Prevent re-review and dismiss review actions on closed and merged PRs (#30065)

Resolves #29965.

---
Manually tested this by:
- Following the
[installation](https://docs.gitea.com/next/installation/install-with-docker#basics)
guide (but built a local Docker image instead)
- Creating 2 users, one who is the `Owner` of a newly-created repository
and the other a `Collaborator`
- Had the `Collaborator` create a PR that the `Owner` reviews
- `Collaborator` resolves conversation and `Owner` merges PR

And with this change we see that we can no longer see re-request review
button for the `Owner`:

<img width="1351" alt="Screenshot 2024-03-25 at 12 39 18 AM"
src="https://github.com/go-gitea/gitea/assets/60799661/bcd9c579-3cf7-474f-a51e-b436fe1a39a4">

2 files changed, 81 insertions, 0 deletions

diff --git a/services/pull/review.go b/services/pull/review.go
index de1021c5c0..5bf1991d13 100644
--- a/services/pull/review.go
+++ b/services/pull/review.go
@@ -20,11 +20,29 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
 	notify_service "code.gitea.io/gitea/services/notify"
 )
 
 var notEnoughLines = regexp.MustCompile(`fatal: file .* has only \d+ lines?`)
 
+// ErrDismissRequestOnClosedPR represents an error when an user tries to dismiss a review associated to a closed or merged PR.
+type ErrDismissRequestOnClosedPR struct{}
+
+// IsErrDismissRequestOnClosedPR checks if an error is an ErrDismissRequestOnClosedPR.
+func IsErrDismissRequestOnClosedPR(err error) bool {
+	_, ok := err.(ErrDismissRequestOnClosedPR)
+	return ok
+}
+
+func (err ErrDismissRequestOnClosedPR) Error() string {
+	return "can't dismiss a review associated to a closed or merged PR"
+}
+
+func (err ErrDismissRequestOnClosedPR) Unwrap() error {
+	return util.ErrPermissionDenied
+}
+
 // checkInvalidation checks if the line of code comment got changed by another commit.
 // If the line got changed the comment is going to be invalidated.
 func checkInvalidation(ctx context.Context, c *issues_model.Comment, doer *user_model.User, repo *git.Repository, branch string) error {
@@ -382,6 +400,21 @@ func DismissReview(ctx context.Context, reviewID, repoID int64, message string,
 		return nil, fmt.Errorf("reviews's repository is not the same as the one we expect")
 	}
 
+	issue := review.Issue
+
+	if issue.IsClosed {
+		return nil, ErrDismissRequestOnClosedPR{}
+	}
+
+	if issue.IsPull {
+		if err := issue.LoadPullRequest(ctx); err != nil {
+			return nil, err
+		}
+		if issue.PullRequest.HasMerged {
+			return nil, ErrDismissRequestOnClosedPR{}
+		}
+	}
+
 	if err := issues_model.DismissReview(ctx, review, isDismiss); err != nil {
 		return nil, err
 	}
diff --git a/services/pull/review_test.go b/services/pull/review_test.go
new file mode 100644
index 0000000000..3bce1e523d
--- /dev/null
+++ b/services/pull/review_test.go
@@ -0,0 +1,48 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package pull_test
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	pull_service "code.gitea.io/gitea/services/pull"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDismissReview(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	pull := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{})
+	assert.NoError(t, pull.LoadIssue(db.DefaultContext))
+	issue := pull.Issue
+	assert.NoError(t, issue.LoadRepo(db.DefaultContext))
+	reviewer := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})
+	review, err := issues_model.CreateReview(db.DefaultContext, issues_model.CreateReviewOptions{
+		Issue:    issue,
+		Reviewer: reviewer,
+		Type:     issues_model.ReviewTypeReject,
+	})
+
+	assert.NoError(t, err)
+	issue.IsClosed = true
+	pull.HasMerged = false
+	assert.NoError(t, issues_model.UpdateIssueCols(db.DefaultContext, issue, "is_closed"))
+	assert.NoError(t, pull.UpdateCols(db.DefaultContext, "has_merged"))
+	_, err = pull_service.DismissReview(db.DefaultContext, review.ID, issue.RepoID, "", &user_model.User{}, false, false)
+	assert.Error(t, err)
+	assert.True(t, pull_service.IsErrDismissRequestOnClosedPR(err))
+
+	pull.HasMerged = true
+	pull.Issue.IsClosed = false
+	assert.NoError(t, issues_model.UpdateIssueCols(db.DefaultContext, issue, "is_closed"))
+	assert.NoError(t, pull.UpdateCols(db.DefaultContext, "has_merged"))
+	_, err = pull_service.DismissReview(db.DefaultContext, review.ID, issue.RepoID, "", &user_model.User{}, false, false)
+	assert.Error(t, err)
+	assert.True(t, pull_service.IsErrDismissRequestOnClosedPR(err))
+}