Fix Relationship.instances cache for thread safety

Taken from randym#477

lib/axlsx/package.rb

@@ -100,11 +100,13 @@ def workbook=(workbook) DataTypeValidator.validate :Package_workbook, Workbook,
     #   File.open('example_streamed.xlsx', 'w') { |f| f.write(s.read) }
     def serialize(output, confirm_valid=false)
       return false unless !confirm_valid || self.validate.empty?
-      Relationship.clear_cached_instances
+      Relationship.initialize_cached_instances
       Zip::OutputStream.open(output) do |zip|
         write_parts(zip)
       end
       true
+    ensure
+      Relationship.clear_cached_instances
     end
 
 
@@ -113,11 +115,13 @@ def serialize(output, confirm_valid=false)
     # @return [StringIO|Boolean] False if confirm_valid and validation errors exist. rewound string IO if not.
     def to_stream(confirm_valid=false)
       return false unless !confirm_valid || self.validate.empty?
-      Relationship.clear_cached_instances
+      Relationship.initialize_cached_instances
       zip = write_parts(Zip::OutputStream.new(StringIO.new, true))
       stream = zip.close_buffer
       stream.rewind
       stream
+    ensure
+      Relationship.clear_cached_instances
     end
 
     # Encrypt the package into a CFB using the password provided

lib/axlsx/rels/relationship.rb

@@ -3,43 +3,51 @@ module Axlsx
   # A relationship defines a reference between package parts.
   # @note Packages automatically manage relationships.
   class Relationship
-    
+
     class << self
       # Keeps track of all instances of this class.
       # @return [Array]
       def instances
-        @instances ||= []
+        Thread.current[:axlsx_relationship_cached_instances] ||= []
       end
-      
-      # Clear cached instances.
-      # 
+
+      # Initialize cached instances.
+      #
       # This should be called before serializing a package (see {Package#serialize} and
-      # {Package#to_stream}) to make sure that serialization is idempotent (i.e. 
+      # {Package#to_stream}) to make sure that serialization is idempotent (i.e.
       # Relationship instances are generated with the same IDs everytime the package
       # is serialized).
-      # 
-      # Also, calling this avoids memory leaks (cached instances lingering around 
-      # forever). 
+      #
+      # Also, calling this avoids memory leaks (cached instances lingering around
+      # forever).
+      def initialize_cached_instances
+        Thread.current[:axlsx_relationship_cached_instances] = []
+      end
+
+      # Clear cached instances.
+      #
+      # This should be called after serializing a package (see {Package#serialize} and
+      # {Package#to_stream}) to free the memory allocated for cache.
       def clear_cached_instances
-        @instances = []
+        Thread.current[:axlsx_relationship_cached_instances] = nil
       end
-      
-      # Generate and return a unique id (eg. `rId123`) Used for setting {#Id}. 
+
+      # Generate and return a unique id (eg. `rId123`) Used for setting {#Id}.
       #
       # The generated id depends on the number of cached instances, so using
       # {clear_cached_instances} will automatically reset the generated ids, too.
       # @return [String]
       def next_free_id
-        "rId#{@instances.size + 1}"
+        "rId#{instances.size + 1}"
       end
     end
 
-    # The id of the relationship (eg. "rId123"). Most instances get their own unique id. 
+    # The id of the relationship (eg. "rId123"). Most instances get their own unique id.
     # However, some instances need to share the same id – see {#should_use_same_id_as?}
     # for details.
     # @return [String]
     attr_reader :Id
-    
+
     # The location of the relationship target
     # @return [String]
     attr_reader :Target
@@ -69,8 +77,8 @@ def next_free_id
     # The source object the relations belongs to (e.g. a hyperlink, drawing, ...). Needed when
     # looking up the relationship for a specific object (see {Relationships#for}).
     attr_reader :source_obj
-    
-    # Initializes a new relationship. 
+
+    # Initializes a new relationship.
     # @param [Object] source_obj see {#source_obj}
     # @param [String] type The type of the relationship
     # @param [String] target The target for the relationship
@@ -105,12 +113,12 @@ def to_xml_string(str = '')
       str << (h.map { |key, value| '' << key.to_s << '="' << Axlsx::coder.encode(value.to_s) << '"'}.join(' '))
       str << '/>'
     end
-    
+
     # Whether this relationship should use the same id as `other`.
     #
     # Instances designating the same relationship need to use the same id. We can not simply
-    # compare the {#Target} attribute, though: `foo/bar.xml`, `../foo/bar.xml`, 
-    # `../../foo/bar.xml` etc. are all different but probably mean the same file (this 
+    # compare the {#Target} attribute, though: `foo/bar.xml`, `../foo/bar.xml`,
+    # `../../foo/bar.xml` etc. are all different but probably mean the same file (this
     # is especially an issue for relationships in the context of pivot tables). So lets
     # just ignore this attribute for now (except when {#TargetMode} is set to `:External` –
     # then {#Target} will be an absolute URL and thus can safely be compared).
@@ -124,6 +132,6 @@ def should_use_same_id_as?(other)
       end
       result
     end
-    
+
   end
 end