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Make System.gc collect under the java-web command #9

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Project516 merged 2 commits into from
Nov 1, 2026
Merged

Make System.gc collect under the java-web command #9

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Make System.gc collect under the java-web command, which node starts …
Project516 Nov 1, 2026
f8cf683
Leave V8 flags alone when they are frozen or gc is renamed, and requi…
Project516 Nov 1, 2026
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15 changes: 15 additions & 0 deletions src/node.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,8 @@ import { isAssertionFlag, parseAssertions } from "./assertions.ts";
import * as fs from "node:fs";
import * as os from "node:os";
import * as path from "node:path";
import { setFlagsFromString } from "node:v8";
import { runInNewContext } from "node:vm";
import { fileURLToPath, pathToFileURL } from "node:url";
import { VFS } from "./fs.ts";
import type { Backend, Stat } from "./fs.ts";
Expand Down Expand Up @@ -180,7 +182,20 @@ function fixedInput(data: Uint8Array) {
const bridges = new WeakMap<object, Bridge>();

// The host file system is the VFS root, with the JDK image mounted at /jdk.
// System.gc() calls globalThis.gc, which node defines only under --expose-gc
function exposeGc() {
const g = globalThis as { gc?: unknown };
if (typeof g.gc === "function") return;
// V8 aborts on a flag change once flags are frozen
if (/(^|\s)--freeze-flags-after-init\b/.test([...process.execArgv, process.env.NODE_OPTIONS ?? ""].join(" "))) return;
setFlagsFromString("--expose-gc");
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// absent when --expose-gc-as gave it another name
const gc = runInNewContext("typeof gc === 'function' ? gc : undefined");
if (gc) g.gc = gc;
}

function nodeVm(o: NodeRunOptions): JVM {
exposeGc();
const vfs = new VFS();
vfs.mount("/", new NodeBackend("/", true));
// built as dist/node.js, it reads the packaged image next to it
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8 changes: 8 additions & 0 deletions test/programs.test.ts
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Expand Up @@ -56,6 +56,14 @@ for (const file of files) {
});
}

// the java-web command, which node starts without --expose-gc
test("System.gc collects under the command", { skip: !files.includes("Refs.java") }, () => {
const ref = spawnSync(hotspot("java"), ["-cp", out, "Refs"], { encoding: "utf8" });
assert.equal(ref.status, 0);
const got = spawnSync(process.execPath, [join(root, "src", "node.ts"), "-cp", out, "Refs"], { encoding: "utf8" });
assert.deepEqual({ stdout: got.stdout, code: got.status }, { stdout: ref.stdout, code: ref.status });
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});

function flags(file: string, tool: string): string[] {
const m = new RegExp(`^// ${tool}: (.*)$`, "m").exec(readFileSync(join(src, file), "utf8"));
return m ? m[1].split(" ") : [];
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