check PR changed files count directly from get pull request endpoint

pull/github.go

@@ -70,6 +70,7 @@ func (loc Locator) IsComplete() bool {
 	case loc.Value.GetHead().GetRepo().GetID() == 0:
 	case loc.Value.GetHead().GetRepo().GetName() == "":
 	case loc.Value.GetHead().GetRepo().GetOwner().GetLogin() == "":
+	case loc.Value.GetChangedFiles() == 0:
 	default:
 		return true
 	}
@@ -108,6 +109,7 @@ func (loc Locator) toV4(ctx context.Context, client *githubv4.Client) (*v4PullRe
 	v4.BaseRefName = loc.Value.GetBase().GetRef()
 	v4.BaseRepository.DatabaseID = loc.Value.GetBase().GetRepo().GetID()
 	v4.IsDraft = loc.Value.GetDraft()
+	v4.ChangedFiles = loc.Value.GetChangedFiles()
 	return &v4, nil
 }
 
@@ -270,6 +272,11 @@ func (ghc *GitHubContext) Branches() (base string, head string) {
 }
 
 func (ghc *GitHubContext) ChangedFiles() ([]*File, error) {
+	// Check if changed files exceeds the limit
+	if ghc.pr.ChangedFiles > MaxPullRequestFiles {
+		return nil, errors.Errorf("number of changed files (%d) exceeds limit (%d)", ghc.pr.ChangedFiles, MaxPullRequestFiles)
+	}
+
 	if ghc.files == nil {
 		opt := github.ListOptions{
 			PerPage: 100,
@@ -318,9 +325,7 @@ func (ghc *GitHubContext) ChangedFiles() ([]*File, error) {
 			})
 		}
 	}
-	if len(ghc.files) >= MaxPullRequestFiles {
-		return nil, errors.Errorf("too many files in pull request, maximum is %d", MaxPullRequestFiles)
-	}
+
 	return ghc.files, nil
 }
 
@@ -1109,6 +1114,8 @@ type v4PullRequest struct {
 	BaseRepository struct {
 		DatabaseID int64
 	}
+
+	ChangedFiles int
 }
 
 type v4PageInfo struct {

pull/github_test.go

@@ -71,6 +71,27 @@ func TestChangedFiles(t *testing.T) {
 	assert.Equal(t, 2, filesRule.Count, "cached files were not used")
 }
 
+func TestChangedFilesExceedsLimit(t *testing.T) {
+	rp := &ResponsePlayer{}
+	rp.AddRule(
+		ExactPathMatcher("/repos/testorg/testrepo/pulls/123/files"),
+		"testdata/responses/pull_files.yml",
+	)
+	rp.AddRule(
+		GraphQLNodePrefixMatcher("repository.pullRequest.changedFiles"),
+		"testdata/responses/pull_changed_files_exceeds_limit.yml",
+	)
+
+	pr := defaultTestPR()
+	*pr.ChangedFiles = 3001
+
+	ctx := makeContext(t, rp, pr, nil)
+
+	_, err := ctx.ChangedFiles()
+	assert.Equal(t, 3001, pr.GetChangedFiles())
+	assert.Contains(t, err.Error(), "number of changed files (3001) exceeds limit (3000)")
+}
+
 func TestChangedFilesNoFiles(t *testing.T) {
 	rp := &ResponsePlayer{}
 	filesRule := rp.AddRule(
@@ -723,6 +744,7 @@ func defaultTestPR() *github.PullRequest {
 				Name: github.String("testrepo"),
 			},
 		},
+		ChangedFiles: github.Int(1),
 	}
 }
 

pull/testdata/responses/pull_changed_files_exceeds_limit.yml

@@ -0,0 +1,18 @@
+- status: 200
+  body: |
+    {
+      "errors": [],
+      "data": {
+        "repository": {
+          "pullRequest": {
+            "body": "/no-platform",
+            "createdAt": "2018-01-12T00:11:50Z",
+            "lastEditedAt": "2018-01-12T00:11:50Z",
+            "author": {
+              "login": "agirlnamedsophia"
+            },
+            "changedFiles": 3001
+          }
+        }
+      }
+    }