Example file locking in Deno (Deno.flock) - filesystems

Question

I realize that it's currently unstable, but does anyone have an example of using exclusive file locking with Deno? Or for that matter, how to get/use the resource id for a specific file path.
Deno.flock

Answer 1

Here's the test that was included in the PR that implemented the feature. It answers both of your questions:
Note that it has evolved since the PR.
flock_test.ts:
// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
import { assertEquals, unitTest } from "./test_util.ts";
unitTest(
{ perms: { read: true, run: true, hrtime: true } },
async function flockFileSync() {
const path = "cli/tests/testdata/fixture.json";
const script = (exclusive: boolean, wait: number) => `
const { rid } = Deno.openSync("${path}");
Deno.flockSync(rid, ${exclusive ? "true" : "false"});
await new Promise(res => setTimeout(res, ${wait}));
Deno.funlockSync(rid);
`;
const run = (e: boolean, w: number) =>
Deno.run({ cmd: [Deno.execPath(), "eval", "--unstable", script(e, w)] });
const firstBlocksSecond = async (
first: boolean,
second: boolean,
): Promise<boolean> => {
const firstPs = run(first, 1000);
await new Promise((res) => setTimeout(res, 250));
const start = performance.now();
const secondPs = run(second, 0);
await secondPs.status();
const didBlock = (performance.now() - start) > 500;
firstPs.close();
secondPs.close();
return didBlock;
};
assertEquals(
await firstBlocksSecond(true, false),
true,
"exclusive blocks shared",
);
assertEquals(
await firstBlocksSecond(false, true),
true,
"shared blocks exclusive",
);
assertEquals(
await firstBlocksSecond(true, true),
true,
"exclusive blocks exclusive",
);
assertEquals(
await firstBlocksSecond(false, false),
false,
"shared does not block shared",
);
},
);
unitTest(
{ perms: { read: true, run: true, hrtime: true } },
async function flockFileAsync() {
const path = "cli/tests/testdata/fixture.json";
const script = (exclusive: boolean, wait: number) => `
const { rid } = await Deno.open("${path}");
await Deno.flock(rid, ${exclusive ? "true" : "false"});
await new Promise(res => setTimeout(res, ${wait}));
await Deno.funlock(rid);
`;
const run = (e: boolean, w: number) =>
Deno.run({ cmd: [Deno.execPath(), "eval", "--unstable", script(e, w)] });
const firstBlocksSecond = async (
first: boolean,
second: boolean,
): Promise<boolean> => {
const firstPs = run(first, 1000);
await new Promise((res) => setTimeout(res, 250));
const start = performance.now();
const secondPs = run(second, 0);
await secondPs.status();
const didBlock = (performance.now() - start) > 500;
firstPs.close();
secondPs.close();
return didBlock;
};
assertEquals(
await firstBlocksSecond(true, false),
true,
"exclusive blocks shared",
);
assertEquals(
await firstBlocksSecond(false, true),
true,
"shared blocks exclusive",
);
assertEquals(
await firstBlocksSecond(true, true),
true,
"exclusive blocks exclusive",
);
assertEquals(
await firstBlocksSecond(false, false),
false,
"shared does not block shared",
);
},
);