fix(test-runner): reject a sibling dir that shares the test dir's prefix

cj/test--do-focus-add-file tested containment with a bare string-prefix-p on two truenames, so tests-old/foo.el passed as being inside tests/ and joined the focus list.

The correct helper was already there. cj/test--file-in-directory-p wraps the directory in file-name-as-directory first, and the other two call sites already used it. This one didn't.

The helper even had a sibling-rejection test. It was proven correct while the call site two hundred lines away never called it, which is why the new test drives the real focus-add path with a tests-old/ directory on disk rather than testing the helper again.

2 files changed, 23 insertions, 1 deletions

diff --git a/modules/test-runner.el b/modules/test-runner.el
index e05145e4..7f157f1c 100644
--- a/modules/test-runner.el
+++ b/modules/test-runner.el
@@ -239,7 +239,10 @@ Returns: \\='success if added successfully,
 Second value is the relative filename if successful."
   (cond
    ((null filepath) (cons 'no-file nil))
-   ((not (string-prefix-p (file-truename testdir) (file-truename filepath)))
+   ;; Route through the helper: it appends the trailing slash, so a sibling
+   ;; sharing the directory's name prefix (tests-old/ against tests/) is
+   ;; rejected.  A bare `string-prefix-p' on the truenames accepts it.
+   ((not (cj/test--file-in-directory-p filepath testdir))
     (cons 'not-in-testdir nil))
    (t
     (let ((relative (file-relative-name filepath testdir)))
diff --git a/tests/test-test-runner.el b/tests/test-test-runner.el
index 0ff66f7f..6854b72e 100644
--- a/tests/test-test-runner.el
+++ b/tests/test-test-runner.el
@@ -152,6 +152,25 @@ FILES is an alist of relative test filenames to file contents."
     (should (eq (car result) 'not-in-testdir)))
   (test-testrunner-teardown))
 
+(ert-deftest test-testrunner-focus-add-file-shared-prefix-sibling-rejected ()
+  "Boundary: a sibling directory sharing the test dir's name prefix is outside it.
+`/tmp/x/tests-old/f.el' starts with `/tmp/x/tests' as a string, but it is not
+in `/tmp/x/tests'.  A raw `string-prefix-p' on the two truenames accepts it;
+comparing against the directory with a trailing slash rejects it."
+  (test-testrunner-setup)
+  (let* ((testdir (file-truename test-testrunner--temp-dir))
+         (sibling (concat (directory-file-name testdir) "-old"))
+         (filepath (expand-file-name "test-foo.el" sibling)))
+    (unwind-protect
+        (progn
+          (make-directory sibling t)
+          (with-temp-file filepath (insert ";; not in the test dir\n"))
+          (let ((result (cj/test--do-focus-add-file
+                         filepath test-testrunner--temp-dir '())))
+            (should (eq (car result) 'not-in-testdir))))
+      (when (file-directory-p sibling) (delete-directory sibling t))))
+  (test-testrunner-teardown))
+
 (ert-deftest test-testrunner-focus-add-file-already-focused ()
   "Should detect already focused file."
   (test-testrunner-setup)