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, 28 insertions, 1 deletions

diff --git a/doc/user/project/import/github.md b/doc/user/project/import/github.md
index 6423beefc77..72def9d1d1d 100644
--- a/doc/user/project/import/github.md
+++ b/doc/user/project/import/github.md
@@ -24,6 +24,8 @@ constrains of a Sidekiq worker.
   - the milestones (GitLab 8.7+)
   - the labels (GitLab 8.7+)
   - the release note descriptions (GitLab 8.12+)
+  - the pull request review comments (GitLab 10.2+)
+  - the regular issue and pull request comments
 - References to pull requests and issues are preserved (GitLab 8.7+)
 - Repository public access is retained. If a repository is private in GitHub
   it will be created as private in GitLab as well.
@@ -43,10 +45,13 @@ the case the namespace is taken, the repository will be imported under the user'
 namespace that started the import process.
 
 The importer will also import branches on forks of projects related to open pull
-requests. These branches will be imported with a naming scheume similar to 
+requests. These branches will be imported with a naming scheme similar to
 GH-SHA-Username/Pull-Request-number/fork-name/branch. This may lead to a discrepency
 in branches compared to the GitHub Repository.
 
+For a more technical description and an overview of the architecture you can
+refer to [Working with the GitHub importer][gh-import-dev-docs].
+
 ## Importing your GitHub repositories
 
 The importer page is visible when you create a new project.
@@ -121,7 +126,29 @@ If you want, you can import all your GitHub projects in one go by hitting
 You can also choose a different name for the project and a different namespace,
 if you have the privileges to do so.
 
+## Making the import process go faster
+
+For large projects it may take a while to import all data. To reduce the time
+necessary you can increase the number of Sidekiq workers that process the
+following queues:
+
+* `github_importer`
+* `github_importer_advance_stage`
+
+For an optimal experience we recommend having at least 4 Sidekiq processes (each
+running a number of threads equal to the number of CPU cores) that _only_
+process these queues. We also recommend that these processes run on separate
+servers. For 4 servers with 8 cores this means you can import up to 32 objects
+(e.g. issues) in parallel.
+
+Reducing the time spent in cloning a repository can be done by increasing
+network throughput, CPU capacity, and disk performance (e.g.  by using high
+performance SSDs) of the disks that store the Git repositories (for your GitLab
+instance). Increasing the number of Sidekiq workers will _not_ reduce the time
+spent cloning repositories.
+
 [gh-import]: ../../../integration/github.md "GitHub integration"
 [gh-rake]: ../../../administration/raketasks/github_import.md "GitHub rake task"
 [gh-integration]: #authorize-access-to-your-repositories-using-the-github-integration
 [gh-token]: #authorize-access-to-your-repositories-using-a-personal-access-token
+[gh-import-dev-docs]: ../../../development/github_importer.md "Working with the GitHub importer"