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added more comments on dir_init.rb git-svn-id: http://theory.cpe.ku.ac.th/grader/web/trunk@383 6386c4cd-e34a-4fa8-8920-d93eb39b512e
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r180:a8ad25e1c3fc - - 1 file changed: 19 inserted, 0 deleted

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@@ -1,107 +1,126
1 1 require 'ftools'
2 2
3 3 # DirInit::Manager handles directory initialization and clean-up when
4 4 # there are many concurrent processes that wants to modify the
5 5 # directory in the same way.
6 6 #
7 7 # An example usage is when each process wants to copy some temporary
8 8 # files to the directory and delete these files after finishing its
9 9 # job. Problems may occur when the first process delete the files
10 10 # while the second process is still using the files.
11 11 #
12 12 # This library maintain a reference counter on the processes using the
13 13 # directory. It locks the dir to manage critical section when
14 14 # updating the reference counter.
15 + #
16 + # Example usage:
17 + #
18 + # dman = DirInit::Manager.new("mydir")
19 + #
20 + # dman.setup do
21 + # # do some initialization
22 + # end
23 + #
24 + # #... do anything you want
25 + #
26 + # dman.teardown do
27 + # # clean up
28 + # end
29 + #
30 + # DirInit::Manager ensures that the block passed to <tt>setup</tt>
31 + # only runs once by the first process in the concurrent dir usage and
32 + # block passed to <tt>teardown</tt> runs once by the last process in
33 + # that concurrent activities leaving that dir.
15 34
16 35 module DirInit
17 36
18 37 class Manager
19 38
20 39 def initialize(dir_name, usage_filename='.usage_counter')
21 40 @dir_name = dir_name
22 41 @usage_filename = usage_filename
23 42 end
24 43
25 44 # Check if someone has initialized the dir. If not, call block.
26 45
27 46 def setup # :yields: block
28 47 dir = File.new(@dir_name)
29 48 dir.flock(File::LOCK_EX)
30 49 begin
31 50 counter_filename = get_counter_filename
32 51 if File.exist? counter_filename
33 52 # someone is here
34 53 f = File.new(counter_filename,"r+")
35 54 counter = f.read.to_i
36 55 f.seek(0)
37 56 f.write("#{counter+1}\n")
38 57 f.close
39 58 else
40 59 # i'm the first, create the counter file
41 60 counter = 0
42 61 f = File.new(counter_filename,"w")
43 62 f.write("1\n")
44 63 f.close
45 64 end
46 65
47 66 # if no one is here
48 67 if counter == 0
49 68 if block_given?
50 69 yield
51 70 end
52 71 end
53 72
54 73 rescue
55 74 raise
56 75
57 76 ensure
58 77 # make sure it unlock the directory
59 78 dir.flock(File::LOCK_UN)
60 79 end
61 80 end
62 81
63 82 # Check if I am the last one using the dir. If true, call block.
64 83
65 84 def teardown
66 85 dir = File.new(@dir_name)
67 86 dir.flock(File::LOCK_EX)
68 87 begin
69 88 counter_filename = get_counter_filename
70 89 if File.exist? counter_filename
71 90 # someone is here
72 91 f = File.new(counter_filename,"r+")
73 92 counter = f.read.to_i
74 93 f.seek(0)
75 94 f.write("#{counter-1}\n")
76 95 f.close
77 96
78 97 if counter == 1
79 98 # i'm the last one
80 99
81 100 File.delete(counter_filename)
82 101 if block_given?
83 102 yield
84 103 end
85 104 end
86 105 else
87 106 # This is BAD
88 107 raise "Error: reference count missing"
89 108 end
90 109
91 110 rescue
92 111 raise
93 112
94 113 ensure
95 114 # make sure it unlock the directory
96 115 dir.flock(File::LOCK_UN)
97 116 end
98 117 end
99 118
100 119 protected
101 120
102 121 def get_counter_filename
103 122 return File.join(@dir_name,@usage_filename)
104 123 end
105 124
106 125 end
107 126 end
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