Note that `ghost IS NOT TRUE` is equivalent to `ghost = 'f' OR ghost is
null`:

(true), (false), (NULL)) AS t (flag);
 flag | ?column? | ?column?
------+----------+----------
 t    | f        | f
 f    | t        | t
      | t        | t
(3 rows)

This makes it a little easier to create partial indexes for this (as only
one condition is needed).

Closes #43304.

This optimises searching for users when using queries consisting out of
one or two characters such as "ab". We optimise such cases by searching
for `LOWER(name)` and `LOWER(username)` instead of using `ILIKE`. Using
`LOWER` produces a _much_ better performing query.

For example, when searching for all users matching the term "a" we'd
produce the following plan:

     Limit  (cost=637.69..637.74 rows=20 width=805) (actual time=41.983..41.995 rows=20 loops=1)
       Buffers: shared hit=8330
       ->  Sort  (cost=637.69..638.61 rows=368 width=805) (actual time=41.982..41.990 rows=20 loops=1)
             Sort Key: (CASE WHEN ((name)::text = 'a'::text) THEN 0 WHEN ((username)::text = 'a'::text) THEN 1 WHEN ((email)::text = 'a'::text) THEN 2 ELSE 3 END), name
             Sort Method: top-N heapsort  Memory: 35kB
             Buffers: shared hit=8330
             ->  Bitmap Heap Scan on users  (cost=75.47..627.89 rows=368 width=805) (actual time=9.452..41.305 rows=277 loops=1)
                   Recheck Cond: (((name)::text ~~* 'a'::text) OR ((username)::text ~~* 'a'::text) OR ((email)::text = 'a'::text))
                   Rows Removed by Index Recheck: 7601
                   Heap Blocks: exact=7636
                   Buffers: shared hit=8327
                   ->  BitmapOr  (cost=75.47..75.47 rows=368 width=0) (actual time=8.290..8.290 rows=0 loops=1)
                         Buffers: shared hit=691
                         ->  Bitmap Index Scan on index_users_on_name_trigram  (cost=0.00..38.85 rows=180 width=0) (actual time=4.369..4.369 rows=4071 loops=1)
                               Index Cond: ((name)::text ~~* 'a'::text)
                               Buffers: shared hit=360
                         ->  Bitmap Index Scan on index_users_on_username_trigram  (cost=0.00..34.41 rows=188 width=0) (actual time=3.896..3.896 rows=4140 loops=1)
                               Index Cond: ((username)::text ~~* 'a'::text)
                               Buffers: shared hit=328
                         ->  Bitmap Index Scan on users_email_key  (cost=0.00..1.94 rows=1 width=0) (actual time=0.022..0.022 rows=0 loops=1)
                               Index Cond: ((email)::text = 'a'::text)
                               Buffers: shared hit=3
     Planning time: 3.912 ms
     Execution time: 42.171 ms

With the changes in this commit we now produce the following plan
instead:

     Limit  (cost=13257.48..13257.53 rows=20 width=805) (actual time=1.567..1.579 rows=20 loops=1)
       Buffers: shared hit=287
       ->  Sort  (cost=13257.48..13280.93 rows=9379 width=805) (actual time=1.567..1.572 rows=20 loops=1)
             Sort Key: (CASE WHEN ((name)::text = 'a'::text) THEN 0 WHEN ((username)::text = 'a'::text) THEN 1 WHEN ((email)::text = 'a'::text) THEN 2 ELSE 3 END), name
             Sort Method: top-N heapsort  Memory: 35kB
             Buffers: shared hit=287
             ->  Bitmap Heap Scan on users  (cost=135.66..13007.91 rows=9379 width=805) (actual time=0.194..1.107 rows=277 loops=1)
                   Recheck Cond: ((lower((name)::text) = 'a'::text) OR (lower((username)::text) = 'a'::text) OR ((email)::text = 'a'::text))
                   Heap Blocks: exact=277
                   Buffers: shared hit=287
                   ->  BitmapOr  (cost=135.66..135.66 rows=9379 width=0) (actual time=0.152..0.152 rows=0 loops=1)
                         Buffers: shared hit=10
                         ->  Bitmap Index Scan on yorick_test_users  (cost=0.00..124.75 rows=9377 width=0) (actual time=0.101..0.101 rows=277 loops=1)
                               Index Cond: (lower((name)::text) = 'a'::text)
                               Buffers: shared hit=4
                         ->  Bitmap Index Scan on index_on_users_lower_username  (cost=0.00..1.94 rows=1 width=0) (actual time=0.035..0.035 rows=1 loops=1)
                               Index Cond: (lower((username)::text) = 'a'::text)
                               Buffers: shared hit=3
                         ->  Bitmap Index Scan on users_email_key  (cost=0.00..1.94 rows=1 width=0) (actual time=0.014..0.014 rows=0 loops=1)
                               Index Cond: ((email)::text = 'a'::text)
                               Buffers: shared hit=3
     Planning time: 0.303 ms
     Execution time: 1.687 ms

Here we can see the new query is 25 times faster compared to the old
query.

With Geo, attempting to view an endpoint with a user could result in an
Error 500 since Devise attempts to update the last sign-in IP and other
details.

Closes gitlab-org/gitlab-ee#4972

As per the discussion with @psimyn, this change does not affect the frontend, so user input will not be validated on the signin screen.

Instead, the value sent to the backend has leading and trailing whitespace stripped before looking up the user with find_by.

Closes #42637

Removing last_sign_in_at in specs

Fixes #32282.

Given no search term, the `search` and `search_with_secondary_emails` methods will yield an empty result set

Closes #41633

This change is required because otherwise if a user is created with a
value for `projects_limit` that matches the DB default, it gets
overwritten by `current_application_settings.default_projects_limit`. By
removing the default we once again can allow a user to be created with a
limit of 10 projects without the risk that it'll change to 10000

(10.2) Avoid partial partial email adresses for matching

See merge request gitlab/gitlabhq!2232

(cherry picked from commit 081aa1e91a777c9acb31be4a1e76b3dd7032fa9a)

There are unresolved conflicts in app/models/user.rb.

fa85a3fd Don't allow searching for partial user emails

assigned_open_merge_requests_count and assigned_open_issues_count, which are used in the top header stats.  Also important for a Geo secondary, so that the pending todo stat gets updated on the same frequency as the users open issues/merge requests.

Comments from code review applied. Also switched forked_from_project and ForkedProjectLinks to ForkNetworkMember

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

By caching the number of personal SSH keys we reduce the number of
queries necessary on pages such as ProjectsController#show (which can
end up querying this data multiple times).

The cache is refreshed/flushed whenever an SSH key is added, removed, or
when a user is removed.

These dropdown options are used for creating and transfering projects.

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.

By using SQL::Union we can return a proper ActiveRecord::Relation,
making it possible to select the columns we're interested in (instead of
all of them).