Merge pull request dependabot#4450 from dependabot/lsep/max-pr-length

Limit the GitHub PR description message to API max

common/lib/dependabot/pull_request_creator/github.rb

@@ -9,6 +9,8 @@ module Dependabot
   class PullRequestCreator
     # rubocop:disable Metrics/ClassLength
     class Github
+      MAX_PR_DESCRIPTION_LENGTH = 65_536 # characters (see #create_pull_request)
+
       attr_reader :source, :branch_name, :base_commit, :credentials,
                   :files, :pr_description, :pr_name, :commit_message,
                   :author_details, :signature_key, :custom_headers,
@@ -347,6 +349,18 @@ def add_milestone_to_pull_request(pull_request)
       end
 
       def create_pull_request
+        # Limit PR description to MAX_PR_DESCRIPTION_LENGTH (65,536) characters
+        # and truncate with message if over. The API limit is 262,144 bytes
+        # (https://github.community/t/maximum-length-for-the-comment-body-in-issues-and-pr/148867/2).
+        # As Ruby strings are UTF-8 encoded, this is a pessimistic limit: it
+        # presumes the case where all characters are 4 bytes.
+        pr_description = @pr_description.dup
+        if pr_description && pr_description.length > MAX_PR_DESCRIPTION_LENGTH
+          truncated_msg = "...\n\n_Description has been truncated_"
+          truncate_length = MAX_PR_DESCRIPTION_LENGTH - truncated_msg.length
+          pr_description = (pr_description[0, truncate_length] + truncated_msg)
+        end
+
         github_client_for_source.create_pull_request(
           source.repo,
           target_branch,

common/spec/dependabot/pull_request_creator/github_spec.rb

@@ -1088,6 +1088,25 @@
           )
         end
       end
+
+      context "the PR description is too long" do
+        let(:pr_description) { "a" * (described_class::MAX_PR_DESCRIPTION_LENGTH + 1) }
+
+        it "truncates the description" do
+          creator.create
+
+          expect(WebMock).
+            to have_requested(:post, "#{repo_api_url}/pulls").
+            with(
+              body: {
+                base: "master",
+                head: "dependabot/bundler/business-1.5.0",
+                title: "PR name",
+                body: ->(body) { expect(body.length).to be <= described_class::MAX_PR_DESCRIPTION_LENGTH }
+              }
+            )
+        end
+      end
     end
   end
 end