Fix pull creation with empty changes (#7920)

* Logs the stderr of git-apply
* Add an integration test
* Skip testPatch when patch is empty

integrations/pull_status_test.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/url"
 	"path"
+	"strings"
 	"testing"
 
 	"code.gitea.io/gitea/models"
@@ -93,3 +94,28 @@ func TestPullCreate_CommitStatus(t *testing.T) {
 		}
 	})
 }
+
+func TestPullCreate_EmptyChangesWithCommits(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, u *url.URL) {
+		session := loginUser(t, "user1")
+		testRepoFork(t, session, "user2", "repo1", "user1", "repo1")
+		testEditFileToNewBranch(t, session, "user1", "repo1", "master", "status1", "README.md", "status1")
+		testEditFileToNewBranch(t, session, "user1", "repo1", "status1", "status1", "README.md", "# repo1\n\nDescription for repo1")
+
+		url := path.Join("user1", "repo1", "compare", "master...status1")
+		req := NewRequestWithValues(t, "POST", url,
+			map[string]string{
+				"_csrf": GetCSRF(t, session, url),
+				"title": "pull request from status1",
+			},
+		)
+		session.MakeRequest(t, req, http.StatusFound)
+
+		req = NewRequest(t, "GET", "/user1/repo1/pulls/1")
+		resp := session.MakeRequest(t, req, http.StatusOK)
+		doc := NewHTMLParser(t, resp.Body)
+
+		text := strings.TrimSpace(doc.doc.Find(".item.text.green").Text())
+		assert.EqualValues(t, "This pull request can be merged automatically.", text)
+	})
+}

models/pull.go

@@ -598,7 +598,7 @@ func (pr *PullRequest) testPatch(e Engine) (err error) {
 	if err != nil {
 		for i := range patchConflicts {
 			if strings.Contains(stderr, patchConflicts[i]) {
-				log.Trace("PullRequest[%d].testPatch (apply): has conflict", pr.ID)
+				log.Trace("PullRequest[%d].testPatch (apply): has conflict: %s", pr.ID, stderr)
 				const prefix = "error: patch failed:"
 				pr.Status = PullRequestStatusConflict
 				pr.ConflictedFiles = make([]string, 0, 5)
@@ -661,14 +661,17 @@ func NewPullRequest(repo *Repository, pull *Issue, labelIDs []int64, uuids []str
 	}
 
 	pr.Index = pull.Index
+	pr.BaseRepo = repo
+	pr.Status = PullRequestStatusChecking
+	if len(patch) > 0 {
 		if err = repo.savePatch(sess, pr.Index, patch); err != nil {
 			return fmt.Errorf("SavePatch: %v", err)
 		}
 
-	pr.BaseRepo = repo
 		if err = pr.testPatch(sess); err != nil {
 			return fmt.Errorf("testPatch: %v", err)
 		}
+	}
 	// No conflict appears after test means mergeable.
 	if pr.Status == PullRequestStatusChecking {
 		pr.Status = PullRequestStatusMergeable