Fix incorrect divergence cache after switching default branch (#34370) (#34406)

Backport #34370 by @GWDx

Issue: After switching the default branch, other branches are still
compared against the old default branch due to outdated divergence
cache.

Change: Clear the divergence cache in SetRepoDefaultBranch to ensure
correct comparisons against the new default branch.

Fixes #34369

Co-authored-by: GWDx <gwdx@mail.ustc.edu.cn>

tests/integration/change_default_branch_test.go

@@ -6,12 +6,16 @@ package integration
 import (
 	"fmt"
 	"net/http"
+	"strconv"
 	"testing"
 
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestChangeDefaultBranch(t *testing.T) {
@@ -38,3 +42,96 @@ func TestChangeDefaultBranch(t *testing.T) {
 	})
 	session.MakeRequest(t, req, http.StatusNotFound)
 }
+
+func checkDivergence(t *testing.T, session *TestSession, branchesURL, expectedDefaultBranch string, expectedBranchToDivergence map[string]git.DivergeObject) {
+	req := NewRequest(t, "GET", branchesURL)
+	resp := session.MakeRequest(t, req, http.StatusOK)
+
+	htmlDoc := NewHTMLParser(t, resp.Body)
+
+	branchNodes := htmlDoc.doc.Find(".branch-name").Nodes
+	branchNames := []string{}
+	for _, node := range branchNodes {
+		branchNames = append(branchNames, node.FirstChild.Data)
+	}
+
+	expectBranchCount := len(expectedBranchToDivergence)
+
+	assert.Len(t, branchNames, expectBranchCount+1)
+	assert.Equal(t, expectedDefaultBranch, branchNames[0])
+
+	allCountBehindNodes := htmlDoc.doc.Find(".count-behind").Nodes
+	allCountAheadNodes := htmlDoc.doc.Find(".count-ahead").Nodes
+
+	assert.Len(t, allCountAheadNodes, expectBranchCount)
+	assert.Len(t, allCountBehindNodes, expectBranchCount)
+
+	for i := range expectBranchCount {
+		branchName := branchNames[i+1]
+		assert.Contains(t, expectedBranchToDivergence, branchName)
+
+		expectedCountAhead := expectedBranchToDivergence[branchName].Ahead
+		expectedCountBehind := expectedBranchToDivergence[branchName].Behind
+		countAhead, err := strconv.Atoi(allCountAheadNodes[i].FirstChild.Data)
+		assert.NoError(t, err)
+		countBehind, err := strconv.Atoi(allCountBehindNodes[i].FirstChild.Data)
+		assert.NoError(t, err)
+
+		assert.Equal(t, expectedCountAhead, countAhead)
+		assert.Equal(t, expectedCountBehind, countBehind)
+	}
+}
+
+func TestChangeDefaultBranchDivergence(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 16})
+	owner := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: repo.OwnerID})
+
+	session := loginUser(t, owner.Name)
+	branchesURL := fmt.Sprintf("/%s/%s/branches", owner.Name, repo.Name)
+	settingsBranchesURL := fmt.Sprintf("/%s/%s/settings/branches", owner.Name, repo.Name)
+
+	// check branch divergence before switching default branch
+	expectedBranchToDivergenceBefore := map[string]git.DivergeObject{
+		"not-signed": {
+			Ahead:  0,
+			Behind: 0,
+		},
+		"good-sign-not-yet-validated": {
+			Ahead:  0,
+			Behind: 1,
+		},
+		"good-sign": {
+			Ahead:  1,
+			Behind: 3,
+		},
+	}
+	checkDivergence(t, session, branchesURL, "master", expectedBranchToDivergenceBefore)
+
+	// switch default branch
+	newDefaultBranch := "good-sign-not-yet-validated"
+	csrf := GetUserCSRFToken(t, session)
+	req := NewRequestWithValues(t, "POST", settingsBranchesURL, map[string]string{
+		"_csrf":  csrf,
+		"action": "default_branch",
+		"branch": newDefaultBranch,
+	})
+	session.MakeRequest(t, req, http.StatusSeeOther)
+
+	// check branch divergence after switching default branch
+	expectedBranchToDivergenceAfter := map[string]git.DivergeObject{
+		"master": {
+			Ahead:  1,
+			Behind: 0,
+		},
+		"not-signed": {
+			Ahead:  1,
+			Behind: 0,
+		},
+		"good-sign": {
+			Ahead:  1,
+			Behind: 2,
+		},
+	}
+	checkDivergence(t, session, branchesURL, newDefaultBranch, expectedBranchToDivergenceAfter)
+}