[RUM-13793] 🐛 Skip potential sanitize updates on unaltered fields

packages/core/src/tools/serialisation/sanitize.ts

@@ -27,7 +27,7 @@ interface SanitizedEvent extends Context {
 
 // The maximum size of a single event is 256KiB. By default, we ensure that user-provided data
 // going through sanitize fits inside our events, while leaving room for other contexts, metadata, ...
-const SANITIZE_DEFAULT_MAX_CHARACTER_COUNT = 220 * ONE_KIBI_BYTE
+export const SANITIZE_DEFAULT_MAX_CHARACTER_COUNT = 220 * ONE_KIBI_BYTE
 
 // Symbol for the root element of the JSONPath used for visited objects
 const JSON_PATH_ROOT_ELEMENT = '$'

packages/rum-core/src/domain/limitModification.spec.ts

@@ -1,5 +1,5 @@
 import type { Context } from '@datadog/browser-core'
-import { objectEntries } from '@datadog/browser-core'
+import { display, noop, objectEntries, SANITIZE_DEFAULT_MAX_CHARACTER_COUNT } from '@datadog/browser-core'
 import type { ModifiableFieldPaths } from './limitModification'
 import { limitModification } from './limitModification'
 
@@ -194,6 +194,38 @@ describe('limitModification', () => {
     limitModification(object, { bar: 'object' }, modifier)
     expect(() => JSON.stringify(object)).not.toThrowError()
   })
+
+  it("should not reset unsafe literal fields that the user didn't alter", () => {
+    spyOn(display, 'warn')
+    const wayTooLongUrl = `/${'a'.repeat(SANITIZE_DEFAULT_MAX_CHARACTER_COUNT + 1)}`
+    const object: Context = { resource: { url: wayTooLongUrl } }
+    const modifier = noop
+    limitModification(object, { 'resource.url': 'string' }, modifier)
+    expect(object).toEqual({ resource: { url: wayTooLongUrl } })
+  })
+
+  it('should sanitize object fields (fieldType "object") even with a noop modifier', () => {
+    spyOn(display, 'warn')
+    const wayTooLongUrl = `/${'a'.repeat(SANITIZE_DEFAULT_MAX_CHARACTER_COUNT + 1)}`
+    const object: Context = {
+      context: {
+        response: {
+          status: 200,
+          url: wayTooLongUrl,
+        },
+      },
+    }
+    const modifier = noop
+    limitModification(object, { context: 'object' }, modifier)
+    expect(object).toEqual({
+      context: {
+        response: {
+          status: 200,
+        },
+      },
+    })
+    expect(display.warn).toHaveBeenCalled()
+  })
 })
 
 function generateModifiableFieldPathsFrom(object: Record<string, string | object>) {

packages/rum-core/src/domain/limitModification.ts

@@ -7,6 +7,10 @@ export type ModifiableFieldPaths = Record<string, 'string' | 'object'>
  * Allows declaring and enforcing modifications to specific fields of an object.
  * Only supports modifying properties of an object (even if nested in an array).
  * Does not support array manipulation (adding/removing items).
+ *
+ * Modifications of the object are sanitized only if the field was actually changed by the modifier function (i.e., value is different).
+ * This ensures consistent SDK behavior regardless of whether limitModification is called.
+ * Only string fields are handled this way, object fields are sanitized regardless of whether the modifier function was called or not.
  */
 export function limitModification<T extends Context, Result>(
   object: T,
@@ -52,6 +56,10 @@ function setNestedValue(
   value: unknown,
   fieldType: 'string' | 'object'
 ) {
+  if (object[field] === value) {
+    return
+  }
+
   const newType = getType(value)
 
   if (newType === fieldType) {