fold: continue processing files after an open error (#12668)

Should make test fold/multiple-files.sh pass

src/uu/fold/src/fold.rs

@@ -13,6 +13,7 @@ use unicode_width::UnicodeWidthChar;
 use uucore::display::Quotable;
 use uucore::error::{FromIo, UResult, USimpleError};
 use uucore::format_usage;
+use uucore::show;
 use uucore::translate;
 
 const TAB_WIDTH: usize = 8;
@@ -164,7 +165,14 @@ fn fold(
             stdin_buf = stdin();
             &mut stdin_buf as &mut dyn Read
         } else {
-            file_buf = File::open(Path::new(filename)).map_err_context(|| filename.to_string())?;
+            // Like GNU, report the error but keep processing the remaining files.
+            match File::open(Path::new(filename)) {
+                Ok(f) => file_buf = f,
+                Err(e) => {
+                    show!(e.map_err_context(|| filename.to_string()));
+                    continue;
+                }
+            }
             &mut file_buf as &mut dyn Read
         });
 

tests/by-util/test_fold.rs

@@ -930,6 +930,21 @@ fn test_bytewise_fold_at_read_buffer_boundary() {
         .stdout_is_bytes(expected);
 }
 
+#[test]
+fn test_continue_after_missing_file() {
+    // A nonexistent operand must not abort the run: the surrounding files are
+    // still folded, the error is reported on stderr, and the exit status is 1.
+    let ts = TestScenario::new(util_name!());
+    ts.fixtures.write("first.txt", "hello\n");
+    ts.fixtures.write("third.txt", "world\n");
+
+    ts.ucmd()
+        .args(&["first.txt", "absent.txt", "third.txt"])
+        .fails_with_code(1)
+        .stdout_is("hello\nworld\n")
+        .stderr_is("fold: absent.txt: No such file or directory\n");
+}
+
 #[test]
 fn test_obsolete_syntax() {
     new_ucmd!()