Rewrite SnippetsFinder to improve performance

This completely rewrites the SnippetsFinder class from the ground up in
order to improve its performance. The old code was beyond salvaging. It
was complex, included various Rails 5 workarounds, comments that
shouldn't be necessary, and most important of all: it produced a really
poorly performing database query.

As a result, I opted for rewriting the finder from scratch, instead of
trying to patch the existing code. Instead of trying to reuse as many
existing methods as possible, I opted for defining new methods
specifically meant for the SnippetsFinder. This requires some extra code
here and there, but allows us to have much more control over the
resulting SQL queries. It is these changes that then allow us to produce
a _much_ more efficient query.

To illustrate how bad the old query was, we will use my own snippets as
an example. Currently I have 52 snippets, most of which are global ones.
To retrieve these, you would run the following Ruby code:

    user = User.find_by(username: 'yorickpeterse')

    SnippetsFinder.new(user, author: user).execute

On GitLab.com the resulting query will take between 10 and 15 seconds to
run, producing the query plan found at
https://explain.depesz.com/s/Y5IX. Apart from the long execution time,
the total number of buffers (the sum of all shared hits) is around 185
GB, though the real number is probably (hopefully) much lower as I doubt
simply summing these numbers produces the true total number of buffers
used.

The new query's plan can be found at https://explain.depesz.com/s/wHdN,
and this query takes between 10 and 100-ish milliseconds to run. The
total number of buffers used is only about 30 MB.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/52639

1 files changed, 29 insertions, 16 deletions

diff --git a/spec/finders/snippets_finder_spec.rb b/spec/finders/snippets_finder_spec.rb
index 1ae0bd988f2..dfeeb3040c6 100644
--- a/spec/finders/snippets_finder_spec.rb
+++ b/spec/finders/snippets_finder_spec.rb
@@ -4,16 +4,13 @@ describe SnippetsFinder do
   include Gitlab::Allowable
   using RSpec::Parameterized::TableSyntax
 
-  context 'filter by visibility' do
-    let!(:snippet1) { create(:personal_snippet, :private) }
-    let!(:snippet2) { create(:personal_snippet, :internal) }
-    let!(:snippet3) { create(:personal_snippet, :public) }
+  describe '#initialize' do
+    it 'raises ArgumentError when a project and author are given' do
+      user = build(:user)
+      project = build(:project)
 
-    it "returns public snippets when visibility is PUBLIC" do
-      snippets = described_class.new(nil, visibility: Snippet::PUBLIC).execute
-
-      expect(snippets).to include(snippet3)
-      expect(snippets).not_to include(snippet1, snippet2)
+      expect { described_class.new(user, author: user, project: project) }
+        .to raise_error(ArgumentError)
     end
   end
 
@@ -66,21 +63,21 @@ describe SnippetsFinder do
     end
 
     it "returns internal snippets" do
-      snippets = described_class.new(user, author: user, visibility: Snippet::INTERNAL).execute
+      snippets = described_class.new(user, author: user, scope: :are_internal).execute
 
       expect(snippets).to include(snippet2)
       expect(snippets).not_to include(snippet1, snippet3)
     end
 
     it "returns private snippets" do
-      snippets = described_class.new(user, author: user, visibility: Snippet::PRIVATE).execute
+      snippets = described_class.new(user, author: user, scope: :are_private).execute
 
       expect(snippets).to include(snippet1)
       expect(snippets).not_to include(snippet2, snippet3)
     end
 
     it "returns public snippets" do
-      snippets = described_class.new(user, author: user, visibility: Snippet::PUBLIC).execute
+      snippets = described_class.new(user, author: user, scope: :are_public).execute
 
       expect(snippets).to include(snippet3)
       expect(snippets).not_to include(snippet1, snippet2)
@@ -98,6 +95,13 @@ describe SnippetsFinder do
       expect(snippets).to include(snippet3)
       expect(snippets).not_to include(snippet2, snippet1)
     end
+
+    it 'returns all snippets for an admin' do
+      admin = create(:user, :admin)
+      snippets = described_class.new(admin, author: user).execute
+
+      expect(snippets).to include(snippet1, snippet2, snippet3)
+    end
   end
 
   context 'filter by project' do
@@ -126,21 +130,21 @@ describe SnippetsFinder do
     end
 
     it "returns public snippets for non project members" do
-      snippets = described_class.new(user, project: project1, visibility: Snippet::PUBLIC).execute
+      snippets = described_class.new(user, project: project1, scope: :are_public).execute
 
       expect(snippets).to include(@snippet3)
       expect(snippets).not_to include(@snippet1, @snippet2)
     end
 
     it "returns internal snippets for non project members" do
-      snippets = described_class.new(user, project: project1, visibility: Snippet::INTERNAL).execute
+      snippets = described_class.new(user, project: project1, scope: :are_internal).execute
 
       expect(snippets).to include(@snippet2)
       expect(snippets).not_to include(@snippet1, @snippet3)
     end
 
     it "does not return private snippets for non project members" do
-      snippets = described_class.new(user, project: project1, visibility: Snippet::PRIVATE).execute
+      snippets = described_class.new(user, project: project1, scope: :are_private).execute
 
       expect(snippets).not_to include(@snippet1, @snippet2, @snippet3)
     end
@@ -156,10 +160,17 @@ describe SnippetsFinder do
     it "returns private snippets for project members" do
       project1.add_developer(user)
 
-      snippets = described_class.new(user, project: project1, visibility: Snippet::PRIVATE).execute
+      snippets = described_class.new(user, project: project1, scope: :are_private).execute
 
       expect(snippets).to include(@snippet1)
     end
+
+    it 'returns all snippets for an admin' do
+      admin = create(:user, :admin)
+      snippets = described_class.new(admin, project: project1).execute
+
+      expect(snippets).to include(@snippet1, @snippet2, @snippet3)
+    end
   end
 
   describe '#execute' do
@@ -184,4 +195,6 @@ describe SnippetsFinder do
       end
     end
   end
+
+  it_behaves_like 'snippet visibility'
 end