Require re-review for amend and reword commits

CONTEXT.md

@@ -11,6 +11,8 @@ Approval applies to the reviewed stack content, even when its commit identities
 **Stack content**:
 The combined change from the original commit's parent to the final commit in the stack.
 Commit metadata and intermediate commit boundaries are not part of this content.
+A fixup! commit carries no message, so patch identity alone decides its review status.
+Appending an amend! or reword! commit still creates a new stack version that requires review, even when the combined change is unchanged.
 
 **Attempt**:
 One recorded review or explicit approval of an exact stack.

internal/review/git.go

@@ -15,6 +15,9 @@ import (
 
 // Stack identifies an original commit and its adjacent feedback commits.
 // Field names retain the Pi state format. ChangeID is optional lineage metadata.
+// AmendCount counts attached amend! and reword! commits, whose messages carry
+// reviewable content. Attached fixup! commits carry no message, so patch
+// identity alone decides their review status.
 type Stack struct {
 	Repo      string   `json:"repo"`
 	Commits   []string `json:"commits"`
@@ -24,6 +27,7 @@ type Stack struct {
 	Key       string   `json:"key"`
 	ChangeID  string   `json:"changeId,omitempty"`
 	PatchHash string   `json:"patchHash,omitempty"`
+	AmendCount int      `json:"amendCount,omitzero"`
 }
 
 func (s Stack) validate() error {
@@ -61,6 +65,12 @@ func (s Stack) validateMetadata() error {
 			return fmt.Errorf("invalid stack Change-Id")
 		}
 	}
+	if s.AmendCount < 0 {
+		return fmt.Errorf("invalid stack amend count")
+	}
+	if s.AmendCount > len(s.Commits) {
+		return fmt.Errorf("invalid stack amend count")
+	}
 	return nil
 }
 
@@ -262,7 +272,7 @@ func (r Repository) stacks(ctx context.Context, repo, base, history string) ([]S
 			return nil, fmt.Errorf("commit %s has an empty subject", sha)
 		}
 		if match := feedbackSubject.FindStringSubmatch(subject); match != nil {
-			if !tryAppendFeedback(stacks, targets, sha, match[1]) {
+			if !tryAppendFeedback(stacks, targets, sha, match[1], match[2]) {
 				id, err := r.changeID(ctx, sha, ids)
 				if err != nil {
 					return nil, err
@@ -282,7 +292,7 @@ func (r Repository) stacks(ctx context.Context, repo, base, history string) ([]S
 	return stacks, nil
 }
 
-func tryAppendFeedback(stacks []Stack, targets map[string][]int, sha, subject string) bool {
+func tryAppendFeedback(stacks []Stack, targets map[string][]int, sha, kind, subject string) bool {
 	matches := targets[subject]
 	if len(matches) != 1 {
 		return false
@@ -294,11 +304,14 @@ func tryAppendFeedback(stacks []Stack, targets map[string][]int, sha, subject st
 	last.Commits = append(last.Commits, sha)
 	last.Tip = sha
 	last.Key = last.Commits[0] + ".." + sha
+	if kind == "amend!" || kind == "reword!" {
+		last.AmendCount++
+	}
 	return true
 }
 
 var (
 	patchHashPattern = regexp.MustCompile(`^(?:v1:[a-f0-9]{64}|v2:(?:[a-f0-9]{40}|[a-f0-9]{64}))$`)
-	feedbackSubject  = regexp.MustCompile(`^(?:fixup!|reword!|amend!) (.+)$`)
+	feedbackSubject  = regexp.MustCompile(`^(fixup!|reword!|amend!) (.+)$`)
 	changeIDPattern  = regexp.MustCompile(`^I[0-9a-fA-F]{40}$`)
 )

internal/review/patch_status_test.go

@@ -10,6 +10,51 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestAppendedFeedbackWithUnchangedPatchRequiresReview(t *testing.T) {
+	for _, tc := range []struct {
+		subject string
+		want    string
+	}{
+		{subject: "amend! Feature", want: "needs_review"},
+		{subject: "reword! Feature", want: "needs_review"},
+		// A fixup! commit carries no message, so patch identity alone decides it.
+		{subject: "fixup! Feature", want: "approved"},
+	} {
+		t.Run(tc.subject, func(t *testing.T) {
+			r := testRepo(t)
+			file := filepath.Join(r.Dir, "file")
+			require.NoError(t, os.WriteFile(file, []byte("one\n"), 0o600))
+			gitTest(t, r.Dir, "add", ".")
+			gitTest(t, r.Dir, "commit", "-m", "Feature")
+			service := Service{Repository: r}
+			_, err := service.Approve(t.Context(), "HEAD", "")
+			require.NoError(t, err)
+			before, err := r.Snapshot(t.Context())
+			require.NoError(t, err)
+			require.NotEmpty(t, before.Stacks[0].PatchHash)
+			gitTest(t, r.Dir, "commit", "--allow-empty", "-m", tc.subject)
+			after, err := r.Snapshot(t.Context())
+			require.NoError(t, err)
+			require.Len(t, after.Stacks, 1)
+			require.Equal(t, before.Stacks[0].PatchHash, after.Stacks[0].PatchHash)
+			if tc.want == "approved" {
+				require.Equal(t, before.Stacks[0].AmendCount, after.Stacks[0].AmendCount)
+				result, err := service.Status(t.Context(), true)
+				require.NoError(t, err)
+				require.Equal(t, "idle", result.Status)
+				require.Empty(t, result.Entries)
+				return
+			}
+			require.Equal(t, before.Stacks[0].AmendCount+1, after.Stacks[0].AmendCount)
+			result, err := service.Status(t.Context(), true)
+			require.NoError(t, err)
+			require.Len(t, result.Entries, 1)
+			require.Equal(t, "needs_review", result.Entries[0].Status)
+			require.Equal(t, "approved", result.Entries[0].Previous.Status)
+		})
+	}
+}
+
 func TestReviewStatusAfterStackRewrite(t *testing.T) {
 	for _, tc := range []struct {
 		name, status, rewrite, want string

internal/review/state.go

@@ -223,7 +223,30 @@ func samePatch(a, b Stack) bool {
 	if a.PatchHash == "" {
 		return false
 	}
-	return a.PatchHash == b.PatchHash
+	if a.PatchHash != b.PatchHash {
+		return false
+	}
+	// An appended amend! or reword! commit is a new version even when the
+	// aggregate diff is unchanged (for example an empty commit). An appended
+	// fixup! commit carries no message, so patch identity alone decides it.
+	if appendedAmends(a, b) || appendedAmends(b, a) {
+		return false
+	}
+	return true
+}
+
+func appendedAmends(old, current Stack) bool {
+	if !extendsStack(old, current) {
+		return false
+	}
+	return current.AmendCount > old.AmendCount
+}
+
+func extendsStack(old, current Stack) bool {
+	if len(current.Commits) <= len(old.Commits) {
+		return false
+	}
+	return slices.Equal(old.Commits, current.Commits[:len(old.Commits)])
 }
 
 func sameStack(a, b Stack) bool {