GKE 1.8 has RBAC on by default, since GKE has bumped the default version
to 1.8.7 RBAC is now on, and we don't support that out of the box.

This was also avoiding cluster applications (tiller, prometheus, etc), to
be installed.

Closes #41619

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.

The frontend code doesn't use this so there's no practical point in
supporting this. We also hardcode the limit to 20 so users can no longer
request their own limit, which could overload the database (depending on
any upper bounds perhaps enforced by Kaminari).

Since the redactor can be run on multiple documents at once and
query results are stored in the request store.

See
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16900#note_60175507

Rugged sometimes chops a context line in between bytes, resulting in the context
line having an invalid encoding: https://github.com/libgit2/rugged/issues/716

When that happens, we will try to detect the encoding for the diff, and
sometimes we'll get it wrong. If that difference in encoding results in a
difference in string lengths between the diff and the underlying blobs, we'd
fail to highlight any inline diffs, and return a 500 status to the user.

As we're using the underlying blobs, the encoding is 'correct' anyway, so if the
string range is invalid, we can just discard the inline diff highlighting. We
still report to Sentry to ensure that we can investigate further in future.

Signed-off-by: NRémy Coutable <remy@rymai.me>

Adds a test where a branch name is also a valid commit id. Git, the
binary should create an error message which is difficult to parse and
leading to errors later, as seen in: gitlab-org/gitlab-ce#43222

To catch these cases in the future,
gitlab-test@1942eed5cc108b19c7405106e81fa96125d0be22 was created. Which
a branch name matching the commit

36847 - add changelog

By extracting a new `filter_items` method, we can override that in the
IssuesFinder and MergeRequestsFinder separately, so we don't need checks that
the model is the correct one, because we can just use the class we're in to know
that.

We can do the same for the VALID_PARAMS constant, by making it a class method.

https://gitlab.com/gitlab-org/gitlab-ce/issues/23036