fix(react-refresh-utils): avoid memory leaks caused by prevExports (#…

…53797)

This fixes memory leaks caused by `prevExports` in react-refresh-utils.
It happens in code like the following:
```tsx
const DATA = Array.from({ length: 100000 }, (_, i) => Math.random());

export const App = () => {
  return (
    <div>
      <div>REWRITE_HERE</div>
      <div>{DATA.length}</div>
    </div>
  );
};
```

After we edit this file to trigger fast refresh, previous `DATA` will be
still retained in the memory since it forms `App(new) -> prevExports ->
App(old) -> DATA` reference chain (there is some screenshots
[here](pmmmwh/react-refresh-webpack-plugin#766)).
I believe there is no reason to retain the whole exports as
`prevExports`. We can just retain "signature" (`string[]`). By only
holding this, we no longer create reference to the old exports, which
fixes the memory leak here. Note that I filed a similar PR in
pmmmwh/react-refresh-webpack-plugin#766 and also
https://github.com/naruaway-sandbox/fast-refresh-hmr-memory-leak-demo is
a reproducible example of this issue, which also explains that
interestingly this issue is not easily solved for Vite.


## Should we fix it?
I think yes, as long as there is no unintended side effect, it's better
to fix it since we cannot predict whether users would load large payload
AND does Fast Refresh many times without reloading the browser or not.
In [this extreme
case](https://github.com/naruaway-sandbox/fast-refresh-hmr-memory-leak-demo),
it eats several hundred mega bytes of RAM.

## Verification
I confirmed that the memory leak is gone with this change by running
https://github.com/naruaway-sandbox/fast-refresh-hmr-memory-leak-demo
with the change.

I am not sure whether new tests are needed but my concern is to
accidentally break Fast Refresh behavior somehow. I believe we have
enough existing test cases 🙏 and I also tested manually.

packages/react-refresh-utils/internal/ReactRefreshModule.runtime.ts

@@ -30,7 +30,8 @@ export default function () {
       // @ts-ignore __webpack_module__ is global
       var currentExports = __webpack_module__.exports
       // @ts-ignore __webpack_module__ is global
-      var prevExports = __webpack_module__.hot.data?.prevExports ?? null
+      var prevSignature: unknown[] | null =
+        __webpack_module__.hot.data?.prevSignature ?? null
 
       // This cannot happen in MainTemplate because the exports mismatch between
       // templating and execution.
@@ -42,10 +43,11 @@ export default function () {
       // A module can be accepted automatically based on its exports, e.g. when
       // it is a Refresh Boundary.
       if (self.$RefreshHelpers$.isReactRefreshBoundary(currentExports)) {
-        // Save the previous exports on update so we can compare the boundary
-        // signatures.
+        // Save the previous exports signature on update so we can compare the boundary
+        // signatures. We avoid saving exports themselves since it causes memory leaks (https://github.com/vercel/next.js/pull/53797)
         __webpack_module__.hot.dispose(function (data) {
-          data.prevExports = currentExports
+          data.prevSignature =
+            self.$RefreshHelpers$.getRefreshBoundarySignature(currentExports)
         })
         // Unconditionally accept an update to this module, we'll check if it's
         // still a Refresh Boundary later.
@@ -55,7 +57,7 @@ export default function () {
         // This field is set when the previous version of this module was a
         // Refresh Boundary, letting us know we need to check for invalidation or
         // enqueue an update.
-        if (prevExports !== null) {
+        if (prevSignature !== null) {
           // A boundary can become ineligible if its exports are incompatible
           // with the previous exports.
           //
@@ -65,8 +67,8 @@ export default function () {
           // function, we want to invalidate the boundary.
           if (
             self.$RefreshHelpers$.shouldInvalidateReactRefreshBoundary(
-              prevExports,
-              currentExports
+              prevSignature,
+              self.$RefreshHelpers$.getRefreshBoundarySignature(currentExports)
             )
           ) {
             __webpack_module__.hot.invalidate()
@@ -79,7 +81,7 @@ export default function () {
         // new exports made it ineligible for being a boundary.
         // We only care about the case when we were _previously_ a boundary,
         // because we already accepted this update (accidental side effect).
-        var isNoLongerABoundary = prevExports !== null
+        var isNoLongerABoundary = prevSignature !== null
         if (isNoLongerABoundary) {
           __webpack_module__.hot.invalidate()
         }

packages/react-refresh-utils/internal/helpers.ts

@@ -122,11 +122,9 @@ function isReactRefreshBoundary(moduleExports: unknown): boolean {
 }
 
 function shouldInvalidateReactRefreshBoundary(
-  prevExports: unknown,
-  nextExports: unknown
+  prevSignature: unknown[],
+  nextSignature: unknown[]
 ): boolean {
-  var prevSignature = getRefreshBoundarySignature(prevExports)
-  var nextSignature = getRefreshBoundarySignature(nextExports)
   if (prevSignature.length !== nextSignature.length) {
     return true
   }