Use CTEs for nested groups and authorizations

This commit introduces the usage of Common Table Expressions (CTEs) to
efficiently retrieve nested group hierarchies, without having to rely on
the "routes" table (which is an _incredibly_ inefficient way of getting
the data). This requires a patch to ActiveRecord (found in the added
initializer) to work properly as ActiveRecord doesn't support WITH
statements properly out of the box.

Unfortunately MySQL provides no efficient way of getting nested groups.
For example, the old routes setup could easily take 5-10 seconds
depending on the amount of "routes" in a database. Providing vastly
different logic for both MySQL and PostgreSQL will negatively impact the
development process. Because of this the various nested groups related
methods return empty relations when used in combination with MySQL.

For project authorizations the logic is split up into two classes:

* Gitlab::ProjectAuthorizations::WithNestedGroups
* Gitlab::ProjectAuthorizations::WithoutNestedGroups

Both classes get the fresh project authorizations (= as they should be
in the "project_authorizations" table), including nested groups if
PostgreSQL is used. The logic of these two classes is quite different
apart from their public interface. This complicates development a bit,
but unfortunately there is no way around this.

This commit also introduces Gitlab::GroupHierarchy. This class can be
used to get the ancestors and descendants of a base relation, or both by
using a UNION. This in turn is used by methods such as:

* Namespace#ancestors
* Namespace#descendants
* User#all_expanded_groups

Again this class relies on CTEs and thus only works on PostgreSQL. The
Namespace methods will return an empty relation when MySQL is used,
while User#all_expanded_groups will return only the groups a user is a
direct member of.

Performance wise the impact is quite large. For example, on GitLab.com
Namespace#descendants used to take around 580 ms to retrieve data for a
particular user. Using CTEs we are able to reduce this down to roughly 1
millisecond, returning the exact same data.

== On The Fly Refreshing

Refreshing of authorizations on the fly (= when
users.authorized_projects_populated was not set) is removed with this
commit. This simplifies the code, and ensures any queries used for
authorizations are not mutated because they are executed in a Rails
scope (e.g. Project.visible_to_user).

This commit includes a migration to schedule refreshing authorizations
for all users, ensuring all of them have their authorizations in place.
Said migration schedules users in batches of 5000, with 5 minutes
between every batch to smear the load around a bit.

== Spec Changes

This commit also introduces some changes to various specs. For example,
some specs for ProjectTeam assumed that creating a personal project
would _not_ lead to the owner having access, which is incorrect. Because
we also no longer refresh authorizations on the fly for new users some
code had to be added to the "empty_project" factory. This chunk of code
ensures that the owner's permissions are refreshed after creating the
project, something that is normally done in Projects::CreateService.

1 files changed, 132 insertions, 0 deletions

diff --git a/config/initializers/postgresql_cte.rb b/config/initializers/postgresql_cte.rb
new file mode 100644
index 00000000000..7f0df8949db
--- /dev/null
+++ b/config/initializers/postgresql_cte.rb
@@ -0,0 +1,132 @@
+# Adds support for WITH statements when using PostgreSQL. The code here is taken
+# from https://github.com/shmay/ctes_in_my_pg which at the time of writing has
+# not been pushed to RubyGems. The license of this repository is as follows:
+#
+# The MIT License (MIT)
+#
+# Copyright (c) 2012 Dan McClain
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+# THE SOFTWARE.
+
+module ActiveRecord
+  class Relation
+    class Merger # :nodoc:
+      def normal_values
+        NORMAL_VALUES + [:with]
+      end
+    end
+  end
+end
+
+module ActiveRecord::Querying
+  delegate :with, to: :all
+end
+
+module ActiveRecord
+  class Relation
+    # WithChain objects act as placeholder for queries in which #with does not have any parameter.
+    # In this case, #with must be chained with #recursive to return a new relation.
+    class WithChain
+      def initialize(scope)
+        @scope = scope
+      end
+
+      # Returns a new relation expressing WITH RECURSIVE
+      def recursive(*args)
+        @scope.with_values += args
+        @scope.recursive_value = true
+        @scope
+      end
+    end
+
+    def with_values
+      @values[:with] || []
+    end
+
+    def with_values=(values)
+      raise ImmutableRelation if @loaded
+      @values[:with] = values
+    end
+
+    def recursive_value=(value)
+      raise ImmutableRelation if @loaded
+      @values[:recursive] = value
+    end
+
+    def recursive_value
+      @values[:recursive]
+    end
+
+    def with(opts = :chain, *rest)
+      if opts == :chain
+        WithChain.new(spawn)
+      elsif opts.blank?
+        self
+      else
+        spawn.with!(opts, *rest)
+      end
+    end
+
+    def with!(opts = :chain, *rest) # :nodoc:
+      if opts == :chain
+        WithChain.new(self)
+      else
+        self.with_values += [opts] + rest
+        self
+      end
+    end
+
+    def build_arel
+      arel = super()
+
+      build_with(arel) if @values[:with]
+
+      arel
+    end
+
+    def build_with(arel)
+      with_statements = with_values.flat_map do |with_value|
+        case with_value
+        when String
+          with_value
+        when Hash
+          with_value.map  do |name, expression|
+            case expression
+            when String
+              select = Arel::Nodes::SqlLiteral.new "(#{expression})"
+            when ActiveRecord::Relation, Arel::SelectManager
+              select = Arel::Nodes::SqlLiteral.new "(#{expression.to_sql})"
+            end
+            Arel::Nodes::As.new Arel::Nodes::SqlLiteral.new("\"#{name}\""), select
+          end
+        when Arel::Nodes::As
+          with_value
+        end
+      end
+
+      unless with_statements.empty?
+        if recursive_value
+          arel.with :recursive, with_statements
+        else
+          arel.with with_statements
+        end
+      end
+    end
+  end
+end