Make MR diff background migration less likely to time out

This version does not use transactions, but individual statements. As we have
unique constraints on the target tables for the inserts, we can just ignore
uniqueness violations there (as long as we always insert the same batch size, in
the same order).

This means the spec now must use truncation, not a transaction, as the
uniqueness violation means that the whole transaction for that spec would be
invalid, which isn't what we'd want. In real-world use, this isn't run in a
transaction anyway.

This commit also wraps unhandled exceptions, for easier finding in Sentry, and
logs with a consistent format, for easier searching.

lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits.rb

@@ -3,6 +3,12 @@ module Gitlab
     class DeserializeMergeRequestDiffsAndCommits
       attr_reader :diff_ids, :commit_rows, :file_rows
 
+      class Error < StandardError
+        def backtrace
+          cause.backtrace
+        end
+      end
+
       class MergeRequestDiff < ActiveRecord::Base
         self.table_name = 'merge_request_diffs'
       end
@@ -34,6 +40,10 @@ module Gitlab
         end
 
         flush_buffers!
+      rescue => e
+        Rails.logger.info("#{self.class.name}: failed for IDs #{merge_request_diffs.map(&:id)} with #{e.class.name}")
+
+        raise Error.new(e.inspect)
       end
 
       private
@@ -46,22 +56,28 @@ module Gitlab
 
       def flush_buffers!
         if diff_ids.any?
-          MergeRequestDiff.transaction do
           commit_rows.each_slice(BUFFER_ROWS).each do |commit_rows_slice|
-              Gitlab::Database.bulk_insert('merge_request_diff_commits', commit_rows_slice)
+            bulk_insert('merge_request_diff_commits', commit_rows_slice)
           end
 
           file_rows.each_slice(DIFF_FILE_BUFFER_ROWS).each do |file_rows_slice|
-              Gitlab::Database.bulk_insert('merge_request_diff_files', file_rows_slice)
+            bulk_insert('merge_request_diff_files', file_rows_slice)
           end
 
           MergeRequestDiff.where(id: diff_ids).update_all(st_commits: nil, st_diffs: nil)
         end
-        end
 
         reset_buffers!
       end
 
+      def bulk_insert(table, rows)
+        Gitlab::Database.bulk_insert(table, rows)
+      rescue ActiveRecord::RecordNotUnique
+        ids = rows.map { |row| row[:merge_request_diff_id] }.uniq.sort
+
+        Rails.logger.info("#{self.class.name}: rows inserted twice for IDs #{ids}")
+      end
+
       def single_diff_rows(merge_request_diff)
         sha_attribute = Gitlab::Database::ShaAttribute.new
         commits = YAML.load(merge_request_diff.st_commits) rescue []

spec/lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits_spec.rb

 require 'spec_helper'
 
-describe Gitlab::BackgroundMigration::DeserializeMergeRequestDiffsAndCommits do
+describe Gitlab::BackgroundMigration::DeserializeMergeRequestDiffsAndCommits, :truncate do
   describe '#perform' do
-    set(:merge_request) { create(:merge_request) }
-    set(:merge_request_diff) { merge_request.merge_request_diff }
+    let(:merge_request) { create(:merge_request) }
+    let(:merge_request_diff) { merge_request.merge_request_diff }
     let(:updated_merge_request_diff) { MergeRequestDiff.find(merge_request_diff.id) }
 
     def diffs_to_hashes(diffs)
@@ -84,12 +84,6 @@ describe Gitlab::BackgroundMigration::DeserializeMergeRequestDiffsAndCommits do
           allow(Gitlab::Database).to receive(:bulk_insert).and_call_original
         end
 
-        it 'updates and continues' do
-          expect(described_class::MergeRequestDiff).to receive(:transaction).twice
-
-          subject.perform(start_id, stop_id)
-        end
-
         it 'inserts commit rows in chunks of BUFFER_ROWS' do
           # There are 29 commits in each diff, so we should have slices of 20 + 9 + 20 + 9.
           stub_const("#{described_class}::BUFFER_ROWS", 20)
@@ -119,27 +113,87 @@ describe Gitlab::BackgroundMigration::DeserializeMergeRequestDiffsAndCommits do
 
       context 'when BUFFER_ROWS is not exceeded' do
         it 'only updates once' do
-          expect(described_class::MergeRequestDiff).to receive(:transaction).once
+          expect(Gitlab::Database).to receive(:bulk_insert)
+                                        .with('merge_request_diff_commits', anything)
+                                        .once
+                                        .and_call_original
+
+          expect(Gitlab::Database).to receive(:bulk_insert)
+                                        .with('merge_request_diff_files', anything)
+                                        .once
+                                        .and_call_original
+
+          subject.perform(start_id, stop_id)
+        end
+      end
+
+      context 'when some rows were already inserted due to a previous failure' do
+        before do
+          subject.perform(start_id, stop_id)
+
+          convert_to_yaml(start_id, merge_request_diff.commits, diffs_to_hashes(merge_request_diff.merge_request_diff_files))
+          convert_to_yaml(stop_id, updated_merge_request_diff.commits, diffs_to_hashes(updated_merge_request_diff.merge_request_diff_files))
+        end
+
+        it 'does not raise' do
+          expect { subject.perform(start_id, stop_id) }.not_to raise_exception
+        end
+
+        it 'logs a message' do
+          expect(Rails.logger).to receive(:info)
+                                    .with(
+                                      a_string_matching(described_class.name).and(matching([start_id, stop_id].inspect))
+                                    )
+                                    .twice
 
           subject.perform(start_id, stop_id)
         end
+
+        it 'ends up with the correct rows' do
+          expect(updated_merge_request_diff.commits.count).to eq(29)
+          expect(updated_merge_request_diff.raw_diffs.count).to eq(20)
         end
       end
 
       context 'when the merge request diff update fails' do
+        let(:exception) { ActiveRecord::RecordNotFound }
+
+        let(:perform_ignoring_exceptions) do
+          begin
+            subject.perform(start_id, stop_id)
+          rescue described_class::Error
+          end
+        end
+
         before do
-        allow(described_class::MergeRequestDiff)
-          .to receive(:update_all).and_raise(ActiveRecord::Rollback)
+          allow_any_instance_of(described_class::MergeRequestDiff::ActiveRecord_Relation)
+            .to receive(:update_all).and_raise(exception)
         end
 
-      it 'does not add any diff commits' do
-        expect { subject.perform(merge_request_diff.id, merge_request_diff.id) }
-          .not_to change { MergeRequestDiffCommit.count }
+        it 'raises an error' do
+          expect { subject.perform(start_id, stop_id) }
+            .to raise_exception(described_class::Error)
         end
 
-      it 'does not add any diff files' do
-        expect { subject.perform(merge_request_diff.id, merge_request_diff.id) }
-          .not_to change { MergeRequestDiffFile.count }
+        it 'logs the error' do
+          expect(Rails.logger).to receive(:info).with(
+            a_string_matching(described_class.name)
+              .and(matching([start_id, stop_id].inspect))
+              .and(matching(exception.name))
+          )
+
+          perform_ignoring_exceptions
+        end
+
+        it 'still adds diff commits' do
+          expect { perform_ignoring_exceptions }
+            .to change { MergeRequestDiffCommit.count }
+        end
+
+        it 'still adds diff files' do
+          expect { perform_ignoring_exceptions }
+            .to change { MergeRequestDiffFile.count }
+        end
       end
     end