Merge branch '11965-geo-prevent-using-non-empty-tracking-database-ce' into 'master'

CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14981

See merge request gitlab-org/gitlab-ce!31493

db/migrate/20190802195602_add_timestamps_columns_to_geo_nodes.rb

+# frozen_string_literal: true
+
+class AddTimestampsColumnsToGeoNodes < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+
+  def change
+    add_column(:geo_nodes, :created_at, :datetime_with_timezone, null: true)
+    add_column(:geo_nodes, :updated_at, :datetime_with_timezone, null: true)
+  end
+end

db/schema.rb

@@ -1450,6 +1450,8 @@ ActiveRecord::Schema.define(version: 2019_08_02_235445) do
     t.string "internal_url"
     t.string "name", null: false
     t.integer "container_repositories_max_capacity", default: 10, null: false
+    t.datetime_with_timezone "created_at"
+    t.datetime_with_timezone "updated_at"
     t.index ["access_key"], name: "index_geo_nodes_on_access_key"
     t.index ["name"], name: "index_geo_nodes_on_name", unique: true
     t.index ["primary"], name: "index_geo_nodes_on_primary"

doc/administration/geo/replication/troubleshooting.md

@@ -230,7 +230,7 @@ sudo gitlab-ctl reconfigure
 This will increase the timeout to three hours (10800 seconds). Choose a time
 long enough to accommodate a full clone of your largest repositories.
 
-### Reseting Geo **secondary** node replication
+### Resetting Geo **secondary** node replication
 
 If you get a **secondary** node in a broken state and want to reset the replication state,
 to start again from scratch, there are a few steps that can help you:
@@ -524,6 +524,20 @@ If it doesn't exist or inadvertent changes have been made to it, run `sudo gitla
 
 If this path is mounted on a remote volume, please check your volume configuration and that it has correct permissions.
 
+### An existing tracking database cannot be reused
+
+Geo cannot reuse an existing tracking database.
+
+It is safest to use a fresh secondary, or reset the whole secondary by following
+[Resetting Geo secondary node replication](#resetting-geo-secondary-node-replication).
+
+If you are not concerned about possible orphaned directories and files, then you
+can simply reset the existing tracking database with:
+
+```sh
+sudo gitlab-rake geo:db:reset
+```
+
 ### Geo node has a database that is writable which is an indication it is not configured for replication with the primary node.
 
 This error refers to a problem with the database replica on a **secondary** node,