first commit

README

@@ -0,0 +1,24 @@
+Just a simple class to determine if another process has already locked a file on a unix filesystem.
+
+Example:
+
+IRB 1>>
+p = ProcessLock.new('example.tmp')
+p.owner?
+=> false
+p.aquire!
+=> true
+p.owner?
+=> true
+
+IRB 2>>
+q = ProcessLock.new('example.tmp')
+p.owner?
+=> false
+p.aquire!
+=> false
+p.alive?
+=> true
+
+Copyright (c) 2008 Simon Engledew, released under the MIT license
+

process_lock.rb

@@ -0,0 +1,42 @@
+class ProcessLock
+
+  def initialize(filename)
+    FileUtils.touch(@filename = filename)
+  end
+
+  def aquire!
+    File.open(@filename, 'r+') do |f|
+      lock(f, false) do
+        f.truncate(f.write(Process.pid))
+        return true
+      end
+    end
+    return false
+  end
+
+  def alive?
+    pid = read
+    return pid > 0 ? Process.kill(0, pid) > 0 : false
+  rescue
+    return false
+  end
+
+  def owner?
+    pid = read
+    pid and pid > 0 and pid == Process.pid
+  end
+
+  def read
+    File.open(@filename, 'r+'){|f|lock(f){f.read.to_i}}
+  end
+
+  private
+
+  def lock(file, blocking = true)
+    file.flock(blocking ? File::LOCK_EX : File::LOCK_EX | File::LOCK_NB)
+    return yield
+  ensure
+    file.flock(File::LOCK_UN)
+  end
+
+end