2 files changed, 128 insertions, 0 deletions
diff --git a/cli/dts/lib.deno.ns.d.ts b/cli/dts/lib.deno.ns.d.ts
index 420a08da4..996aff363 100644
--- a/cli/dts/lib.deno.ns.d.ts
+++ b/cli/dts/lib.deno.ns.d.ts
@@ -787,6 +787,32 @@ declare namespace Deno {
    */
   export function fdatasync(rid: number): Promise<void>;
 
+  /** **UNSTABLE**: New API should be tested first.
+   *
+   * Acquire an advisory file-system lock for the provided file. `exclusive`
+   * defaults to `false`.
+   */
+  export function flock(rid: number, exclusive?: boolean): Promise<void>;
+
+  /** **UNSTABLE**: New API should be tested first.
+   *
+   * Acquire an advisory file-system lock for the provided file. `exclusive`
+   * defaults to `false`.
+   */
+  export function flockSync(rid: number, exclusive?: boolean): void;
+
+  /** **UNSTABLE**: New API should be tested first.
+   *
+   * Release an advisory file-system lock for the provided file.
+   */
+  export function funlock(rid: number): Promise<void>;
+
+  /** **UNSTABLE**: New API should be tested first.
+   *
+   * Release an advisory file-system lock for the provided file.
+   */
+  export function funlockSync(rid: number): void;
+
   /** Close the given resource ID (rid) which has been previously opened, such
    * as via opening or creating a file.  Closing a file when you are finished
    * with it is important to avoid leaking resources.
diff --git a/cli/tests/unit/flock_test.ts b/cli/tests/unit/flock_test.ts
new file mode 100644
index 000000000..13d09bcf5
--- /dev/null
+++ b/cli/tests/unit/flock_test.ts
@@ -0,0 +1,102 @@
+// 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",
+    );
+  },
+);