fix(merge gate): an unknown head commit never merges; pr_merge refuses closed and draft PRs (review of PR #76)

Review found a bypass: a PR reporting an empty head sha made the runs query lose its head_sha filter (the repo's 50
most recent runs of any commit were classified) and dropped head_commit_id (omitempty), so the merge was unbound.
Both tools now go through evaluateMergeGate, which refuses without a head sha, and pollRuns makes no query without
one. pr_merge also reports merged:false for a PR that is not open or is a draft (Gitea refuses closed ones itself;
the gate no longer depends on that). Tests: unit (gate, counting-server pollRuns with positive control), end to end
for both tools, CI scoped to head_sha, closed/merged/draft. The two caller-wiring mutants are equivalent to the
pollRuns guard and survive by design.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZkhi49hfeSqBp5aZJNXsM

internal/tools/pr_merge_test.go

@@ -34,6 +34,9 @@ type prMergeFakeOpts struct {
 	protectionStatus  int // 0 → 404 (unprotected); 200 with required approvals when requiredApprovals>0
 	requiredApprovals int
 	mergeStatus       int
+	emptyHeadSha      bool   // the PR reports head.sha == ""
+	prState           string // default "open"
+	prDraft           bool
 }
 
 type prMergeFake struct {
@@ -41,6 +44,7 @@ type prMergeFake struct {
 	mu         sync.Mutex
 	mergeCalls int
 	mergeBody  []byte
+	runsQuery  string // raw query of the last workflow-runs request
 }
 
 func (f *prMergeFake) merged() bool { f.mu.Lock(); defer f.mu.Unlock(); return f.mergeCalls > 0 }
@@ -62,6 +66,13 @@ func newPRMergeFake(t *testing.T, o prMergeFakeOpts) *prMergeFake {
 	if o.mergeStatus == 0 {
 		o.mergeStatus = http.StatusNoContent
 	}
+	if o.prState == "" {
+		o.prState = "open"
+	}
+	sha := "abc"
+	if o.emptyHeadSha {
+		sha = ""
+	}
 	f := &prMergeFake{}
 	f.Server = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		p := r.URL.Path
@@ -69,7 +80,7 @@ func newPRMergeFake(t *testing.T, o prMergeFakeOpts) *prMergeFake {
 		switch {
 		case r.Method == http.MethodGet && p == "/api/v1/repos/owner/repo/pulls/7":
 			w.WriteHeader(o.prStatus)
-			_, _ = w.Write([]byte(`{"number":7,"title":"t","state":"open","html_url":"http://x/pulls/7","head":{"ref":"feat","sha":"abc"},"base":{"ref":"main"}}`))
+			_, _ = w.Write([]byte(`{"number":7,"title":"t","state":"` + o.prState + `","draft":` + boolJSON(o.prDraft) + `,"html_url":"http://x/pulls/7","head":{"ref":"feat","sha":"` + sha + `"},"base":{"ref":"main"}}`))
 		case r.Method == http.MethodGet && strings.Contains(p, "/branch_protections/"):
 			switch {
 			case o.protectionStatus != 0:
@@ -82,6 +93,9 @@ func newPRMergeFake(t *testing.T, o prMergeFakeOpts) *prMergeFake {
 				_, _ = w.Write([]byte(`{"message":"not found"}`))
 			}
 		case r.Method == http.MethodGet && strings.Contains(p, "/actions/runs"):
+			f.mu.Lock()
+			f.runsQuery = r.URL.RawQuery
+			f.mu.Unlock()
 			_, _ = w.Write([]byte(`{"total_count":0,"workflow_runs":` + o.runs + `}`))
 		case r.Method == http.MethodGet && p == "/api/v1/repos/owner/repo/issues/42":
 			w.WriteHeader(o.issueStatus)
@@ -103,6 +117,13 @@ func newPRMergeFake(t *testing.T, o prMergeFakeOpts) *prMergeFake {
 	return f
 }
 
+func boolJSON(b bool) string {
+	if b {
+		return "true"
+	}
+	return "false"
+}
+
 func callMerge(t *testing.T, f *prMergeFake, args string) (map[string]any, error) {
 	t.Helper()
 	tool := tools.NewPRMerge(gitea.NewClient(f.URL, "tok"), allowlist.New([]string{"owner"}))
@@ -327,3 +348,49 @@ func TestPRMerge_TheTierIsReadFromTheNamedIssueOnly(t *testing.T) {
 	assert.Equal(t, "high", res["risk"])
 	assert.False(t, f.merged())
 }
+
+// ---- review of PR #76 ----
+
+// An empty head sha used to disable BOTH the CI scoping (the runs query lost its head_sha filter, so the repo's most
+// recent runs of any commit were classified) and the merge binding (head_commit_id is omitempty). Fail closed.
+func TestPRMerge_AnEmptyHeadShaFailsClosed(t *testing.T) {
+	f := newPRMergeFake(t, prMergeFakeOpts{emptyHeadSha: true})
+
+	res, err := callMerge(t, f, mergeArgs)
+
+	require.NoError(t, err)
+	assert.Equal(t, false, res["merged"])
+	assert.Contains(t, res["reason"], "head commit")
+	assert.False(t, f.merged(), "an unknown head commit must never merge")
+	assert.Empty(t, f.runsQuery, "no unscoped CI query may be made when the head sha is unknown")
+}
+
+// CI is read for the PR's head commit only: the query names it.
+func TestPRMerge_CIIsScopedToTheHeadCommit(t *testing.T) {
+	f := newPRMergeFake(t, prMergeFakeOpts{})
+
+	_, err := callMerge(t, f, mergeArgs)
+
+	require.NoError(t, err)
+	assert.Contains(t, f.runsQuery, "head_sha=abc")
+}
+
+// Defence in depth: Gitea refuses closed/merged PRs itself, but the gate should not depend on that.
+func TestPRMerge_ClosedOrDraftPRsAreNotMerged(t *testing.T) {
+	for name, o := range map[string]prMergeFakeOpts{
+		"closed": {prState: "closed"},
+		"merged": {prState: "merged"},
+		"draft":  {prDraft: true},
+	} {
+		t.Run(name, func(t *testing.T) {
+			f := newPRMergeFake(t, o)
+
+			res, err := callMerge(t, f, mergeArgs)
+
+			require.NoError(t, err)
+			assert.Equal(t, false, res["merged"])
+			assert.NotEmpty(t, res["reason"])
+			assert.False(t, f.merged())
+		})
+	}
+}