pr: stop merged output after the last line without a page trailer

src/uu/pr/src/pr.rs

@@ -1501,7 +1501,14 @@ fn write_columns(
     };
 
     let blank_line = FileLine::default();
+    // Without a page trailer there is nothing to pad the page out to.
+    let no_trailer = feed_line_present || !options.display_header_and_trailer;
     for row in table {
+        // Merged files are padded with empty cells to fill the page; once
+        // every file has run out there is nothing left to print.
+        if merge && no_trailer && row.iter().all(Option::is_none) {
+            break;
+        }
         // On a page that is not completely filled the last row stops part way
         // through, and its final cell must not be followed by a column
         // separator. Merging prints an empty cell for every column instead, so
@@ -1532,10 +1539,7 @@ fn write_columns(
         // The last row of a partly filled page has content in its leading
         // columns and nothing in the rest, so it still needs to be terminated.
         // Only a row without any content at all means the page ran out.
-        if cells_written == 0
-            && not_found_break
-            && (feed_line_present || !options.display_header_and_trailer)
-        {
+        if cells_written == 0 && not_found_break && no_trailer {
             break;
         }
         writer.write_all(line_separator)?;

tests/by-util/test_pr.rs

@@ -701,6 +701,29 @@ fn test_join_lines_keeps_column_separator() {
         .stdout_is("1\t3\n2\t4\n");
 }
 
+#[test]
+fn test_merge_stops_after_last_line_without_header() {
+    // without a page trailer there is nothing to pad to: like GNU pr, stop
+    // after the last merged line (-F still prints the 5 header lines and a form feed)
+    // -J keeps the columns from being padded, so the lines are short
+    let body = "a\tc\nb\td\n";
+    let header = format!("\n\nX{}Page 1\n\n\n", " ".repeat(65));
+    for (args, expected) in [
+        (vec!["-J", "-t"], body.to_string()),
+        (vec!["-J", "-T"], body.to_string()),
+        (vec!["-J", "-F", "-D", "X"], format!("{header}{body}\x0c")),
+    ] {
+        let (at, mut ucmd) = at_and_ucmd!();
+        at.write("one", "a\nb\n");
+        at.write("two", "c\nd\n");
+        ucmd.arg("-m")
+            .args(&args)
+            .args(&["one", "two"])
+            .succeeds()
+            .stdout_is(expected);
+    }
+}
+
 #[test]
 fn test_value_for_number_lines() {
     // *5 is of the form [SEP[NUMBER]] so is accepted and succeeds