chmod: accept modes like -=r, -+x and -,u+x

extract_negative_modes only took an argument starting with '-' as a mode when its second character was a permission letter or a digit. Modes such as "-=r", "-+x" and "-,u+x" went on to clap, which has no such option and rejected them. Also accept '=', '+' and ',' there; no option starts with those.

Closes #15665

src/uu/chmod/src/chmod.rs

@@ -81,11 +81,13 @@ fn extract_negative_modes(args: impl uucore::Args) -> (Option<String>, Vec<OsStr
         let Some(arg) = arg.to_str() else {
             return false;
         };
+        // A mode can also be "-=r", "-+x" or "-,u+x". No option starts with '=', '+' or ',',
+        // and a second '-' (a long option) is deliberately not listed.
         arg.len() >= 2
             && arg.starts_with('-')
             && matches!(
                 arg.chars().nth(1).unwrap(),
-                'r' | 'w' | 'x' | 'X' | 's' | 't' | 'u' | 'g' | 'o' | '0'..='7'
+                'r' | 'w' | 'x' | 'X' | 's' | 't' | 'u' | 'g' | 'o' | '=' | '+' | ',' | '0'..='7'
             )
     };
 

tests/by-util/test_chmod.rs

@@ -364,6 +364,31 @@ fn test_chmod_many_options() {
     run_tests(tests);
 }
 
+#[test]
+fn test_chmod_option_like_mode_starting_with_operator() {
+    let tests = vec![
+        TestCase {
+            args: vec!["-=r", TEST_FILE],
+            before: 0o100_644,
+            after: 0o100_444,
+            umask: Some(0o022),
+        },
+        TestCase {
+            args: vec!["-+x", TEST_FILE],
+            before: 0o100_644,
+            after: 0o100_755,
+            umask: Some(0o022),
+        },
+        TestCase {
+            args: vec!["-,u+x", TEST_FILE],
+            before: 0o100_644,
+            after: 0o100_744,
+            umask: Some(0o022),
+        },
+    ];
+    run_tests(tests);
+}
+
 #[test]
 #[allow(clippy::unreadable_literal)]
 fn test_chmod_reference_file() {