A test binary's directory is one it made fresh, and a same-pid name already there when it started is not counted as its leftover

TODO.org

@@ -1708,7 +1708,8 @@ flake and a flake is how a watchdog gets deleted.
 
 ** DONE A test binary leaves nothing in the temporary directory it was given
 CLOSED: [2026-09-25]
-Every test binary makes =flan-t<pid>= under its =TMPDIR=, exports it as
+Every test binary makes a fresh =flan-t<pid>-<hex>= under its =TMPDIR=, never
+adopting an existing entry, exports it as
 =TMPDIR= to everything it starts, and removes it on exit, on the watchdog, and
 on SIGINT or SIGTERM (=test/own_tmp.ml=). A binary exits nonzero if that
 directory survives or if a =flan-<pid>= or =flan-dev-<pid>= for its own pid

test/own_tmp.ml

@@ -37,11 +37,25 @@ let owner = Unix.getpid ()
 
 (* Short, because a Unix socket path has 108 bytes and the sockets the tests
    bind are under here: [flan-dev-<pid>/agent.sock], and names such as
-   [flan-devtest-halfwrite--llvm.sock]. *)
+   [flan-devtest-halfwrite--llvm.sock].
+
+   Made fresh, never adopted: [mkdir] fails on any existing entry, a symlink
+   included, and a name that is taken is passed over for another. So the
+   directory this binary removes at exit is one it provably created, and
+   nothing someone else put at that name is followed or deleted. *)
 let dir =
-  let d = Filename.concat outer (Printf.sprintf "flan-t%d" owner) in
-  (try Unix.mkdir d 0o700 with Unix.Unix_error (Unix.EEXIST, _, _) -> ());
-  d
+  Random.self_init ();
+  let rec make tries =
+    let d =
+      Filename.concat outer
+        (Printf.sprintf "flan-t%d-%04x" owner (Random.bits () land 0xffff))
+    in
+    match Unix.mkdir d 0o700 with
+    | () -> d
+    | exception Unix.Unix_error (Unix.EEXIST, _, _) when tries > 0 ->
+      make (tries - 1)
+  in
+  make 100
 
 let () =
   Unix.putenv "TMPDIR" dir;
@@ -61,13 +75,19 @@ let rec remove path =
    if removing it failed, and a session or build directory named for this
    process in the directory it was given. The second is an in-process session
    or build that did not go through [dir] — something that read the temporary
-   directory before this module set it. *)
-let leftovers () =
-  let mine =
+   directory before this module set it. A name already there when this binary
+   started belongs to an earlier process that had the same pid, and is not
+   counted. *)
+let named_for_me =
   [ Filename.concat outer (Printf.sprintf "flan-dev-%d" owner);
     Filename.concat outer (Printf.sprintf "flan-%d" owner) ]
-  in
-  List.filter Sys.file_exists (dir :: mine)
+
+let already = List.filter Sys.file_exists named_for_me
+
+let leftovers () =
+  List.filter
+    (fun d -> Sys.file_exists d && not (List.mem d already))
+    (dir :: named_for_me)
 
 (* Only the process that made the directory removes it. The binaries fork
    (test_acceptance's compile pool, the daemons before they exec), and a