Replace recursive values instead of omitting them

Instead of ignoring recursive values, replace them with a placeholder
string.

.changesets/ignore-recursively-nested-values-on-sanitize.md

@@ -3,4 +3,4 @@ bump: "patch"
 type: "change"
 ---
 
-When sanitizing an array or hash, ignore recursively nested values. This fixes a SystemStackError issue when sanitising arrays and hashes.
+When sanitizing an array or hash, replace recursively nested values with a placeholder string. This fixes a SystemStackError issue when sanitising arrays and hashes.

lib/appsignal/utils/hash_sanitizer.rb

@@ -5,6 +5,7 @@ module Utils
     # @api private
     class HashSanitizer
       FILTERED = "[FILTERED]"
+      RECURSIVE = "[RECURSIVE VALUE]"
 
       class << self
         def sanitize(value, filter_keys = [])
@@ -31,10 +32,10 @@ def sanitize_hash(source, filter_keys, seen)
 
           {}.tap do |hash|
             source.each_pair do |key, value|
-              next if seen.include?(value.object_id)
-
               hash[key] =
-                if filter_keys.include?(key.to_s)
+                if seen.include?(value.object_id)
+                  RECURSIVE
+                elsif filter_keys.include?(key.to_s)
                   FILTERED
                 else
                   sanitize_value(value, filter_keys, seen)
@@ -48,9 +49,12 @@ def sanitize_array(source, filter_keys, seen)
 
           [].tap do |array|
             source.each_with_index do |item, index|
-              next if seen.include?(item.object_id)
-
-              array[index] = sanitize_value(item, filter_keys, seen)
+              array[index] =
+                if seen.include?(item.object_id)
+                  RECURSIVE
+                else
+                  sanitize_value(item, filter_keys, seen)
+                end
             end
           end
         end

spec/lib/appsignal/utils/hash_sanitizer_spec.rb

@@ -103,22 +103,22 @@
             expect(subject[:file]).to include "::UploadedFile"
           end
 
-          it "ignores a recursive array" do
-            expect(subject[:recursive_array]).to be_nil
+          it "replaces a recursive array" do
+            expect(subject[:recursive_array]).to eq("[RECURSIVE VALUE]")
           end
 
-          it "ignores a recursive hash" do
-            expect(subject[:recursive_hash]).to be_nil
+          it "replaces a recursive hash" do
+            expect(subject[:recursive_hash]).to eq("[RECURSIVE VALUE]")
           end
         end
 
         describe "nested array" do
-          it "ignores a recursive array" do
-            expect(sanitized_params[:hash][:nested_array][4]).to be_nil
+          it "replaces a recursive array" do
+            expect(sanitized_params[:hash][:nested_array][4]).to eq("[RECURSIVE VALUE]")
           end
 
-          it "ignores a recursive hash" do
-            expect(sanitized_params[:hash][:nested_array][5]).to be_nil
+          it "replaces a recursive hash" do
+            expect(sanitized_params[:hash][:nested_array][5]).to eq("[RECURSIVE VALUE]")
           end
         end
       end