Shortcut concurrent index creation/removal if no effect.

Index creation does not have an effect if the index is present already.
Index removal does not have an affect if the index is not present.

This helps to avoid patterns like this in migrations:
```
if index_exists?(...)
  remove_concurrent_index(...)
end
```

doc/development/migration_style_guide.md

@@ -136,11 +136,14 @@ class MyMigration < ActiveRecord::Migration
   disable_ddl_transaction!
   def up
-    remove_concurrent_index :table_name, :column_name if index_exists?(:table_name, :column_name)
+    remove_concurrent_index :table_name, :column_name
   end
 end
 ```
+Note that it is not necessary to check if the index exists prior to
+removing it.
+
 ## Adding indexes
 If you need to add a unique index please keep in mind there is the possibility

lib/gitlab/database/migration_helpers.rb

@@ -59,6 +59,11 @@ module Gitlab
           disable_statement_timeout
         end
+        if index_exists?(table_name, column_name, options)
+          Rails.logger.warn "Index not created because it already exists (this may be due to an aborted migration or similar): table_name: #{table_name}, column_name: #{column_name}"
+          return
+        end
+
         add_index(table_name, column_name, options)
       end
@@ -83,6 +88,11 @@ module Gitlab
           disable_statement_timeout
         end
+        unless index_exists?(table_name, column_name, options)
+          Rails.logger.warn "Index not removed because it does not exist (this may be due to an aborted migration or similar): table_name: #{table_name}, column_name: #{column_name}"
+          return
+        end
+
         remove_index(table_name, options.merge({ column: column_name }))
       end
@@ -107,6 +117,11 @@ module Gitlab
           disable_statement_timeout
         end
+        unless index_exists_by_name?(table_name, index_name)
+          Rails.logger.warn "Index not removed because it does not exist (this may be due to an aborted migration or similar): table_name: #{table_name}, index_name: #{index_name}"
+          return
+        end
+
         remove_index(table_name, options.merge({ name: index_name }))
       end

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -67,17 +67,35 @@ describe Gitlab::Database::MigrationHelpers do
           model.add_concurrent_index(:users, :foo, unique: true)
         end
+
+        it 'does nothing if the index exists already' do
+          expect(model).to receive(:index_exists?)
+            .with(:users, :foo, { algorithm: :concurrently, unique: true }).and_return(true)
+          expect(model).not_to receive(:add_index)
+
+          model.add_concurrent_index(:users, :foo, unique: true)
+        end
       end
       context 'using MySQL' do
-        it 'creates a regular index' do
-          expect(Gitlab::Database).to receive(:postgresql?).and_return(false)
+        before do
+          allow(Gitlab::Database).to receive(:postgresql?).and_return(false)
+        end
+        it 'creates a regular index' do
           expect(model).to receive(:add_index)
             .with(:users, :foo, {})
           model.add_concurrent_index(:users, :foo)
         end
+
+        it 'does nothing if the index exists already' do
+          expect(model).to receive(:index_exists?)
+            .with(:users, :foo, { unique: true }).and_return(true)
+          expect(model).not_to receive(:add_index)
+
+          model.add_concurrent_index(:users, :foo, unique: true)
+        end
       end
     end
@@ -95,6 +113,7 @@ describe Gitlab::Database::MigrationHelpers do
     context 'outside a transaction' do
       before do
         allow(model).to receive(:transaction_open?).and_return(false)
+        allow(model).to receive(:index_exists?).and_return(true)
       end
       context 'using PostgreSQL' do
@@ -103,18 +122,41 @@ describe Gitlab::Database::MigrationHelpers do
           allow(model).to receive(:disable_statement_timeout)
         end
-        it 'removes the index concurrently by column name' do
-          expect(model).to receive(:remove_index)
-            .with(:users, { algorithm: :concurrently, column: :foo })
-          model.remove_concurrent_index(:users, :foo)
+        describe 'by column name' do
+          it 'removes the index concurrently' do
+            expect(model).to receive(:remove_index)
+              .with(:users, { algorithm: :concurrently, column: :foo })
+            model.remove_concurrent_index(:users, :foo)
+          end
+
+          it 'does nothing if the index does not exist' do
+            expect(model).to receive(:index_exists?)
+              .with(:users, :foo, { algorithm: :concurrently, unique: true }).and_return(false)
+            expect(model).not_to receive(:remove_index)
+
+            model.remove_concurrent_index(:users, :foo, unique: true)
+          end
         end
-        it 'removes the index concurrently by index name' do
-          expect(model).to receive(:remove_index)
-            .with(:users, { algorithm: :concurrently, name: "index_x_by_y" })
-          model.remove_concurrent_index_by_name(:users, "index_x_by_y")
+        describe 'by index name' do
+          before do
+            allow(model).to receive(:index_exists_by_name?).with(:users, "index_x_by_y").and_return(true)
+          end
+
+          it 'removes the index concurrently by index name' do
+            expect(model).to receive(:remove_index)
+              .with(:users, { algorithm: :concurrently, name: "index_x_by_y" })
+
+            model.remove_concurrent_index_by_name(:users, "index_x_by_y")
+          end
+
+          it 'does nothing if the index does not exist' do
+            expect(model).to receive(:index_exists_by_name?).with(:users, "index_x_by_y").and_return(false)
+            expect(model).not_to receive(:remove_index)
+            model.remove_concurrent_index_by_name(:users, "index_x_by_y")
+          end
         end
       end