Fix #4244 - Cached Attributes is not thread safe

The underlying hash is not protected and thus two threads accessing
the cached value at the same time and one expiring the value can result
in a race condition.

This patch synchronizes the access to the value_cache underlying hash.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>

1 files changed, 15 insertions, 10 deletions

diff --git a/lib/puppet/util/cacher.rb b/lib/puppet/util/cacher.rb
index 8785c694f..3dddec0d4 100644
--- a/lib/puppet/util/cacher.rb
+++ b/lib/puppet/util/cacher.rb
@@ -1,3 +1,5 @@
+require 'monitor'
+
 module Puppet::Util::Cacher
   module Expirer
     attr_reader :timestamp
@@ -49,7 +51,7 @@ module Puppet::Util::Cacher
       define_method(name.to_s + "=") do |value|
         # Make sure the cache timestamp is set
         cache_timestamp
-        value_cache[name] = value
+        value_cache.synchronize { value_cache[name] = value }
       end
 
       if ttl = options[:ttl]
@@ -70,6 +72,7 @@ module Puppet::Util::Cacher
 
   # Methods that get added to instances.
   module InstanceMethods
+
     def expire
       # Only expire if we have an expirer.  This is
       # mostly so that we can comfortably handle cases
@@ -92,15 +95,17 @@ module Puppet::Util::Cacher
     end
 
     def cached_value(name)
-      # Allow a nil expirer, in which case we regenerate the value every time.
-      if expired_by_expirer?(name)
-        value_cache.clear
-        @cache_timestamp = Time.now
-      elsif expired_by_ttl?(name)
-        value_cache.delete(name)
+      value_cache.synchronize do
+        # Allow a nil expirer, in which case we regenerate the value every time.
+        if expired_by_expirer?(name)
+          value_cache.clear
+          @cache_timestamp = Time.now
+        elsif expired_by_ttl?(name)
+          value_cache.delete(name)
+        end
+        value_cache[name] = send("init_#{name}") unless value_cache.include?(name)
+        value_cache[name]
       end
-      value_cache[name] = send("init_#{name}") unless value_cache.include?(name)
-      value_cache[name]
     end
 
     def expired_by_expirer?(name)
@@ -121,7 +126,7 @@ module Puppet::Util::Cacher
     end
 
     def value_cache
-      @value_cache ||= {}
+      @value_cache ||= {}.extend(MonitorMixin)
     end
   end
 end