Extract allowed chars for GUIDs to constant

Also: disallow special chars at the end of a GUID

docs/federation/types.md

@@ -31,6 +31,8 @@ A network-wide, unique identifier. A random string of at least 16 and at most 25
 * Numbers: `0-9`
 * Special chars: `-`, `_`, `@`, `.` and `:`
 
+ Special chars aren't allowed at the end.
+
 Example: `298962a0b8dc0133e40d406c8f31e210`
 
 ## String

lib/diaspora_federation/validators/rules/guid.rb

@@ -6,7 +6,11 @@ module Validation
     # * Letters: a-z
     # * Numbers: 0-9
     # * Special chars: '-', '_', '@', '.' and ':'
+    # Special chars aren't allowed at the end.
     class Guid
+      # Allowed chars to validate a GUID with a regex
+      VALID_CHARS = "[0-9A-Za-z\\-_@.:]{15,254}[0-9a-z]".freeze
+
       # The error key for this rule
       # @return [Symbol] error key
       def error_key
@@ -15,7 +19,7 @@ module Validation
 
       # Determines if value is a valid +GUID+
       def valid_value?(value)
-        value.is_a?(String) && value.downcase =~ /\A[0-9a-z\-_@.:]{16,255}\z/
+        value.is_a?(String) && value =~ /\A#{VALID_CHARS}\z/
       end
 
       # This rule has no params.

spec/lib/diaspora_federation/validators/rules/guid_spec.rb

@@ -45,6 +45,14 @@ describe Validation::Rule::Guid do
       expect(validator.errors).to include(:guid)
     end
 
+    it "fails if the string contains special chars at the end" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: "abcdef0123456789."))
+      validator.rule(:guid, :guid)
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:guid)
+    end
+
     it "fails if the string contains invalid chars" do
       validator = Validation::Validator.new(OpenStruct.new(guid: "ghijklmnopqrstuvwxyz++"))
       validator.rule(:guid, :guid)