DEV: Support per-unicorn-worker DB variable overrides (#35615)

This commit adds support for setting ActiveRecord's database
configuration variables for unicorn workers via ENVs with the
`DISCOURSE_UNICORN_WORKER_DB_VARIABLES_` prefix.

For example to set `statement_timeout`, one would add the
`DISCOURSE_UNICORN_WORKER_DB_VARIABLES_STATEMENT_TIMEOUT` env with its
corresponding value. Do note that envs with the
`DISCOURSE_UNICORN_WORKER_DB_VARIABLES_` prefix will take precendent
over envs with the `DISCOURSE_DB_VARIABLES_` prefix.

config/database.yml

@@ -1,3 +1,10 @@
+<%
+  db_variables =
+    ENV
+      .filter { |key, _| key.start_with?("DISCOURSE_DB_VARIABLES_") }
+      .transform_keys { |key| key.slice(("DISCOURSE_DB_VARIABLES_".length)..).downcase }
+%>
+
 development:
   prepared_statements: false
   adapter: postgresql
@@ -11,9 +18,7 @@ development:
     ###   - restart sidekiq if you change this setting
     ###   - rebake all to posts using: `RAILS_ENV=production bundle exec rake posts:rebake`
     - "<%= ENV['DISCOURSE_HOSTNAME'] || 'localhost' %>"
-  variables: <%= ENV.filter { |k,v| k.start_with? 'DISCOURSE_DB_VARIABLES_' }
-                    .transform_keys { |k| k.slice(('DISCOURSE_DB_VARIABLES_'.length)..).downcase }
-                    .to_json %>
+  variables: <%= db_variables.to_json %>
 
 # Warning: The database defined as "test" will be erased and
 # re-generated from your development database when you run "rake".
@@ -41,6 +46,7 @@ test:
   checkout_timeout: <%= ENV["CHECKOUT_TIMEOUT"] || 5 %>
   host_names:
     - test.localhost
+  variables: <%= db_variables.to_json %>
 
 # profile db is used for benchmarking using the script/bench.rb script
 profile:

config/unicorn.conf.rb

@@ -194,6 +194,7 @@ end
 
 after_fork do |server, worker|
   DiscourseEvent.trigger(:web_fork_started)
+  Discourse.after_unicorn_worker_fork
   Discourse.after_fork
   SignalTrapLogger.instance.after_fork
 

lib/discourse.rb

@@ -934,6 +934,26 @@ module Discourse
     Process.warmup
   end
 
+  def self.after_unicorn_worker_fork
+    unicorn_worker_db_variables_prefix = "DISCOURSE_UNICORN_WORKER_DB_VARIABLES_"
+    db_variables_overrides = {}
+
+    ENV.filter do |key, value|
+      if key.start_with?(unicorn_worker_db_variables_prefix)
+        db_variables_overrides[
+          "DISCOURSE_DB_VARIABLES_#{key.sub(unicorn_worker_db_variables_prefix, "")}"
+        ] = value
+      end
+    end
+
+    if db_variables_overrides.any?
+      db_variables_overrides.each { |key, value| ENV[key] = value }
+      ActiveRecord::Base.configurations = Rails.application.config.database_configuration
+      ActiveRecord::Base.connection_handler.clear_all_connections!(:all)
+      ActiveRecord::Base.establish_connection
+    end
+  end
+
   # all forking servers must call this
   # after fork, otherwise Discourse will be
   # in a bad state

spec/lib/discourse_spec.rb

@@ -67,6 +67,41 @@ RSpec.describe Discourse do
     end
   end
 
+  describe ".after_unicorn_worker_fork" do
+    around do |example|
+      original_env = ENV.to_hash
+      original_config = ActiveRecord::Base.configurations
+      original_show_statement_timeout =
+        ActiveRecord::Base.connection.execute("SHOW statement_timeout").first["statement_timeout"]
+
+      begin
+        example.run
+      ensure
+        ENV.replace(original_env)
+        ActiveRecord::Base.configurations = original_config
+        ActiveRecord::Base.connection_handler.clear_all_connections!(:all)
+        ActiveRecord::Base.establish_connection
+
+        expect(
+          ActiveRecord::Base.connection.execute("SHOW statement_timeout").first[
+            "statement_timeout"
+          ],
+        ).to eq(original_show_statement_timeout)
+      end
+    end
+
+    it "applies worker-specific database variable overrides" do
+      ENV["DISCOURSE_DB_VARIABLES_STATEMENT_TIMEOUT"] = "5s"
+      ENV["DISCOURSE_UNICORN_WORKER_DB_VARIABLES_STATEMENT_TIMEOUT"] = "100s"
+
+      Discourse.after_unicorn_worker_fork
+
+      expect(
+        ActiveRecord::Base.connection.execute("SHOW statement_timeout").first["statement_timeout"],
+      ).to eq("100s")
+    end
+  end
+
   describe ".plugins_sorted_by_name" do
     before do
       Discourse.stubs(:visible_plugins).returns(