pr: ignore empty line after newline char (#10332)

Fix a bug in `pr` where a form feed character (`\f`) immediately
following a newline character (`\n`) was incorrectly rendered as an
extra blank line in the output when running with the `-f` option. After
this commit, the empty line is correctly ignored.

Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>

Author: jfinkels

src/uu/pr/src/pr.rs

@@ -836,8 +836,13 @@ fn get_pages(
         if buf[i] == FF {
             // Treat everything up to (but not including) the form feed
             // character as the last line of the page.
-            let file_line = FileLine::from_buf(file_id, page_num, line_num, &buf[prev..i])?;
-            page.push(file_line);
+            if i > 0 && i == prev && buf[i - 1] == NL {
+                // If the file has the pattern `\n\f`, don't treat the
+                // `\f` as its own line; instead ignore the empty line.
+            } else {
+                let file_line = FileLine::from_buf(file_id, page_num, line_num, &buf[prev..i])?;
+                page.push(file_line);
+            }
 
             // Remember where the last line ended.
             prev = i + 1;

tests/by-util/test_pr.rs

@@ -632,6 +632,22 @@ fn test_form_feed_newlines() {
         .stdout_matches(&regex);
 }
 
+#[test]
+fn test_new_line_followed_by_form_feed() {
+    // Here we define the expected output.
+    let whitespace = " ".repeat(50);
+    let datetime_pattern = r"\d\d\d\d-\d\d-\d\d \d\d:\d\d";
+    let pattern = format!("\n\n{datetime_pattern}{whitespace}Page 1\n\n\nabc\n\x0c");
+    let regex = Regex::new(&pattern).unwrap();
+
+    // Command line: `printf "abc\n\f" | pr -f`.
+    new_ucmd!()
+        .arg("-f")
+        .pipe_in("abc\n\x0c")
+        .succeeds()
+        .stdout_matches(&regex);
+}
+
 #[test]
 fn test_form_feed_followed_by_new_line() {
     // Here we define the expected output.