Fix Connection#exists? when using the DB LB

Database::Connection#exists? would call `connection` to determine if
the database exists. Without load balancing enabled, this would call
`ActiveRecord::Base.connection`, which in turn calls
`ActiveRecord::Base.retrieve_connection`. If no connection could be
established or the database doesn't exist, this will raise an error.

When using the load balancer, the `Connection#connection` method instead
just returns a `ConnectionProxy`, without running any database queries.
The result is that `exists?` would return `true` even if the database
didn't exist.

To work around this, `Connection#exists?` obtains the database version
and caches this using Rails' schema cache. This ensures we run an actual
database query the first time, without running a query _every_ time.
Using the schema cache means we don't need to implement our own caching
logic.

Changelog: fixed

lib/gitlab/database/connection.rb

@@ -174,8 +174,11 @@ module Gitlab
       end
 
       def exists?
-        connection
-
+        # We can't _just_ check if `connection` raises an error, as it will
+        # point to a `ConnectionProxy`, and obtaining those doesn't involve any
+        # database queries. So instead we obtain the database version, which is
+        # cached after the first call.
+        connection.schema_cache.database_version
         true
       rescue StandardError
         false

spec/lib/gitlab/database/connection_spec.rb

@@ -393,34 +393,28 @@ RSpec.describe Gitlab::Database::Connection do
   end
 
   describe '#cached_column_exists?' do
-    it 'only retrieves data once' do
-      expect(connection.scope.connection)
-        .to receive(:columns)
-        .once.and_call_original
-
-      2.times do
-        expect(connection.cached_column_exists?(:projects, :id)).to be_truthy
-        expect(connection.cached_column_exists?(:projects, :bogus_column)).to be_falsey
+    it 'only retrieves the data from the schema cache' do
+      queries = ActiveRecord::QueryRecorder.new do
+        2.times do
+          expect(connection.cached_column_exists?(:projects, :id)).to be_truthy
+          expect(connection.cached_column_exists?(:projects, :bogus_column)).to be_falsey
+        end
       end
+
+      expect(queries.count).to eq(0)
     end
   end
 
   describe '#cached_table_exists?' do
-    it 'only retrieves data once per table' do
-      expect(connection.scope.connection)
-        .to receive(:data_source_exists?)
-        .with(:projects)
-        .once.and_call_original
-
-      expect(connection.scope.connection)
-        .to receive(:data_source_exists?)
-        .with(:bogus_table_name)
-        .once.and_call_original
-
-      2.times do
-        expect(connection.cached_table_exists?(:projects)).to be_truthy
-        expect(connection.cached_table_exists?(:bogus_table_name)).to be_falsey
+    it 'only retrieves the data from the schema cache' do
+      queries = ActiveRecord::QueryRecorder.new do
+        2.times do
+          expect(connection.cached_table_exists?(:projects)).to be_truthy
+          expect(connection.cached_table_exists?(:bogus_table_name)).to be_falsey
+        end
       end
+
+      expect(queries.count).to eq(0)
     end
 
     it 'returns false when database does not exist' do
@@ -433,16 +427,14 @@ RSpec.describe Gitlab::Database::Connection do
   end
 
   describe '#exists?' do
-    it 'returns true if `ActiveRecord::Base.connection` succeeds' do
-      expect(connection.scope).to receive(:connection)
-
+    it 'returns true if the database exists' do
       expect(connection.exists?).to be(true)
     end
 
-    it 'returns false if `ActiveRecord::Base.connection` fails' do
-      expect(connection.scope).to receive(:connection) do
-        raise ActiveRecord::NoDatabaseError, 'broken'
-      end
+    it "returns false if the database doesn't exist" do
+      expect(connection.scope.connection.schema_cache)
+        .to receive(:database_version)
+        .and_raise(ActiveRecord::NoDatabaseError)
 
       expect(connection.exists?).to be(false)
     end