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
```

3 files changed, 71 insertions, 11 deletions

diff --git a/doc/development/migration_style_guide.md b/doc/development/migration_style_guide.md
index 243ac7f0c98..1e060ffd941 100644
--- a/doc/development/migration_style_guide.md
+++ b/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
diff --git a/lib/gitlab/database/migration_helpers.rb b/lib/gitlab/database/migration_helpers.rb
index 21287a8efd0..55160ca8708 100644
--- a/lib/gitlab/database/migration_helpers.rb
+++ b/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
 
diff --git a/spec/lib/gitlab/database/migration_helpers_spec.rb b/spec/lib/gitlab/database/migration_helpers_spec.rb
index 1de3a14b809..9074e17ae80 100644
--- a/spec/lib/gitlab/database/migration_helpers_spec.rb
+++ b/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 })
+        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)
+            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" })
+        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")
+            model.remove_concurrent_index_by_name(:users, "index_x_by_y")
+          end
         end
       end