Include PostgreSQL system identifier in usage ping

We see a lot of the same UUIDs in the usage ping. Include the PostgreSQL
system ID to see if there is any more randomness there.

changelogs/unreleased/sh-usage-data-pg-system-id.yml

+---
+title: Include PostgreSQL system identifier in usage ping
+merge_request: 44972
+author:
+type: added

doc/development/product_analytics/usage_ping.md

@@ -750,7 +750,8 @@ The following is example content of the Usage Ping payload.
   },
   "database": {
     "adapter": "postgresql",
-    "version": "9.6.15"
+    "version": "9.6.15",
+    "pg_system_id": 6842684531675334351
   },
   "avg_cycle_analytics": {
     "issue": {
@@ -910,6 +911,10 @@ The following is example content of the Usage Ping payload.
 }
 ```
 
+## Notable changes
+
+In GitLab 13.5, `pg_system_id` was added to send the [PostgreSQL system identifier](https://www.2ndquadrant.com/en/blog/support-for-postgresqls-system-identifier-in-barman/).
+
 ## Exporting Usage Ping SQL queries and definitions
 
 Two Rake tasks exist to export Usage Ping definitions.

lib/gitlab/database.rb

@@ -250,6 +250,12 @@ module Gitlab
       false
     end
 
+    def self.system_id
+      row = connection.execute('SELECT system_identifier FROM pg_control_system()').first
+
+      row['system_identifier']
+    end
+
     def self.get_write_location(ar_connection)
       row = ar_connection
         .select_all("SELECT pg_current_wal_insert_lsn()::text AS location")

lib/gitlab/usage_data.rb

@@ -290,7 +290,8 @@ module Gitlab
           },
           database: {
             adapter: alt_usage_data { Gitlab::Database.adapter_name },
-            version: alt_usage_data { Gitlab::Database.version }
+            version: alt_usage_data { Gitlab::Database.version },
+            pg_system_id: alt_usage_data { Gitlab::Database.system_id }
           },
           mail: {
             smtp_server: alt_usage_data { ActionMailer::Base.smtp_settings[:address] }

spec/lib/gitlab/database_spec.rb

@@ -39,6 +39,12 @@ RSpec.describe Gitlab::Database do
     end
   end
 
+  describe '.system_id' do
+    it 'returns the PostgreSQL system identifier' do
+      expect(described_class.system_id).to be_an_instance_of(Integer)
+    end
+  end
+
   describe '.postgresql?' do
     subject { described_class.postgresql? }
 

spec/lib/gitlab/usage_data_spec.rb

@@ -734,6 +734,7 @@ RSpec.describe Gitlab::UsageData, :aggregate_failures do
         expect(subject[:git][:version]).to eq(Gitlab::Git.version)
         expect(subject[:database][:adapter]).to eq(Gitlab::Database.adapter_name)
         expect(subject[:database][:version]).to eq(Gitlab::Database.version)
+        expect(subject[:database][:pg_system_id]).to eq(Gitlab::Database.system_id)
         expect(subject[:mail][:smtp_server]).to eq(ActionMailer::Base.smtp_settings[:address])
         expect(subject[:gitaly][:version]).to be_present
         expect(subject[:gitaly][:servers]).to be >= 1