Add migration to fix a merge_requests index

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52116 we
introduced a new index used when generating changelogs using the GitLab
API. This migration failed in staging and production, but due to
https://github.com/lfittl/pg_query/issues/184 the original error is
masked, and an INVALID index is left behind.

This commit turns the old migration into a noop, and creates a new
migration. This new migration catches any CREATE INDEX errors and raises
a regular error, preventing the pg_query bug from surfacing. It also
removes the existing index if it's marked as INVALID. In addition, we
disable statement timeouts when creating the index; something we forgot
to do in the previous migration.

db/migrate/20210127143025_add_oldest_merge_requests_index.rb

 # frozen_string_literal: true
 
 class AddOldestMergeRequestsIndex < ActiveRecord::Migration[6.0]
-  include Gitlab::Database::SchemaHelpers
-  include Gitlab::Database::MigrationHelpers
-
-  disable_ddl_transaction!
-
-  # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
-  INDEX = 'index_on_merge_requests_for_latest_diffs'
-
   def up
-    return if index_exists_by_name?('merge_requests', INDEX)
-
-    execute "CREATE INDEX CONCURRENTLY #{INDEX} ON merge_requests " \
-      'USING btree (target_project_id) INCLUDE (id, latest_merge_request_diff_id)'
-
-    create_comment(
-      'INDEX',
-      INDEX,
-      'Index used to efficiently obtain the oldest merge request for a commit SHA'
-    )
+    # replaced by db/migrate/20210201140434_add_oldest_merge_requests_index_again.rb
   end
 
   def down
-    return unless index_exists_by_name?('merge_requests', INDEX)
-
-    execute "DROP INDEX CONCURRENTLY #{INDEX}"
+    # replaced by db/migrate/20210201140434_add_oldest_merge_requests_index_again.rb
   end
 end

db/migrate/20210201140434_add_oldest_merge_requests_index_again.rb

+# frozen_string_literal: true
+
+class AddOldestMergeRequestsIndexAgain < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::SchemaHelpers
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  DOWNTIME = false
+
+  INDEX = 'index_on_merge_requests_for_latest_diffs'
+
+  def up
+    execute "DROP INDEX CONCURRENTLY #{INDEX}" if invalid_index?
+
+    return if index_exists_by_name?('merge_requests', INDEX)
+
+    begin
+      disable_statement_timeout do
+        execute "CREATE INDEX CONCURRENTLY #{INDEX} ON merge_requests " \
+          'USING btree (target_project_id) INCLUDE (id, latest_merge_request_diff_id)'
+      end
+    rescue ActiveRecord::StatementInvalid => ex
+      # Due to https://github.com/lfittl/pg_query/issues/184, if the CREATE
+      # INDEX statement fails, we trigger a separate error due to the Gem not
+      # supporting the INCLUDE syntax.
+      #
+      # To work around this, we raise a custom error instead, as these won't
+      # have a query context injected.
+      raise "The index #{INDEX} couldn't be added: #{ex.message}"
+    end
+
+    create_comment(
+      'INDEX',
+      INDEX,
+      'Index used to efficiently obtain the oldest merge request for a commit SHA'
+    )
+  end
+
+  def down
+    return unless index_exists_by_name?('merge_requests', INDEX)
+
+    disable_statement_timeout do
+      execute "DROP INDEX CONCURRENTLY #{INDEX}"
+    end
+  end
+
+  def invalid_index?
+    result = execute(<<~SQL)
+      SELECT pg_class.relname
+      FROM pg_class, pg_index
+      WHERE pg_index.indisvalid = false
+      AND pg_index.indexrelid = pg_class.oid
+      AND pg_class.relname = '#{INDEX}';
+    SQL
+
+    result.values.any?
+  end
+end

db/schema_migrations/20210201140434

+71220cfc36215f6c22d22d1fb0b74389e90c58733214b5fa36dcb8da0377a120
\ No newline at end of file