# daemon_controller, library for robust daemon management
# Copyright (c) 2010 Phusion
# 
# 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.

require 'fcntl'

class DaemonController

# A lock file is a synchronization mechanism, like a Mutex, but it also allows
# inter-process synchronization (as opposed to only inter-thread synchronization
# within a single process).
#
# Processes can obtain either a shared lock or an exclusive lock. It's possible
# for multiple processes to obtain a shared lock on a file as long as no
# exclusive lock has been obtained by a process. If a process has obtained an
# exclusive lock, then no other processes can lock the file, whether they're
# trying to obtain a shared lock or an exclusive lock.
#
# Note that on JRuby, LockFile can only guarantee synchronization between
# threads if the different threads use the same LockFile object. Specifying the
# same filename is not enough.
class LockFile
	class AlreadyLocked < StandardError
	end
	
	# Create a LockFile object. The lock file is initially not locked.
	#
	# +filename+ may point to a nonexistant file. In that case, the lock
	# file will not be created until one's trying to obtain a lock.
	#
	# Note that LockFile will use this exact filename. So if +filename+
	# is a relative filename, then the actual lock file that will be used
	# depends on the current working directory.
	def initialize(filename)
		@filename = filename
	end
	
	# Obtain an exclusive lock on the lock file, yield the given block,
	# then unlock the lockfile. If the lock file was already locked (whether
	# shared or exclusively) by another process/thread then this method will
	# block until the lock file has been unlocked.
	#
	# The lock file *must* be writable, otherwise an Errno::EACCESS
	# exception will be raised.
	def exclusive_lock
		File.open(@filename, 'w') do |f|
			if Fcntl.const_defined? :F_SETFD
				f.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
			end
			f.flock(File::LOCK_EX)
			yield
		end
	end
	
	# Obtain an exclusive lock on the lock file, yield the given block,
	# then unlock the lockfile. If the lock file was already exclusively
	# locked by another process/thread then this method will
	# block until the exclusive lock has been released. This method will not
	# block if only shared locks have been obtained.
	#
	# The lock file *must* be writable, otherwise an Errno::EACCESS
	# exception will be raised.
	def shared_lock
		File.open(@filename, 'w+') do |f|
			if Fcntl.const_defined? :F_SETFD
				f.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
			end
			f.flock(File::LOCK_SH)
			yield
		end
	end
	
	# Try to obtain a shared lock on the lock file, similar to #shared_lock.
	# But unlike #shared_lock, this method will raise AlreadyLocked if
	# no lock can be obtained, instead of blocking.
	#
	# If a lock can be obtained, then the given block will be yielded.
	def try_shared_lock
		File.open(@filename, 'w+') do |f|
			if Fcntl.const_defined? :F_SETFD
				f.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
			end
			if f.flock(File::LOCK_SH | File::LOCK_NB)
				yield
			else
				raise AlreadyLocked
			end
		end
	end
	
	# Try to obtain an exclusive lock on the lock file, similar to #exclusive_lock.
	# But unlike #exclusive_lock, this method will raise AlreadyLocked if
	# no lock can be obtained, instead of blocking.
	#
	# If a lock can be obtained, then the given block will be yielded.
	def try_exclusive_lock
		File.open(@filename, 'w') do |f|
			if Fcntl.const_defined? :F_SETFD
				f.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
			end
			if f.flock(File::LOCK_EX | File::LOCK_NB)
				yield
			else
				raise AlreadyLocked
			end
		end
	end
end # class LockFile
end # class DaemonController
