Created
May 31, 2017 01:56
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| class HashRace | |
| def initialize | |
| @clients = {} | |
| 50.times {add_client} | |
| end | |
| def call | |
| Thread.new do | |
| purge | |
| end | |
| main | |
| end | |
| private | |
| def main | |
| i = 0 | |
| loop do | |
| snooze | |
| if rand(2) == 1 | |
| k = add_client | |
| puts "new! #{k}" | |
| end | |
| snooze | |
| @clients.each { |key, client| | |
| puts "* #{i} hi #{key}" | |
| } | |
| puts '' | |
| i += 1 | |
| end | |
| end | |
| def purge | |
| loop do | |
| snooze | |
| @clients.each { |key, client| | |
| snooze | |
| if rand(10) == 1 | |
| puts "* deleting key: #{key}" | |
| @clients.delete(key) | |
| end | |
| } | |
| end | |
| end | |
| def add_client | |
| k = rand_string | |
| v = rand_string | |
| @clients[k] = v | |
| k | |
| end | |
| def rand(n) | |
| [*1..n].sample | |
| end | |
| def snooze | |
| sleep (1.0 / ([*1..5].sample)) | |
| end | |
| def rand_string | |
| (0...8).map { ('A'.ord + rand(26)).chr }.join | |
| end | |
| end | |
| t = HashRace.new | |
| t.() |
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This is an example to illustrate a race condition that can occur between two threads. Running it (eventually) leads to an error
hash_race.rb:51:in `add_client': can't add a new key into hash during iteration (RuntimeError).There are two threads: a main thread that occasionally adds a new item ("client") to a hash and iterates over the hash to print out names of clients in the hash; and a purge thread that iterates over the hash of clients and occasionally deletes an element.
The problem occurs when the main thread adds an element to the hash while the purge thread is iterating over it. One way to solve this is by using a mutex, like this.
Here's some sample output: