fix(github): prevent null pointer dereferencing when using AllowMergeableBypassApply with no required checks on branch protection

runatlantis.io/docs/server-configuration.md

@@ -564,11 +564,6 @@ This is useful when you have many projects and want to keep the pull request cle
   ```
   Feature flag to enable ability to use `mergeable` mode with required apply status check.
 
-  ::: warning NOTE
-  If there aren't any required checks set in the Github branch protection settings then this will cause atlantis to fail.
-  See issue https://github.com/runatlantis/atlantis/issues/2663.
-  :::
-
 ### `--gitlab-hostname`
   ```bash
   atlantis server --gitlab-hostname="my.gitlab.enterprise.com"

server/events/vcs/github_client.go

@@ -418,6 +418,10 @@ func (g *GithubClient) GetCombinedStatusMinusApply(repo models.Repo, pull *githu
 		return false, errors.Wrap(err, "getting required status checks")
 	}
 
+	if required.RequiredStatusChecks == nil {
+		return true, nil
+	}
+
 	//check check suite/check run api
 	checksuites, _, err := g.client.Checks.ListCheckSuitesForRef(context.Background(), *pull.Head.Repo.Owner.Login, repo.Name, *pull.Head.Ref, nil)
 	if err != nil {

server/events/vcs/github_client_test.go

@@ -737,6 +737,114 @@ func TestGithubClient_PullIsMergeableWithAllowMergeableBypassApply(t *testing.T)
 	}
 }
 
+func TestGithubClient_PullIsMergeableWithAllowMergeableBypassApplyButWithNoBranchProtectionChecks(t *testing.T) {
+	vcsStatusName := "atlantis"
+	cases := []struct {
+		state          string
+		reviewDecision string
+		expMergeable   bool
+	}{
+		{
+			"blocked",
+			`"REVIEW_REQUIRED"`,
+			false,
+		},
+	}
+
+	// Use a real GitHub json response and edit the mergeable_state field.
+	jsBytes, err := os.ReadFile("testdata/github-pull-request.json")
+	Ok(t, err)
+	prJSON := string(jsBytes)
+
+	// Status Check Response
+	jsBytes, err = os.ReadFile("testdata/github-commit-status-full.json")
+	Ok(t, err)
+	commitJSON := string(jsBytes)
+
+	// Branch protection Response
+	jsBytes, err = os.ReadFile("testdata/github-branch-protection-no-required-checks.json")
+	Ok(t, err)
+	branchProtectionJSON := string(jsBytes)
+
+	// List check suites Response
+	jsBytes, err = os.ReadFile("testdata/github-commit-check-suites-completed.json")
+	Ok(t, err)
+	checkSuites := string(jsBytes)
+
+	// List check runs in a check suite
+	jsBytes, err = os.ReadFile("testdata/github-commit-check-suites-check-runs-completed.json")
+	Ok(t, err)
+	checkRuns := string(jsBytes)
+
+	for _, c := range cases {
+		t.Run(c.state, func(t *testing.T) {
+			response := strings.Replace(prJSON,
+				`"mergeable_state": "clean"`,
+				fmt.Sprintf(`"mergeable_state": "%s"`, c.state),
+				1,
+			)
+
+			// reviewDecision Response
+			reviewDecision := fmt.Sprintf(`{
+				"data": {
+					"repository": {
+						"pullRequest": {
+							"reviewDecision": %s
+						}
+					}
+				}
+			}`, c.reviewDecision)
+
+			testServer := httptest.NewTLSServer(
+				http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+					switch r.RequestURI {
+					case "/api/v3/repos/octocat/Hello-World/pulls/1":
+						w.Write([]byte(response)) // nolint: errcheck
+						return
+					case "/api/v3/repos/octocat/Hello-World/pulls/1/reviews?per_page=300":
+						w.Write([]byte("[]")) // nolint: errcheck
+						return
+					case "/api/v3/repos/octocat/Hello-World/commits/new-topic/status":
+						w.Write([]byte(commitJSON)) // nolint: errcheck
+					case "/api/graphql":
+						w.Write([]byte(reviewDecision)) // nolint: errcheck
+					case "/api/v3/repos/octocat/Hello-World/branches/main/protection":
+						w.Write([]byte(branchProtectionJSON)) // nolint: errcheck
+					case "/api/v3/repos/octocat/Hello-World/commits/new-topic/check-suites":
+						w.Write([]byte(checkSuites)) // nolint: errcheck
+					case "/api/v3/repos/octocat/Hello-World/check-suites/1234567890/check-runs":
+						w.Write([]byte(checkRuns)) // nolint: errcheck
+					default:
+						t.Errorf("got unexpected request at %q", r.RequestURI)
+						http.Error(w, "not found", http.StatusNotFound)
+						return
+					}
+				}))
+			testServerURL, err := url.Parse(testServer.URL)
+			Ok(t, err)
+			client, err := vcs.NewGithubClient(testServerURL.Host, &vcs.GithubUserCredentials{"user", "pass"}, vcs.GithubConfig{AllowMergeableBypassApply: true}, logging.NewNoopLogger(t))
+			Ok(t, err)
+			defer disableSSLVerification()()
+
+			actMergeable, err := client.PullIsMergeable(models.Repo{
+				FullName:          "octocat/Hello-World",
+				Owner:             "octocat",
+				Name:              "Hello-World",
+				CloneURL:          "",
+				SanitizedCloneURL: "",
+				VCSHost: models.VCSHost{
+					Type:     models.Github,
+					Hostname: "github.com",
+				},
+			}, models.PullRequest{
+				Num: 1,
+			}, vcsStatusName)
+			Ok(t, err)
+			Equals(t, c.expMergeable, actMergeable)
+		})
+	}
+}
+
 func TestGithubClient_MergePullHandlesError(t *testing.T) {
 	cases := []struct {
 		code    int

server/events/vcs/testdata/github-branch-protection-no-required-checks.json

@@ -0,0 +1,10 @@
+{
+    "url": "https://api.github.com/repos/octocat/repo/branches/master/protection",
+    "required_pull_request_reviews": {
+        "url": "https://api.github.com/repos/octocat/repo/branches/master/protection/required_pull_request_reviews",
+        "dismiss_stale_reviews": false,
+        "require_code_owner_reviews": false,
+        "require_last_push_approval": false,
+        "required_approving_review_count": 1
+    }
+}

server/events/vcs/testdata/github-commit-check-suites-check-runs-completed.json

@@ -0,0 +1,95 @@
+{
+    "total_count": 1,
+    "check_runs": [
+        {
+            "id": 4,
+            "head_sha": "ce587453ced02b1526dfb4cb910479d431683101",
+            "node_id": "MDg6Q2hlY2tSdW40",
+            "external_id": "",
+            "url": "https://api.github.com/repos/github/hello-world/check-runs/4",
+            "html_url": "https://github.com/github/hello-world/runs/4",
+            "details_url": "https://example.com",
+            "status": "completed",
+            "conclusion": "success",
+            "started_at": "2018-05-04T01:14:52Z",
+            "completed_at": "2018-05-04T01:14:52Z",
+            "output": {
+                "title": "Mighty Readme report",
+                "summary": "There are 0 failures, 2 warnings, and 1 notice.",
+                "text": "You may have some misspelled words on lines 2 and 4. You also may want to add a section in your README about how to install your app.",
+                "annotations_count": 2,
+                "annotations_url": "https://api.github.com/repos/github/hello-world/check-runs/4/annotations"
+            },
+            "name": "mighty_readme",
+            "check_suite": {
+                "id": 5
+            },
+            "app": {
+                "id": 1,
+                "slug": "octoapp",
+                "node_id": "MDExOkludGVncmF0aW9uMQ==",
+                "owner": {
+                    "login": "github",
+                    "id": 1,
+                    "node_id": "MDEyOk9yZ2FuaXphdGlvbjE=",
+                    "url": "https://api.github.com/orgs/github",
+                    "repos_url": "https://api.github.com/orgs/github/repos",
+                    "events_url": "https://api.github.com/orgs/github/events",
+                    "avatar_url": "https://github.com/images/error/octocat_happy.gif",
+                    "gravatar_id": "",
+                    "html_url": "https://github.com/octocat",
+                    "followers_url": "https://api.github.com/users/octocat/followers",
+                    "following_url": "https://api.github.com/users/octocat/following{/other_user}",
+                    "gists_url": "https://api.github.com/users/octocat/gists{/gist_id}",
+                    "starred_url": "https://api.github.com/users/octocat/starred{/owner}{/repo}",
+                    "subscriptions_url": "https://api.github.com/users/octocat/subscriptions",
+                    "organizations_url": "https://api.github.com/users/octocat/orgs",
+                    "received_events_url": "https://api.github.com/users/octocat/received_events",
+                    "type": "User",
+                    "site_admin": true
+                },
+                "name": "Octocat App",
+                "description": "",
+                "external_url": "https://example.com",
+                "html_url": "https://github.com/apps/octoapp",
+                "created_at": "2017-07-08T16:18:44-04:00",
+                "updated_at": "2017-07-08T16:18:44-04:00",
+                "permissions": {
+                    "metadata": "read",
+                    "contents": "read",
+                    "issues": "write",
+                    "single_file": "write"
+                },
+                "events": [
+                    "push",
+                    "pull_request"
+                ]
+            },
+            "pull_requests": [
+                {
+                    "url": "https://api.github.com/repos/github/hello-world/pulls/1",
+                    "id": 1934,
+                    "number": 3956,
+                    "head": {
+                        "ref": "say-hello",
+                        "sha": "3dca65fa3e8d4b3da3f3d056c59aee1c50f41390",
+                        "repo": {
+                            "id": 526,
+                            "url": "https://api.github.com/repos/github/hello-world",
+                            "name": "hello-world"
+                        }
+                    },
+                    "base": {
+                        "ref": "master",
+                        "sha": "e7fdf7640066d71ad16a86fbcbb9c6a10a18af4f",
+                        "repo": {
+                            "id": 526,
+                            "url": "https://api.github.com/repos/github/hello-world",
+                            "name": "hello-world"
+                        }
+                    }
+                }
+            ]
+        }
+    ]
+}