fix: last --jobserver-auth wins

src/lib.rs

@@ -279,16 +279,10 @@ impl Client {
             }
         };
 
-        let (arg, pos) = match ["--jobserver-fds=", "--jobserver-auth="]
-            .iter()
-            .map(|&arg| var.find(arg).map(|pos| (arg, pos)))
-            .find_map(|pos| pos)
-        {
-            Some((arg, pos)) => (arg, pos),
+        let s = match find_jobserver_auth(var) {
+            Some(s) => s,
             None => return FromEnv::new_err(FromEnvErrorInner::NoJobserver, env, var_os),
         };
-
-        let s = var[pos + arg.len()..].split(' ').next().unwrap();
         match imp::Client::open(s, check_pipe) {
             Ok(c) => FromEnv::new_ok(Client { inner: Arc::new(c) }, env, var_os),
             Err(err) => FromEnv::new_err(err, env, var_os),
@@ -588,9 +582,74 @@ impl HelperState {
     }
 }
 
+/// Finds and returns the value of `--jobserver-auth=<VALUE>` in the given
+/// environment variable.
+///
+/// Precedence rules:
+///
+/// * The last instance wins [^1].
+/// * `--jobserver-fds=` as a fallback when no `--jobserver-auth=` is present [^2].
+///
+/// [^1]: See ["GNU `make` manual: Sharing Job Slots with GNU `make`"](https://www.gnu.org/software/make/manual/make.html#Job-Slots)
+/// _"Be aware that the `MAKEFLAGS` variable may contain multiple instances of
+/// the `--jobserver-auth=` option. Only the last instance is relevant."_
+///
+/// [^2]: Refer to [the release announcement](https://git.savannah.gnu.org/cgit/make.git/tree/NEWS?h=4.2#n31)
+/// of GNU Make 4.2, which states that `--jobserver-fds` was initially an
+/// internal-only flag and was later renamed to `--jobserver-auth`.
+fn find_jobserver_auth(var: &str) -> Option<&str> {
+    ["--jobserver-auth=", "--jobserver-fds="]
+        .iter()
+        .find_map(|&arg| var.rsplit_once(arg).map(|(_, s)| s))
+        .and_then(|s| s.split(' ').next())
+}
+
 #[test]
 fn no_helper_deadlock() {
     let x = crate::Client::new(32).unwrap();
     let _y = x.clone();
     std::mem::drop(x.into_helper_thread(|_| {}).unwrap());
 }
+
+#[test]
+fn test_find_jobserver_auth() {
+    let cases = [
+        ("", None),
+        ("-j2", None),
+        ("-j2 --jobserver-auth=3,4", Some("3,4")),
+        ("--jobserver-auth=3,4 -j2", Some("3,4")),
+        ("--jobserver-auth=3,4", Some("3,4")),
+        ("--jobserver-auth=fifo:/myfifo", Some("fifo:/myfifo")),
+        ("--jobserver-auth=", Some("")),
+        ("--jobserver-auth", None),
+        ("--jobserver-fds=3,4", Some("3,4")),
+        ("--jobserver-fds=fifo:/myfifo", Some("fifo:/myfifo")),
+        ("--jobserver-fds=", Some("")),
+        ("--jobserver-fds", None),
+        (
+            "--jobserver-auth=auth-a --jobserver-auth=auth-b",
+            Some("auth-b"),
+        ),
+        (
+            "--jobserver-auth=auth-b --jobserver-auth=auth-a",
+            Some("auth-a"),
+        ),
+        ("--jobserver-fds=fds-a --jobserver-fds=fds-b", Some("fds-b")),
+        ("--jobserver-fds=fds-b --jobserver-fds=fds-a", Some("fds-a")),
+        (
+            "--jobserver-auth=auth-a --jobserver-fds=fds-a --jobserver-auth=auth-b",
+            Some("auth-b"),
+        ),
+        (
+            "--jobserver-fds=fds-a --jobserver-auth=auth-a --jobserver-fds=fds-b",
+            Some("auth-a"),
+        ),
+    ];
+    for (var, expected) in cases {
+        let actual = find_jobserver_auth(var);
+        assert_eq!(
+            actual, expected,
+            "expect {expected:?}, got {actual:?}, input `{var:?}`"
+        );
+    }
+}