JavaScript heap out of memory during next build (Next 15.1)

[pchab]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/distracted-forest-vhlczj?workspaceId=ws_LGaEwH7iNTCahs4YK1orjP

To Reproduce

1. Have a moderately complex page to build (generateStaticParams + parallel routes)
2. next build
3. process will crash JavaScript heap out of memory (usually during the 'Collecting page data' step)

Current vs. Expected behavior

next build process doesn't crash.

Provide environment information

Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 15.1.1-canary.0 // Latest available version is detected (15.1.1-canary.0).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.3.3
Next.js Config:
  output: N/A


(Also tested with node 20)

Which area(s) are affected? (Select all that apply)

Not sure

Which stage(s) are affected? (Select all that apply)

next build (local), Vercel (Deployed)

Additional context

The crash doesn't happen if I reduce the size of the array returned by generateStaticParams or if I remove a couple parallel routes. This happens on all platform I tested: Vercel, locally and on codesandbox.io.
This used to work correctly with [email protected].

[icyJoseph]

Hi, could you move this to a Stackblitz instead of codesandbox? Just type next.new into your browser address bar, and redo the example there.

[pchab]

https://stackblitz.com/edit/nextjs-vo1ozofj

[pchab]

On stackblitz, the build doesn't crash but never completes and stay indefinitely on the collecting page data step

[icyJoseph]

I tried to use workers to get the build going, but...

uncaughtException [Error: Worker terminated due to reaching memory limit: JS heap out of memory] {
  code: 'ERR_WORKER_OUT_OF_MEMORY'
}

Otherwise, you get the:

<--- Last few GCs --->

[26998:0x138040000]    21645 ms: Scavenge 4051.8 (4120.5) -> 4049.6 (4123.7) MB, 6.1 / 0.0 ms  (average mu = 0.452, current mu = 0.157) allocation failure; 
[26998:0x138040000]    21653 ms: Scavenge 4055.0 (4123.7) -> 4052.4 (4142.0) MB, 6.5 / 0.0 ms  (average mu = 0.452, current mu = 0.157) allocation failure; 
[26998:0x138040000]    23958 ms: Mark-sweep 4065.0 (4142.0) -> 4056.9 (4150.2) MB, 2298.2 / 0.0 ms  (average mu = 0.253, current mu = 0.026) allocation failure; scavenge might not succeed


<--- JS stacktrace --->

And so on... so I guess, we have to debug the build script? 🤔

[icyJoseph]

Alright, I've done a binary search/bisect, which ever, and found that this has been introduced at 15.0.4-canary.50:

• https://github.com/vercel/next.js/releases/tag/v15.0.4-canary.50

Now it is time to check the changes introduced on that canary.

[icyJoseph]

Now I've been able to make a build work, by locally checking out this commit, and building Next.js up to that point:

• 1ed81f7

Which happens right before, this was merged #73358

Checking out, 92b3dd2, where #73358 sits, triggers the error, so in all likelihood, the error comes from that ISR fix for empty generateStaticParams

[icyJoseph]

🤔

next.js/packages/next/src/build/segment-config/app/app-segments.ts

Lines 115 to 120 in 7061f94

	// Recursively process parallel routes
	for (const parallelRouteKey in parallelRoutes) {
	const parallelRoute = parallelRoutes[parallelRouteKey]
	await processLoaderTree(parallelRoute, [...currentSegments])
	}
	}

Could that be the issue? But why, does it build up to 2x or so pages, but fails at several

It looks like the recursion does end... so nope

[icyJoseph]

Alright, so I found the issue I think,

    { slug: 'grault' },  { slug: 'garply' },  { slug: 'waldo' },
    { slug: 'fred' },    { slug: 'plugh' },   { slug: 'xyzzy' },
    { slug: 'thud' },    { slug: 'foo2' },    { slug: 'bar2' },
    { slug: 'baz2' },
    ... 17210268 more items

Within, buildAppStaticPaths, at the builtRouteParams, that can't be right... some checks are failing to contain this madness

I noticed that your repository example, can handle up to 33 routes, 34 kills it... probably the Array at hand goes way over the Array max length and boom

[icyJoseph]

Just to keep on dumping info, if I take again, 1ed81f7, and run a build, the max I see is 28 items in the parentsParams, whereas, taking the 92b3dd2, causes this massive growth

[icyJoseph]

One more thing before signing off.

I think these lines:

next.js/packages/next/src/build/segment-config/app/app-segments.ts

Lines 111 to 113 in 2de9722

	if (name === PAGE_SEGMENT_KEY) {
	segments.push(...currentSegments)
	}

Are introducing a lot of repeated routes, which ends up with the,

next.js/packages/next/src/build/utils.ts

Lines 1331 to 1334 in 2de9722

	async function builtRouteParams(
	parentsParams: Params[] = [],
	idx = 0
	): Promise<Params[]> {

parentParams growing exponentially.

If I do this tweak:

    // If this is a page segment, we know we've reached a leaf node associated with the
    // page we're collecting segments for. We can add the collected segments to our final result.
    if (name === PAGE_SEGMENT_KEY) {
      currentSegments.forEach((seg) => {
        if (
          segments.some(
            (other) =>
              other.name === seg.name &&
              other.filePath === seg.filePath &&
              other.param === seg.param
          )
        ) {
          return
        }
        segments.push(seg)
      })
    }

Then things build fine.

[pchab]

@icyJoseph Thanks a lot for your investigation so far. To give you an order of magnitude, I noticed the issue on an application where generateStaticParams is an array of 14 elements with 7 parallel routes (in a dashboard like page).
So not a massive amount of static params.

[icyJoseph]

@pchab yeah, I saw that it gets extremely bad very quickly with the number of parallel routes. This must be fixed soon!