[miniflare] Support workerd V8 flags via MINIFLARE_WORKERD_V8_FLAGS environment variable

[Sipixer]

Relates to #14701 (immediate mitigation — the underlying heap retention and crash recovery are tracked there and in #13045).

Support passing V8 flags to workerd via a MINIFLARE_WORKERD_V8_FLAGS environment variable, mirroring the existing MINIFLARE_WORKERD_AUTOGATES passthrough (#8431). The generated workerd config already supports v8Flags; Miniflare just never populated it, so the runtime always runs with V8's default ~1.4 GB heap limit and there is no way to raise it.

Large dev applications served through @cloudflare/vite-plugin can reach that limit during a normal editing session (each server-file edit re-evaluates the invalidated module graph inside the isolate — measurements in #14701). When it happens, workerd aborts with V8 fatal error; location = Reached heap limit and every subsequent dispatchFetch() returns fetch failed until the dev server is manually restarted. Setting e.g. MINIFLARE_WORKERD_V8_FLAGS="--max-old-space-size=4096" keeps those sessions alive.

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• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows: patched a local install and verified the flag reaches V8 — with MINIFLARE_WORKERD_V8_FLAGS="--max-old-space-size=256" workerd OOMs at exactly 255.9 MB on first render (vs the ~1.4 GB default); with --max-old-space-size=4096, 70 consecutive edit/request cycles that previously crashed the runtime complete with zero failures. Also verified the variable being unset produces a config identical to before (empty v8Flags list).
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: this mirrors the undocumented MINIFLARE_WORKERD_AUTOGATES escape hatch; both are internal/debugging environment variables.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 4136d01

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
miniflare	Minor
@cloudflare/deploy-helpers	Patch
@cloudflare/pages-shared	Patch
@cloudflare/runtime-types	Patch
@cloudflare/vite-plugin	Patch
@cloudflare/vitest-pool-workers	Patch
wrangler	Patch

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[workers-devprod]

Codeowners approval required for this PR:

• @cloudflare/wrangler

Show detailed file reviewers

• .changeset/miniflare-v8-flags-env.md: [@cloudflare/wrangler]
• packages/miniflare/src/index.ts: [@cloudflare/wrangler]
• packages/miniflare/types/env.d.ts: [@cloudflare/wrangler]

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[petebacondarwin]

I think that the fact that the memory keeps growing in a Vite setup on each build indicates we have a real memory leak in the system somewhere. So far we have not been able to identify the cause, but we would rather try to fix it than patch over it with an increased heap size.

We're going to have a dig around to see if we can fix the leak, but if not then we can return to this fix later next week.

[Sipixer]

I dug into the leak you mentioned and prototyped a fix (13 lines in the bundled runner-worker, −58% live memory, stack traces unaffected) — details and measurements in #14701 (comment).