From 5d4951a37308e3a4d31cff52ee995c903044f8ce Mon Sep 17 00:00:00 2001
From: Jeroen Soeters <wfhsoeters@gmail.com>
Date: Fri, 21 Aug 2026 13:19:52 -0700
Subject: [PATCH] fix(scripts): mutate a module-root package from its own
 directory

When a changed package is itself a module root, such as pkg/plugin, the
relative-path computation never matched: the prefix strip carries a
trailing slash, so a path equal to the module root came through intact
and gremlins was invoked on a doubled absolute path like
./home/runner/work/formae/formae/pkg/plugin, failing setup before
coverage could run. The check then reported the package as producing no
usable result.

A package that equals its module root is now addressed as the current
directory, so gremlins runs in that module on the module itself.
---
 scripts/mutation-test-changed.sh            | 11 +++++--
 scripts/tests/mutation-test-changed_test.sh | 32 +++++++++++++++++++++
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/scripts/mutation-test-changed.sh b/scripts/mutation-test-changed.sh
index d1fc149c7..fd8b3c70a 100755
--- a/scripts/mutation-test-changed.sh
+++ b/scripts/mutation-test-changed.sh
@@ -307,9 +307,16 @@ main() {
   for pkg in "${testable_packages[@]}"; do
     pkg_index=$((pkg_index + 1))
     module_root=$(find_module_root "$pkg")
-    # Compute relative package path from the module root
+    # Compute relative package path from the module root. A package that is
+    # itself the module root has nothing to strip — the prefix pattern below
+    # carries a trailing slash and would leave the absolute path intact — so
+    # it is addressed as the current directory instead.
     rel_pkg="${REPO_ROOT}/${pkg#/}"
-    rel_pkg="${rel_pkg#"$module_root"/}"
+    if [[ "$rel_pkg" == "$module_root" ]]; then
+      rel_pkg="."
+    else
+      rel_pkg="${rel_pkg#"$module_root"/}"
+    fi
 
     echo ""
     echo "=== $pkg (module: ${module_root#"$REPO_ROOT"/}) ==="
diff --git a/scripts/tests/mutation-test-changed_test.sh b/scripts/tests/mutation-test-changed_test.sh
index 55e3e53fe..ae021fe78 100755
--- a/scripts/tests/mutation-test-changed_test.sh
+++ b/scripts/tests/mutation-test-changed_test.sh
@@ -371,6 +371,37 @@ test_a_report_with_a_zero_exit_is_ok() {
   assert_status 0 "a completed run passes"
 }
 
+# A changed package that is its own module root (it holds a go.mod) is invoked
+# on itself: gremlins runs in that directory targeting the directory, not a
+# path re-rooted below it.
+test_a_module_root_package_is_invoked_on_itself() {
+  local work repo bin invocation
+  work=$(new_workdir)
+  repo="$work/repo"
+  bin="$work/bin"
+  invocation="$work/invocation"
+
+  make_fixture_repo "$repo"
+  add_changed_package "$repo" "pkg/plugin"
+  printf 'module example/plugin\n\ngo 1.26\n' > "$repo/pkg/plugin/go.mod"
+  fixture_commit "$repo" "make pkg/plugin its own module"
+
+  stub_gremlins "$bin" "pwd > '$invocation'; echo \"\$target\" >> '$invocation'
+printf '%s' '$(mutation_report KILLED)' > \"\$report_path\"; exit 0"
+
+  run_script "$repo" "$bin"
+
+  assert_status 0 "a module-root package produces a usable result"
+  assert_output_matches '\| `pkg/plugin` \| ok \|' \
+    "the module-root package row is ok"
+  if [[ "$(sed -n 1p "$invocation" 2>/dev/null)" != "$repo/pkg/plugin" ]]; then
+    fail "gremlins did not run in the package's own module root (got '$(sed -n 1p "$invocation" 2>/dev/null)')"
+  fi
+  if [[ "$(sed -n 2p "$invocation" 2>/dev/null)" != "./." ]]; then
+    fail "gremlins was not invoked on the module root itself (got '$(sed -n 2p "$invocation" 2>/dev/null)')"
+  fi
+}
+
 # Surviving mutants make gremlins exit non-zero: advisory content, not a crash.
 test_surviving_mutants_are_not_a_failure() {
   local work repo bin
@@ -883,6 +914,7 @@ run_test() {
 main() {
   run_test test_exit_zero_without_report_is_a_failure
   run_test test_a_report_with_a_zero_exit_is_ok
+  run_test test_a_module_root_package_is_invoked_on_itself
   run_test test_surviving_mutants_are_not_a_failure
   run_test test_an_unknown_non_zero_exit_with_a_report_is_ok
   run_test test_a_zero_mutant_report_is_ok_without_a_score