Rewrite the GitHub importer from scratch

Prior to this MR there were two GitHub related importers:

* Github::Import: the main importer used for GitHub projects
* Gitlab::GithubImport: importer that's somewhat confusingly used for
  importing Gitea projects (apparently they have a compatible API)

This MR renames the Gitea importer to Gitlab::LegacyGithubImport and
introduces a new GitHub importer in the Gitlab::GithubImport namespace.
This new GitHub importer uses Sidekiq for importing multiple resources
in parallel, though it also has the ability to import data sequentially
should this be necessary.

The new code is spread across the following directories:

* lib/gitlab/github_import: this directory contains most of the importer
  code such as the classes used for importing resources.
* app/workers/gitlab/github_import: this directory contains the Sidekiq
  workers, most of which simply use the code from the directory above.
* app/workers/concerns/gitlab/github_import: this directory provides a
  few modules that are included in every GitHub importer worker.

== Stages

The import work is divided into separate stages, with each stage
importing a specific set of data. Stages will schedule the work that
needs to be performed, followed by scheduling a job for the
"AdvanceStageWorker" worker. This worker will periodically check if all
work is completed and schedule the next stage if this is the case. If
work is not yet completed this worker will reschedule itself.

Using this approach we don't have to block threads by calling `sleep()`,
as doing so for large projects could block the thread from doing any
work for many hours.

== Retrying Work

Workers will reschedule themselves whenever necessary. For example,
hitting the GitHub API's rate limit will result in jobs rescheduling
themselves. These jobs are not processed until the rate limit has been
reset.

== User Lookups

Part of the importing process involves looking up user details in the
GitHub API so we can map them to GitLab users. The old importer used
an in-memory cache, but this obviously doesn't work when the work is
spread across different threads.

The new importer uses a Redis cache and makes sure we only perform
API/database calls if absolutely necessary.  Frequently used keys are
refreshed, and lookup misses are also cached; removing the need for
performing API/database calls if we know we don't have the data we're
looking for.

== Performance & Models

The new importer in various places uses raw INSERT statements (as
generated by `Gitlab::Database.bulk_insert`) instead of using Rails
models. This allows us to bypass any validations and callbacks,
drastically reducing the number of SQL queries and Gitaly RPC calls
necessary to import projects.

To ensure the code produces valid data the corresponding tests check if
the produced rows are valid according to the model validation rules.

