Fix ActiveRecord::ReadOnlyError with Active Record 8.1

README.md

@@ -20,6 +20,33 @@ Or install it yourself as:
 
     $ gem install activerecord-debug_errors
 
+## Prerequisites
+
+### MySQL permissions
+
+To collect complete MySQL diagnostics, the account used by Active Record needs the
+global [`PROCESS`](https://dev.mysql.com/doc/refman/8.4/en/privileges-provided.html#priv_process)
+privilege:
+
+```sql
+GRANT PROCESS ON *.* TO 'application_user'@'application_host';
+```
+
+The gem runs `SHOW ENGINE INNODB STATUS` for lock wait timeouts and deadlocks.
+MySQL requires `PROCESS` to execute this statement. Without the privilege, the
+gem logs the resulting permission error instead of the InnoDB diagnostic
+section.
+
+For lock wait timeouts, the gem also runs `SHOW FULL PROCESSLIST`. This
+statement works without `PROCESS`, but only shows threads owned by the current
+MySQL account. With `PROCESS`, it shows threads for all accounts, which may be
+necessary to identify the session holding a lock.
+
+Because `PROCESS` can expose statements executed by other users on the same
+server, grant it only when the additional diagnostic visibility is acceptable.
+The privilege is global and cannot be limited to the application's database.
+
+
 ## Usage
 
 You only have to load the gem:
@@ -118,8 +145,6 @@ Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 *** WE ROLL BACK TRANSACTION (2)
 ```
 
-Note that the user requires the PROCESS priviledge to collect the information.
-
 ### ActiveRecord::ConnectionTimeoutError
 
 When `ActiveRecord::ConnectionTimeoutError` occurs, you can see the information of connection owners (threads):

spec/activerecord/debug_errors/ext/connection_adapters/abstract_mysql_adapter_spec.rb

@@ -59,9 +59,7 @@ def cause_deadlock(role:)
       context "when the user doesn't have the permission to execute 'SHOW ENGINE INNODB STATUS'" do
         it "displays an error message" do
           expect {
-            ActiveRecord::Base.connected_to(role: :reading) do
-              cause_deadlock(role: :reading)
-            end
+            cause_deadlock(role: :restricted)
           }.to raise_error(ActiveRecord::Deadlocked)
           expect(log.string).to include("Failed to execute")
         end

spec/spec_helper.rb

@@ -29,11 +29,11 @@
       },
     }
 
-    user_for_replica = 'activerecord-debug_errors'
+    restricted_user = 'activerecord-debug_errors'
     ActiveRecord::Base.configurations = {
       default_env: {
         primary: base_db_config,
-        primary_replica: base_db_config.merge(username: user_for_replica, replica: true),
+        restricted: base_db_config.merge(username: restricted_user),
       }
     }
 
@@ -48,18 +48,18 @@
 
     class ApplicationRecord < ActiveRecord::Base
       self.abstract_class = true
-      connects_to database: { writing: :primary, reading: :primary_replica }
+      connects_to database: { writing: :primary, restricted: :restricted }
     end
     class User < ApplicationRecord; end
 
     User.find_or_create_by!(name: 'foo')
     User.find_or_create_by!(name: 'bar')
 
     ActiveRecord::Base.connection.execute(<<~SQL)
-      CREATE USER IF NOT EXISTS '#{user_for_replica}'@'%' IDENTIFIED BY '#{ENV['MYSQL_PASSWORD']}'
+      CREATE USER IF NOT EXISTS '#{restricted_user}'@'%' IDENTIFIED BY '#{ENV['MYSQL_PASSWORD']}'
     SQL
     ActiveRecord::Base.connection.execute(<<~SQL)
-      GRANT SELECT, LOCK TABLES ON *.* To '#{user_for_replica}'@'%'
+      GRANT SELECT, LOCK TABLES ON *.* To '#{restricted_user}'@'%'
     SQL
   end
 end