witht a line separator we have to insert one,
otherwise the last line will be combined with the first line of the next
file.

src/uu/sort/src/chunks.rs

@@ -175,6 +175,7 @@ fn read_to_buffer(
     separator: u8,
 ) -> (usize, bool) {
     let mut read_target = &mut buffer[start_offset..];
+    let mut last_file_target_size = read_target.len();
     loop {
         match file.read(read_target) {
             Ok(0) => {
@@ -208,14 +209,27 @@ fn read_to_buffer(
                         read_target = &mut buffer[len..];
                     }
                 } else {
-                    // This file is empty.
+                    // This file has been fully read.
+                    let mut leftover_len = read_target.len();
+                    if last_file_target_size != leftover_len {
+                        // The file was not empty.
+                        let read_len = buffer.len() - leftover_len;
+                        if buffer[read_len - 1] != separator {
+                            // The file did not end with a separator. We have to insert one.
+                            buffer[read_len] = separator;
+                            leftover_len -= 1;
+                        }
+                        let read_len = buffer.len() - leftover_len;
+                        read_target = &mut buffer[read_len..];
+                    }
                     if let Some(next_file) = next_files.next() {
                         // There is another file.
+                        last_file_target_size = leftover_len;
                         *file = next_file;
                     } else {
                         // This was the last file.
-                        let leftover_len = read_target.len();
-                        return (buffer.len() - leftover_len, false);
+                        let read_len = buffer.len() - leftover_len;
+                        return (read_len, false);
                     }
                 }
             }

tests/by-util/test_sort.rs

@@ -792,3 +792,11 @@ fn test_nonexistent_file() {
 fn test_blanks() {
     test_helper("blanks", &["-b", "--ignore-blanks"]);
 }
+
+#[test]
+fn sort_multiple() {
+    new_ucmd!()
+        .args(&["no_trailing_newline1.txt", "no_trailing_newline2.txt"])
+        .succeeds()
+        .stdout_is("a\nb\nb\n");
+}

tests/fixtures/sort/no_trailing_newline1.txt

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

tests/fixtures/sort/no_trailing_newline2.txt

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