Stop WAL checkpointer before fork; one thread owns the flock

Unify startup: initializer starts every non-test process, Puma and Resque
stop before fork and start again in the child. Only the contender thread
releases the lock. Drop Puma::CLI / single_puma_process? special cases.

Co-authored-by: Thomas Klemm <github@tklemm.eu>

config/database.yml

@@ -9,8 +9,9 @@ default: &default
   pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 10 } %>
   timeout: 5000
   default_transaction_mode: immediate
-  # Checkpoint on a background connection instead (SqliteWalCheckpoint). Every
-  # non-test writer process runs a contender so this is never left without one.
+  # Checkpoint on a background connection instead (SqliteWalCheckpoint).
+  # Non-test processes start a contender; forking servers stop before fork and
+  # start again in the child so the flock is never inherited.
   pragmas:
     wal_autocheckpoint: 0
 

config/initializers/sqlite_wal_checkpoint.rb

@@ -1,10 +1,5 @@
-# Start a checkpoint contender in console, runner, rake and other non-Puma writers.
-# Puma skips this path: config/puma.rb and config/puma_dev.rb start after the
-# correct process is chosen (worker boot vs single-process), so the master that
-# only forks workers never holds the lock alone. Resque starts after_prefork.
+# Start a checkpoint contender in every non-test process. Puma and Resque pool
+# stop before fork and start again in the child so the flock is never inherited.
 Rails.application.config.after_initialize do
-  next if Rails.env.test?
-  next if defined?(Puma::CLI)
-
-  SqliteWalCheckpoint.start
+  SqliteWalCheckpoint.start unless Rails.env.test?
 end

config/puma.rb

@@ -33,10 +33,7 @@ pidfile ENV.fetch("PIDFILE") { "tmp/pids/server.pid" }
 # processes).
 #
 worker_count = (Concurrent.processor_count * 0.666).ceil
-# Keep the raw setting: WEB_CONCURRENCY=auto must not be coerced with to_i (that
-# is 0 and would look like single-process mode while Puma still forks workers).
-configured_workers = ENV.fetch("WEB_CONCURRENCY") { worker_count }
-workers configured_workers
+workers ENV.fetch("WEB_CONCURRENCY") { worker_count }
 
 ENV["JOB_CONCURRENCY"] ||= worker_count.to_s
 
@@ -53,14 +50,11 @@ plugin :tmp_restart
 # Reset all membership connections
 Membership.disconnect_all
 
-# Only the literal 0 is single-process. "auto" and positive counts fork workers;
-# those must start the checkpointer after boot so a surviving worker can take
-# the lock if the previous leader exits.
-if SqliteWalCheckpoint.single_puma_process?(configured_workers)
-  SqliteWalCheckpoint.start
-else
-  on_worker_boot { SqliteWalCheckpoint.start }
-end
+# Initializer starts a contender in this process. Stop before fork so workers
+# do not inherit the flock; each worker starts its own contender. Single-process
+# mode never forks, so the initializer's contender keeps running.
+before_fork { SqliteWalCheckpoint.stop }
+on_worker_boot { SqliteWalCheckpoint.start }
 
 Signal.trap :SIGPROF do
   Thread.list.each do |t|

config/puma_dev.rb

@@ -1,7 +1,5 @@
 require File.expand_path("../config/environment", File.dirname(__FILE__))
 
-SqliteWalCheckpoint.start
-
 Signal.trap :SIGPROF do
   Thread.list.each do |t|
     puts t