Masking: Avoid the repeated calls to closest when recursing through the DOM (#1349)

* masking performance: avoid the repeated calls to `closest` when recursing through the DOM
 - needsMask===true means that an ancestor has tested positively for masking, and so this node and all descendents should be masked
 - needsMask===false means that no ancestors have tested positively for masking, we should check each node encountered
 - needsMask===undefined means that we don't know whether ancestors are masked or not (e.g. after a mutation) and should look up the tree
* Add tests including an explicit characterData mutation tests 
* Further performance improvement: avoid calls to `el.matches` when on a leaf node, e.g. a `<br/>`
---------

Authored-by: eoghanmurray <eoghan@getthere.ie>
Based on initial PR #1338 by Alexey Babik <alexey.babik@noibu.com>

packages/rrweb-snapshot/src/snapshot.ts

@@ -310,6 +310,7 @@ export function needMaskingText(
   node: Node,
   maskTextClass: string | RegExp,
   maskTextSelector: string | null,
+  checkAncestors: boolean,
 ): boolean {
   try {
     const el: HTMLElement | null =
@@ -317,17 +318,21 @@ export function needMaskingText(
         ? (node as HTMLElement)
         : node.parentElement;
     if (el === null) return false;
-
     if (typeof maskTextClass === 'string') {
-      if (el.classList.contains(maskTextClass)) return true;
-      if (el.closest(`.${maskTextClass}`)) return true;
+      if (checkAncestors) {
+        if (el.closest(`.${maskTextClass}`)) return true;
+      } else {
+        if (el.classList.contains(maskTextClass)) return true;
+      }
     } else {
-      if (classMatchesRegex(el, maskTextClass, true)) return true;
+      if (classMatchesRegex(el, maskTextClass, checkAncestors)) return true;
     }
-
     if (maskTextSelector) {
-      if (el.matches(maskTextSelector)) return true;
-      if (el.closest(maskTextSelector)) return true;
+      if (checkAncestors) {
+        if (el.closest(maskTextSelector)) return true;
+      } else {
+        if (el.matches(maskTextSelector)) return true;
+      }
     }
   } catch (e) {
     //
@@ -426,8 +431,7 @@ function serializeNode(
     mirror: Mirror;
     blockClass: string | RegExp;
     blockSelector: string | null;
-    maskTextClass: string | RegExp;
-    maskTextSelector: string | null;
+    needsMask: boolean | undefined;
     inlineStylesheet: boolean;
     maskInputOptions: MaskInputOptions;
     maskTextFn: MaskTextFn | undefined;
@@ -447,8 +451,7 @@ function serializeNode(
     mirror,
     blockClass,
     blockSelector,
-    maskTextClass,
-    maskTextSelector,
+    needsMask,
     inlineStylesheet,
     maskInputOptions = {},
     maskTextFn,
@@ -500,8 +503,7 @@ function serializeNode(
       });
     case n.TEXT_NODE:
       return serializeTextNode(n as Text, {
-        maskTextClass,
-        maskTextSelector,
+        needsMask,
         maskTextFn,
         rootId,
       });
@@ -531,13 +533,12 @@ function getRootId(doc: Document, mirror: Mirror): number | undefined {
 function serializeTextNode(
   n: Text,
   options: {
-    maskTextClass: string | RegExp;
-    maskTextSelector: string | null;
+    needsMask: boolean | undefined;
     maskTextFn: MaskTextFn | undefined;
     rootId: number | undefined;
   },
 ): serializedNode {
-  const { maskTextClass, maskTextSelector, maskTextFn, rootId } = options;
+  const { needsMask, maskTextFn, rootId } = options;
   // The parent node may not be a html element which has a tagName attribute.
   // So just let it be undefined which is ok in this use case.
   const parentTagName = n.parentNode && (n.parentNode as HTMLElement).tagName;
@@ -568,12 +569,7 @@ function serializeTextNode(
   if (isScript) {
     textContent = 'SCRIPT_PLACEHOLDER';
   }
-  if (
-    !isStyle &&
-    !isScript &&
-    textContent &&
-    needMaskingText(n, maskTextClass, maskTextSelector)
-  ) {
+  if (!isStyle && !isScript && textContent && needsMask) {
     textContent = maskTextFn
       ? maskTextFn(textContent, n.parentElement)
       : textContent.replace(/[\S]/g, '*');
@@ -935,6 +931,7 @@ export function serializeNodeWithId(
     inlineStylesheet: boolean;
     newlyAddedElement?: boolean;
     maskInputOptions?: MaskInputOptions;
+    needsMask?: boolean;
     maskTextFn: MaskTextFn | undefined;
     maskInputFn: MaskInputFn | undefined;
     slimDOMOptions: SlimDOMOptions;
@@ -980,14 +977,29 @@ export function serializeNodeWithId(
     keepIframeSrcFn = () => false,
     newlyAddedElement = false,
   } = options;
+  let { needsMask } = options;
   let { preserveWhiteSpace = true } = options;
+
+  if (
+    !needsMask &&
+    n.childNodes // we can avoid the check on leaf elements, as masking is applied to child text nodes only
+  ) {
+    // perf: if needsMask = true, children won't also need to check
+    const checkAncestors = needsMask === undefined; // if false, we've already checked ancestors
+    needsMask = needMaskingText(
+      n as Element,
+      maskTextClass,
+      maskTextSelector,
+      checkAncestors,
+    );
+  }
+
   const _serializedNode = serializeNode(n, {
     doc,
     mirror,
     blockClass,
     blockSelector,
-    maskTextClass,
-    maskTextSelector,
+    needsMask,
     inlineStylesheet,
     maskInputOptions,
     maskTextFn,
@@ -1058,6 +1070,7 @@ export function serializeNodeWithId(
       mirror,
       blockClass,
       blockSelector,
+      needsMask,
       maskTextClass,
       maskTextSelector,
       skipChild,
@@ -1118,6 +1131,7 @@ export function serializeNodeWithId(
             mirror,
             blockClass,
             blockSelector,
+            needsMask,
             maskTextClass,
             maskTextSelector,
             skipChild: false,
@@ -1165,6 +1179,7 @@ export function serializeNodeWithId(
             mirror,
             blockClass,
             blockSelector,
+            needsMask,
             maskTextClass,
             maskTextSelector,
             skipChild: false,