Perform pull request IO work outside a transaction

When importing a GitHub pull request we would perform all work in a
single database transaction. This is less than ideal, because we perform
various slow Git operations when creating a merge request. This in turn
can lead to many DB connections being used, while just waiting for an IO
operation to complete.

To work around this, we now move most of the heavy lifting out of the
database transaction. Some extra error handling is added to ensure we
can resume importing a partially imported pull request, instead of just
throwing an error.

This commit also changes the specs for IssueImporter so they don't rely
on deprecated RSpec methods.

changelogs/unreleased/gh-importer-transactions.yml

+---
+title: Move PR IO operations out of a transaction
+merge_request:
+author:
+type: performance

lib/gitlab/github_import/importer/pull_request_importer.rb

@@ -22,15 +22,22 @@ module Gitlab
         end
 
         def execute
-          if (mr_id = create_merge_request)
-            issuable_finder.cache_database_id(mr_id)
+          mr, already_exists = create_merge_request
+
+          if mr
+            insert_git_data(mr, already_exists)
+            issuable_finder.cache_database_id(mr.id)
           end
         end
 
         # Creates the merge request and returns its ID.
         #
         # This method will return `nil` if the merge request could not be
-        # created.
+        # created, otherwise it will return an Array containing the following
+        # values:
+        #
+        # 1. A MergeRequest instance.
+        # 2. A boolean indicating if the MR already exists.
         def create_merge_request
           author_id, author_found = user_finder.author_id_for(pull_request)
 
@@ -69,21 +76,42 @@ module Gitlab
             merge_request_id = GithubImport
               .insert_and_return_id(attributes, project.merge_requests)
 
-            merge_request = project.merge_requests.find(merge_request_id)
-
-            # These fields are set so we can create the correct merge request
-            # diffs.
-            merge_request.source_branch_sha = pull_request.source_branch_sha
-            merge_request.target_branch_sha = pull_request.target_branch_sha
-
-            merge_request.keep_around_commit
-            merge_request.merge_request_diffs.create
-
-            merge_request.id
+            [project.merge_requests.find(merge_request_id), false]
           end
         rescue ActiveRecord::InvalidForeignKey
           # It's possible the project has been deleted since scheduling this
           # job. In this case we'll just skip creating the merge request.
+          []
+        rescue ActiveRecord::RecordNotUnique
+          # It's possible we previously created the MR, but failed when updating
+          # the Git data. In this case we'll just continue working on the
+          # existing row.
+          [project.merge_requests.find_by(iid: pull_request.iid), true]
+        end
+
+        def insert_git_data(merge_request, already_exists = false)
+          # These fields are set so we can create the correct merge request
+          # diffs.
+          merge_request.source_branch_sha = pull_request.source_branch_sha
+          merge_request.target_branch_sha = pull_request.target_branch_sha
+
+          merge_request.keep_around_commit
+
+          # MR diffs normally use an "after_save" hook to pull data from Git.
+          # All of this happens in the transaction started by calling
+          # create/save/etc. This in turn can lead to these transactions being
+          # held open for much longer than necessary. To work around this we
+          # first save the diff, then populate it.
+          diff =
+            if already_exists
+              merge_request.merge_request_diffs.take
+            else
+              merge_request.merge_request_diffs.build
+            end
+
+          diff.importing = true
+          diff.save
+          diff.save_git_content
         end
       end
     end

spec/lib/gitlab/github_import/importer/issue_importer_spec.rb

@@ -180,12 +180,12 @@ describe Gitlab::GithubImport::Importer::IssueImporter, :clean_gitlab_redis_cach
 
       allow(importer.user_finder)
         .to receive(:user_id_for)
-        .ordered.with(issue.assignees[0])
+        .with(issue.assignees[0])
         .and_return(4)
 
       allow(importer.user_finder)
         .to receive(:user_id_for)
-        .ordered.with(issue.assignees[1])
+        .with(issue.assignees[1])
         .and_return(5)
 
       expect(Gitlab::Database)

spec/lib/gitlab/github_import/importer/pull_request_importer_spec.rb

@@ -40,13 +40,19 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
 
   describe '#execute' do
     it 'imports the pull request' do
+      mr = double(:merge_request, id: 10)
+
       expect(importer)
         .to receive(:create_merge_request)
-        .and_return(10)
+        .and_return([mr, false])
+
+      expect(importer)
+        .to receive(:insert_git_data)
+        .with(mr, false)
 
       expect_any_instance_of(Gitlab::GithubImport::IssuableFinder)
         .to receive(:cache_database_id)
-        .with(10)
+        .with(mr.id)
 
       importer.execute
     end
@@ -99,18 +105,11 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
         importer.create_merge_request
       end
 
-      it 'returns the ID of the created merge request' do
-        id = importer.create_merge_request
-
-        expect(id).to be_a_kind_of(Numeric)
-      end
-
-      it 'creates the merge request diffs' do
-        importer.create_merge_request
-
-        mr = project.merge_requests.take
+      it 'returns the created merge request' do
+        mr, exists = importer.create_merge_request
 
-        expect(mr.merge_request_diffs.exists?).to eq(true)
+        expect(mr).to be_instance_of(MergeRequest)
+        expect(exists).to eq(false)
       end
     end
 
@@ -217,5 +216,65 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
         expect { importer.create_merge_request }.not_to raise_error
       end
     end
+
+    context 'when the merge request already exists' do
+      before do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .with(pull_request)
+          .and_return([user.id, true])
+
+        allow(importer.user_finder)
+          .to receive(:assignee_id_for)
+          .with(pull_request)
+          .and_return(user.id)
+      end
+
+      it 'returns the existing merge request' do
+        mr1, exists1 = importer.create_merge_request
+        mr2, exists2 = importer.create_merge_request
+
+        expect(mr2).to eq(mr1)
+        expect(exists1).to eq(false)
+        expect(exists2).to eq(true)
+      end
+    end
+  end
+
+  describe '#insert_git_data' do
+    before do
+      allow(importer.milestone_finder)
+        .to receive(:id_for)
+        .with(pull_request)
+        .and_return(milestone.id)
+
+      allow(importer.user_finder)
+        .to receive(:author_id_for)
+        .with(pull_request)
+        .and_return([user.id, true])
+
+      allow(importer.user_finder)
+        .to receive(:assignee_id_for)
+        .with(pull_request)
+        .and_return(user.id)
+    end
+
+    it 'creates the merge request diffs' do
+      mr, exists = importer.create_merge_request
+
+      importer.insert_git_data(mr, exists)
+
+      expect(mr.merge_request_diffs.exists?).to eq(true)
+    end
+
+    it 'creates the merge request diff commits' do
+      mr, exists = importer.create_merge_request
+
+      importer.insert_git_data(mr, exists)
+
+      diff = mr.merge_request_diffs.take
+
+      expect(diff.merge_request_diff_commits.exists?).to eq(true)
+    end
   end
 end