1 files changed, 296 insertions, 0 deletions

diff --git a/spec/lib/gitlab/github_import/parallel_scheduling_spec.rb b/spec/lib/gitlab/github_import/parallel_scheduling_spec.rb
new file mode 100644
index 00000000000..98205d3ee25
--- /dev/null
+++ b/spec/lib/gitlab/github_import/parallel_scheduling_spec.rb
@@ -0,0 +1,296 @@
+require 'spec_helper'
+
+describe Gitlab::GithubImport::ParallelScheduling do
+  let(:importer_class) do
+    Class.new do
+      include(Gitlab::GithubImport::ParallelScheduling)
+
+      def collection_method
+        :issues
+      end
+    end
+  end
+
+  let(:project) { double(:project, id: 4, import_source: 'foo/bar') }
+  let(:client) { double(:client) }
+
+  describe '#parallel?' do
+    it 'returns true when running in parallel mode' do
+      expect(importer_class.new(project, client)).to be_parallel
+    end
+
+    it 'returns false when running in sequential mode' do
+      importer = importer_class.new(project, client, parallel: false)
+
+      expect(importer).not_to be_parallel
+    end
+  end
+
+  describe '#execute' do
+    it 'imports data in parallel when running in parallel mode' do
+      importer = importer_class.new(project, client)
+      waiter = double(:waiter)
+
+      expect(importer)
+        .to receive(:parallel_import)
+        .and_return(waiter)
+
+      expect(importer.execute)
+        .to eq(waiter)
+    end
+
+    it 'imports data in parallel when running in sequential mode' do
+      importer = importer_class.new(project, client, parallel: false)
+
+      expect(importer)
+        .to receive(:sequential_import)
+        .and_return([])
+
+      expect(importer.execute)
+        .to eq([])
+    end
+
+    it 'expires the cache used for tracking already imported objects' do
+      importer = importer_class.new(project, client)
+
+      expect(importer).to receive(:parallel_import)
+
+      expect(Gitlab::GithubImport::Caching)
+        .to receive(:expire)
+        .with(importer.already_imported_cache_key, a_kind_of(Numeric))
+
+      importer.execute
+    end
+  end
+
+  describe '#sequential_import' do
+    let(:importer) { importer_class.new(project, client, parallel: false) }
+
+    it 'imports data in sequence' do
+      repr_class = double(:representation_class)
+      repr_instance = double(:representation_instance)
+      gh_importer = double(:github_importer)
+      gh_importer_instance = double(:github_importer_instance)
+      object = double(:object)
+
+      expect(importer)
+        .to receive(:each_object_to_import)
+        .and_yield(object)
+
+      expect(importer)
+        .to receive(:representation_class)
+        .and_return(repr_class)
+
+      expect(repr_class)
+        .to receive(:from_api_response)
+        .with(object)
+        .and_return(repr_instance)
+
+      expect(importer)
+        .to receive(:importer_class)
+        .and_return(gh_importer)
+
+      expect(gh_importer)
+        .to receive(:new)
+        .with(repr_instance, project, client)
+        .and_return(gh_importer_instance)
+
+      expect(gh_importer_instance)
+        .to receive(:execute)
+
+      importer.sequential_import
+    end
+  end
+
+  describe '#parallel_import' do
+    let(:importer) { importer_class.new(project, client) }
+
+    it 'imports data in parallel' do
+      repr_class = double(:representation)
+      worker_class = double(:worker)
+      object = double(:object)
+
+      expect(importer)
+        .to receive(:each_object_to_import)
+        .and_yield(object)
+
+      expect(importer)
+        .to receive(:representation_class)
+        .and_return(repr_class)
+
+      expect(importer)
+        .to receive(:sidekiq_worker_class)
+        .and_return(worker_class)
+
+      expect(repr_class)
+        .to receive(:from_api_response)
+        .with(object)
+        .and_return({ title: 'Foo' })
+
+      expect(worker_class)
+        .to receive(:perform_async)
+        .with(project.id, { title: 'Foo' }, an_instance_of(String))
+
+      expect(importer.parallel_import)
+        .to be_an_instance_of(Gitlab::JobWaiter)
+    end
+  end
+
+  describe '#each_object_to_import' do
+    let(:importer) { importer_class.new(project, client) }
+    let(:object) { double(:object) }
+
+    before do
+      expect(importer)
+        .to receive(:collection_options)
+        .and_return({ state: 'all' })
+    end
+
+    it 'yields every object to import' do
+      page = double(:page, objects: [object], number: 1)
+
+      expect(client)
+        .to receive(:each_page)
+        .with(:issues, 'foo/bar', { state: 'all', page: 1 })
+        .and_yield(page)
+
+      expect(importer.page_counter)
+        .to receive(:set)
+        .with(1)
+        .and_return(true)
+
+      expect(importer)
+        .to receive(:already_imported?)
+        .with(object)
+        .and_return(false)
+
+      expect(importer)
+        .to receive(:mark_as_imported)
+        .with(object)
+
+      expect { |b| importer.each_object_to_import(&b) }
+        .to yield_with_args(object)
+    end
+
+    it 'resumes from the last page' do
+      page = double(:page, objects: [object], number: 2)
+
+      expect(importer.page_counter)
+        .to receive(:current)
+        .and_return(2)
+
+      expect(client)
+        .to receive(:each_page)
+        .with(:issues, 'foo/bar', { state: 'all', page: 2 })
+        .and_yield(page)
+
+      expect(importer.page_counter)
+        .to receive(:set)
+        .with(2)
+        .and_return(true)
+
+      expect(importer)
+        .to receive(:already_imported?)
+        .with(object)
+        .and_return(false)
+
+      expect(importer)
+        .to receive(:mark_as_imported)
+        .with(object)
+
+      expect { |b| importer.each_object_to_import(&b) }
+        .to yield_with_args(object)
+    end
+
+    it 'does not yield any objects if the page number was not set' do
+      page = double(:page, objects: [object], number: 1)
+
+      expect(client)
+        .to receive(:each_page)
+        .with(:issues, 'foo/bar', { state: 'all', page: 1 })
+        .and_yield(page)
+
+      expect(importer.page_counter)
+        .to receive(:set)
+        .with(1)
+        .and_return(false)
+
+      expect { |b| importer.each_object_to_import(&b) }
+        .not_to yield_control
+    end
+
+    it 'does not yield the object if it was already imported' do
+      page = double(:page, objects: [object], number: 1)
+
+      expect(client)
+        .to receive(:each_page)
+        .with(:issues, 'foo/bar', { state: 'all', page: 1 })
+        .and_yield(page)
+
+      expect(importer.page_counter)
+        .to receive(:set)
+        .with(1)
+        .and_return(true)
+
+      expect(importer)
+        .to receive(:already_imported?)
+        .with(object)
+        .and_return(true)
+
+      expect(importer)
+        .not_to receive(:mark_as_imported)
+
+      expect { |b| importer.each_object_to_import(&b) }
+        .not_to yield_control
+    end
+  end
+
+  describe '#already_imported?', :clean_gitlab_redis_cache do
+    let(:importer) { importer_class.new(project, client) }
+
+    it 'returns false when an object has not yet been imported' do
+      object = double(:object, id: 10)
+
+      expect(importer)
+        .to receive(:id_for_already_imported_cache)
+        .with(object)
+        .and_return(object.id)
+
+      expect(importer.already_imported?(object))
+        .to eq(false)
+    end
+
+    it 'returns true when an object has already been imported' do
+      object = double(:object, id: 10)
+
+      allow(importer)
+        .to receive(:id_for_already_imported_cache)
+        .with(object)
+        .and_return(object.id)
+
+      importer.mark_as_imported(object)
+
+      expect(importer.already_imported?(object))
+        .to eq(true)
+    end
+  end
+
+  describe '#mark_as_imported', :clean_gitlab_redis_cache do
+    it 'marks an object as already imported' do
+      object = double(:object, id: 10)
+      importer = importer_class.new(project, client)
+
+      expect(importer)
+        .to receive(:id_for_already_imported_cache)
+        .with(object)
+        .and_return(object.id)
+
+      expect(Gitlab::GithubImport::Caching)
+        .to receive(:set_add)
+        .with(importer.already_imported_cache_key, object.id)
+        .and_call_original
+
+      importer.mark_as_imported(object)
+    end
+  end
+end