Option to delay conflict checking of old pull requests until page view (#27779)

`[repository.pull-request] DELAY_CHECK_FOR_INACTIVE_DAYS` is a new
setting to delay the mergeable check for pull requests that have been
inactive for the specified number of days.

This avoids potentially long delays for big repositories with many pull
requests. and reduces system load overall when there are many
repositories or pull requests.

When viewing the PR, checking will start immediately and the PR merge
box will automatically reload when complete. Accessing the PR through
the API will also start checking immediately.

The default value of `7` provides a balance between system load, and
keeping behavior similar to what it was before both for users and API
access. With `0` all conflict checking will be delayed, while `-1`
always checks immediately to restore the previous behavior.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

tests/integration/pull_status_test.go

@@ -13,9 +13,13 @@ import (
 
 	auth_model "code.gitea.io/gitea/models/auth"
 	git_model "code.gitea.io/gitea/models/git"
+	"code.gitea.io/gitea/models/issues"
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/setting"
 	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/test"
+	"code.gitea.io/gitea/services/pull"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -165,3 +169,75 @@ func TestPullCreate_EmptyChangesWithSameCommits(t *testing.T) {
 		assert.Contains(t, text, "This branch is already included in the target branch. There is nothing to merge.")
 	})
 }
+
+func TestPullStatusDelayCheck(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, u *url.URL) {
+		defer test.MockVariableValue(&setting.IsProd)()
+		defer test.MockVariableValue(&setting.Repository.PullRequest.DelayCheckForInactiveDays, 1)()
+		defer test.MockVariableValue(&pull.AddPullRequestToCheckQueue)()
+
+		session := loginUser(t, "user2")
+
+		run := func(t *testing.T, fn func(*testing.T)) (issue3 *issues.Issue, checkedPrID int64) {
+			pull.AddPullRequestToCheckQueue = func(prID int64) {
+				checkedPrID = prID
+			}
+			fn(t)
+			issue3 = unittest.AssertExistsAndLoadBean(t, &issues.Issue{RepoID: 1, Index: 3})
+			_ = issue3.LoadPullRequest(t.Context())
+			return issue3, checkedPrID
+		}
+
+		assertReloadingInterval := func(t *testing.T, interval string) {
+			req := NewRequest(t, "GET", "/user2/repo1/pulls/3")
+			resp := session.MakeRequest(t, req, http.StatusOK)
+			attr := "data-pull-merge-box-reloading-interval"
+			if interval == "" {
+				assert.NotContains(t, resp.Body.String(), attr)
+			} else {
+				assert.Contains(t, resp.Body.String(), fmt.Sprintf(`%s="%v"`, attr, interval))
+			}
+		}
+
+		// PR issue3 is merageable at the beginning
+		issue3, checkedPrID := run(t, func(t *testing.T) {})
+		assert.Equal(t, issues.PullRequestStatusMergeable, issue3.PullRequest.Status)
+		assert.Zero(t, checkedPrID)
+		setting.IsProd = true
+		assertReloadingInterval(t, "") // the PR is mergeable, so no need to reload the merge box
+		setting.IsProd = false
+		assertReloadingInterval(t, "1") // make sure dev mode always do merge box reloading, to make sure the UI logic won't break
+		setting.IsProd = true
+
+		// when base branch changes, PR status should be updated, but it is inactive for long time, so no real check
+		issue3, checkedPrID = run(t, func(t *testing.T) {
+			testEditFile(t, session, "user2", "repo1", "master", "README.md", "new content 1")
+		})
+		assert.Equal(t, issues.PullRequestStatusChecking, issue3.PullRequest.Status)
+		assert.Zero(t, checkedPrID)
+		assertReloadingInterval(t, "2000") // the PR status is "checking", so try to reload the merge box
+
+		// view a PR with status=checking, it starts the real check
+		issue3, checkedPrID = run(t, func(t *testing.T) {
+			req := NewRequest(t, "GET", "/user2/repo1/pulls/3")
+			session.MakeRequest(t, req, http.StatusOK)
+		})
+		assert.Equal(t, issues.PullRequestStatusChecking, issue3.PullRequest.Status)
+		assert.Equal(t, issue3.PullRequest.ID, checkedPrID)
+
+		// when base branch changes, still so no real check
+		issue3, checkedPrID = run(t, func(t *testing.T) {
+			testEditFile(t, session, "user2", "repo1", "master", "README.md", "new content 2")
+		})
+		assert.Equal(t, issues.PullRequestStatusChecking, issue3.PullRequest.Status)
+		assert.Zero(t, checkedPrID)
+
+		// then allow to check PRs without delay, when base branch changes, the PRs will be checked
+		setting.Repository.PullRequest.DelayCheckForInactiveDays = -1
+		issue3, checkedPrID = run(t, func(t *testing.T) {
+			testEditFile(t, session, "user2", "repo1", "master", "README.md", "new content 3")
+		})
+		assert.Equal(t, issues.PullRequestStatusChecking, issue3.PullRequest.Status)
+		assert.Equal(t, issue3.PullRequest.ID, checkedPrID)
+	})
+}