first commit!

.gitignore

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+*.gem
+*.rbc
+*.idea
+.bundle
+.config
+.yardoc
+Gemfile.lock
+InstalledFiles
+_yardoc
+coverage
+doc/
+lib/bundler/man
+pkg
+rdoc
+spec/reports
+test/tmp
+test/version_tmp
+tmp

.travis.yml

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+language: ruby
+
+rvm:
+  - 1.8.7
+  - 1.9.3
+
+env:
+  - DB=sqlite
+  - DB=mysql
+  - DB=pg
+
+script: bundle exec rake
+
+before_script:
+  - mysql -e 'create database with_advisory_lock_test'
+  - psql -c 'create database with_advisory_lock_test' -U postgres
+

Gemfile

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+source 'https://rubygems.org'
+
+gemspec

LICENSE.txt

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+Copyright (c) 2013 Matthew McEachen
+
+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# with_advisory_lock [![Build Status](https://api.travis-ci.org/mceachen/with_advisory_lock.png?branch=master)](https://travis-ci.org/mceachen/with_advisory_lock)
+
+Adds advisory locking to ActiveRecord 3.x. MySQL and PostgreSQL are supported natively.
+SQLite resorts to file locking (which won't span hosts, of course).
+
+## Usage
+
+```ruby
+Tag.with_advisory_lock(lock_name) do
+  do_something_that_needs_locking
+end
+```
+
+### What happens
+
+1. The thread will wait indefinitely until the lock is acquired.
+2. While inside the block, you will exclusively own the advisory lock.
+3. The lock will be released after your block ends, even if an exception is raised in the block.
+
+### Lock wait timeouts
+
+The second parameter for ```with_advisory_lock``` is ```timeout_seconds```, and defaults to ```nil```,
+which means wait indefinitely for the lock.
+
+If a non-nil value is provided, the block may not be invoked.
+
+The return value of ```with_advisory_lock``` will be the result of the yielded block,
+if the lock was able to be acquired and the block yielded, or ```false```, if you provided
+a timeout_seconds value and the lock was not able to be acquired in time.
+
+### When to use
+
+If you want to prevent duplicate inserts, and there isn't a row to lock yet, you need
+a [shared mutex](http://en.wikipedia.org/wiki/Mutual_exclusion), either though
+a [table-level lock](https://github.com/mceachen/monogamy), or through an advisory lock.
+
+When possible, use [optimistic](http://api.rubyonrails.org/classes/ActiveRecord/Locking/Optimistic.html)
+or [pessimistic](http://api.rubyonrails.org/classes/ActiveRecord/Locking/Pessimistic.html) row locking.
+
+## Installation
+
+Add this line to your application's Gemfile:
+
+``` ruby
+gem 'with_advisory_lock'
+```
+
+And then execute:
+
+    $ bundle
+
+## Changelog
+
+### 0.0.1
+
+* First whack

Rakefile

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+require "bundler/gem_tasks"
+
+require 'yard'
+YARD::Rake::YardocTask.new do |t|
+  t.files = ['lib/**/*.rb', 'README.md']
+end
+
+require 'rake/testtask'
+
+Rake::TestTask.new do |t|
+  t.libs.push "lib"
+  t.libs.push "test"
+  t.pattern = 'test/**/*_test.rb'
+  t.verbose = true
+end
+
+task :default => :test

lib/with_advisory_lock.rb

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+require 'with_advisory_lock/concern'
+
+ActiveSupport.on_load :active_record do
+  ActiveRecord::Base.send :include, WithAdvisoryLock::Concern
+end

lib/with_advisory_lock/base.rb

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+module WithAdvisoryLock
+  class Base
+    attr_reader :connection, :lock_name, :timeout_seconds
+
+    def initialize(connection, lock_name, timeout_seconds)
+      @connection = connection
+      @lock_name = lock_name
+      @timeout_seconds = timeout_seconds
+    end
+
+    def quoted_lock_name
+      connection.quote(lock_name)
+    end
+
+    def with_advisory_lock(&block)
+      give_up_at = Time.now + @timeout_seconds if @timeout_seconds
+      while @timeout_seconds.nil? || Time.now < give_up_at do
+        if try_lock
+          begin
+            return yield
+          ensure
+            release_lock
+          end
+        else
+          sleep(0.1)
+        end
+      end
+      false # failed to get lock in time.
+    end
+  end
+end

lib/with_advisory_lock/concern.rb

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+# Tried desperately to monkeypatch the polymorphic connection object,
+# but rails autoloading is too clever by half. Pull requests are welcome.
+
+# Think of this module as a hipster, using "case" ironically.
+
+require 'with_advisory_lock/base'
+require 'with_advisory_lock/mysql'
+require 'with_advisory_lock/postgresql'
+require 'with_advisory_lock/flock'
+require 'active_support/concern'
+
+module WithAdvisoryLock
+  module Concern
+    extend ActiveSupport::Concern
+
+    def with_advisory_lock(lock_name, timeout_seconds=nil, &block)
+      self.class.with_advisory_lock(lock_name, timeout_seconds, &block)
+    end
+
+    module ClassMethods
+      def with_advisory_lock(lock_name, timeout_seconds=nil, &block)
+        case (connection.adapter_name.downcase)
+          when "postgresql"
+            WithAdvisoryLock::PostgreSQL
+          when "mysql", "mysql2"
+            WithAdvisoryLock::MySQL
+          else
+            WithAdvisoryLock::Flock
+        end.new(connection, lock_name, timeout_seconds).with_advisory_lock(&block)
+      end
+    end
+  end
+end

