[miniflare] Support workerd V8 flags via MINIFLARE_WORKERD_V8_FLAGS environment variable

.changeset/miniflare-v8-flags-env.md

@@ -0,0 +1,9 @@
+---
+"miniflare": minor
+---
+
+Support passing V8 flags to `workerd` via the `MINIFLARE_WORKERD_V8_FLAGS` environment variable
+
+The generated `workerd` config already supports `v8Flags`, but Miniflare never populated it, so the runtime always ran with V8's default heap limit (~1.4 GB). Large dev applications (e.g. big SSR module graphs under `@cloudflare/vite-plugin`, where each server-file edit grows the runner isolate's heap) can reach that limit, at which point `workerd` aborts with `V8 fatal error; location = Reached heap limit` and every subsequent `dispatchFetch()` fails with `fetch failed` until the dev server is manually restarted.
+
+Setting e.g. `MINIFLARE_WORKERD_V8_FLAGS="--max-old-space-size=4096"` raises the limit and keeps long dev sessions alive. The variable follows the same space-separated format as `MINIFLARE_WORKERD_AUTOGATES`.

packages/miniflare/src/index.ts

@@ -2456,6 +2456,9 @@ export class Miniflare {
 			autogates: process.env.MINIFLARE_WORKERD_AUTOGATES
 				? process.env.MINIFLARE_WORKERD_AUTOGATES.split(" ")
 				: [],
+			v8Flags: process.env.MINIFLARE_WORKERD_V8_FLAGS
+				? process.env.MINIFLARE_WORKERD_V8_FLAGS.split(" ")
+				: [],
 		};
 	}
 

packages/miniflare/types/env.d.ts

@@ -6,5 +6,6 @@ declare namespace NodeJS {
 		MINIFLARE_WORKERD_CONFIG_DEBUG?: string;
 		MINIFLARE_ASSERT_BODIES_CONSUMED?: "true";
 		MINIFLARE_WORKERD_AUTOGATES?: string;
+		MINIFLARE_WORKERD_V8_FLAGS?: string;
 	}
 }