fix: address logging infinite recursion issue

This cropped up as a bug recently where Content Discovery were trying to
log an entire Request object, which references itself. This resulted in
Node.js running out of memory because we were infinitely recursing over
the properties.

This is a bug that's always been present in n-mask-logger.

I've addressed the issue with my first legit use of `WeakSet` and I'm
quite pleased with myself about it 😁

packages/logger/lib/transforms/legacy-mask.js

@@ -16,6 +16,8 @@
  *     A regular expression which applies masking to a string.
  * @property {string} maskString
  *     The mask string to apply to discovered sensitive values.
+ * @property {WeakSet} references
+ *     An internal store of references used to avoid masking the same object infinitely.
  */
 
 /**
@@ -69,6 +71,15 @@ function mask(value, settings) {
 	if (typeof value === 'string') {
 		return maskString(value, settings);
 	}
+
+	// Handle circular references
+	if (value && typeof value === 'object') {
+		if (settings.references.has(value)) {
+			return value;
+		}
+		settings.references.add(value);
+	}
+
 	if (Array.isArray(value)) {
 		return value.map((item) => mask(item, settings));
 	}
@@ -177,7 +188,8 @@ function createLegacyMaskTransform({
 		return maskObject(logData, {
 			maskedFields,
 			maskRegExp,
-			maskString
+			maskString,
+			references: new WeakSet()
 		});
 	};
 }

packages/logger/test/unit/lib/transforms/legacy-mask.spec.js

@@ -217,6 +217,18 @@ describe('@dotcom-reliability-kit/logger', () => {
 					expect(transform({ mock: 123 })).toEqual({ mock: 123 });
 				});
 			});
+
+			describe('when called with an object that references itself', () => {
+				it('does not recurse infinitely', () => {
+					const logData = {
+						email: 'mock'
+					};
+					logData.naughtyLittleSelfReference = logData;
+					const result = transform(logData);
+					expect(result.email).toEqual('*****');
+					expect(result.naughtyLittleSelfReference.email).toEqual('*****');
+				});
+			});
 		});
 
 		describe('when `options.denyList` is set', () => {