lib/with_advisory_lock/flock.rb

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+require 'fileutils'
+
+module WithAdvisoryLock
+  class Flock < Base
+
+    def filename
+      @filename ||= begin
+        safe = @lock_name.gsub(/[^a-z0-9]/i, '')
+        fn = ".lock-#{safe}-#{@lock_name.to_s.hash}"
+        # Let the user specify a directory besides CWD.
+        ENV['FLOCK_DIR'] ? File.expand_path(fn, ENV['FLOCK_DIR']) : fn
+      end
+    end
+
+    def file_io
+      @file_io ||= begin
+        FileUtils.touch(filename)
+        File.open(filename, 'r+')
+      end
+    end
+
+    def try_lock
+      0 == file_io.flock(File::LOCK_EX|File::LOCK_NB)
+    end
+
+    def release_lock
+      0 == file_io.flock(File::LOCK_UN)
+    end
+  end
+end

lib/with_advisory_lock/mysql.rb

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+module WithAdvisoryLock
+  class MySQL < Base
+
+    # See http://dev.mysql.com/doc/refman/5.0/en/miscellaneous-functions.html#function_get-lock
+
+    def try_lock
+      # Returns 1 if the lock was obtained successfully,
+      # 0 if the attempt timed out (for example, because another client has
+      # previously locked the name), or NULL if an error occurred
+      # (such as running out of memory or the thread was killed with mysqladmin kill).
+      1 == connection.select_value("SELECT GET_LOCK(#{quoted_lock_name}, 0)")
+    end
+
+    def release_lock
+      # Returns 1 if the lock was released,
+      # 0 if the lock was not established by this thread (
+      # in which case the lock is not released), and
+      # NULL if the named lock did not exist.
+      1 == connection.select_value("SELECT RELEASE_LOCK(#{quoted_lock_name})")
+    end
+  end
+end

lib/with_advisory_lock/postgresql.rb

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+module WithAdvisoryLock
+  class PostgreSQL < Base
+
+    # See http://www.postgresql.org/docs/9.1/static/functions-admin.html#FUNCTIONS-ADVISORY-LOCKS
+
+    def try_lock
+      # pg_try_advisory_lock will either obtain the lock immediately
+      # and return true, or return false if the lock cannot be acquired immediately
+      "t" == connection.select_value("SELECT pg_try_advisory_lock(#{numeric_lock})")
+    end
+
+    def release_lock
+      "t" == connection.select_value("SELECT pg_advisory_unlock(#{numeric_lock})")
+    end
+
+    def numeric_lock
+      @numeric_lock ||= begin
+        if lock_name.is_a? Numeric
+          lock_name.to_i
+        else
+          lock_name.to_s.hash
+        end
+      end
+    end
+  end
+end

lib/with_advisory_lock/version.rb

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+module WithAdvisoryLock
+  VERSION = "0.0.1"
+end

test/database.yml

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+sqlite:
+  adapter: <%= "jdbc" if defined? JRUBY_VERSION %>sqlite3
+  database: test.sqlite3.db
+  pool: 50
+pg:
+  adapter: postgresql
+  username: postgres
+  database: with_advisory_lock_test
+  min_messages: ERROR
+  pool: 50
+mysql:
+  adapter: mysql2
+  host: localhost
+  username: root
+  database: with_advisory_lock_test
+  pool: 50

test/minitest_helper.rb

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+require 'erb'
+require 'active_record'
+require 'with_advisory_lock'
+require 'database_cleaner'
+require 'tmpdir'
+
+db_config = File.expand_path("database.yml", File.dirname(__FILE__))
+ActiveRecord::Base.configurations = YAML::load(ERB.new(IO.read(db_config)).result)
+ActiveRecord::Base.establish_connection(ENV["DB"] || "sqlite")
+ActiveRecord::Migration.verbose = false
+
+require 'test_models'
+require 'minitest/autorun'
+
+Thread.abort_on_exception = true
+
+DatabaseCleaner.strategy = :deletion
+class MiniTest::Spec
+  before :each do
+    DatabaseCleaner.start
+  end
+  after :each do
+    DatabaseCleaner.clean
+  end
+  before :all do
+    ENV['FLOCK_DIR'] = Dir.mktmpdir
+  end
+  after :all do
+    FileUtils.remove_entry_secure ENV['FLOCK_DIR']
+  end
+end
+

test/test_models.rb

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+ActiveRecord::Schema.define(:version => 0) do
+  create_table "tags", :force => true do |t|
+    t.string "name"
+  end
+  create_table "tag_audits", :id => false, :force => true do |t|
+    t.string "tag_name"
+  end
+  create_table "labels", :id => false, :force => true do |t|
+    t.string "name"
+  end
+end
+
+class Tag < ActiveRecord::Base
+  after_save do
+    TagAudit.create { |ea| ea.tag_name = name }
+    Label.create { |ea| ea.name = name }
+  end
+end
+
+class TagAudit < ActiveRecord::Base
+end
+
+class Label < ActiveRecord::Base
+end