fix(std/node): Stop callbacks being called twice when callback throws error (#8867)

std/node/_fs/_fs_exists_test.ts

@@ -1,5 +1,9 @@
 // Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
-import { assertEquals } from "../../testing/asserts.ts";
+import {
+  assert,
+  assertEquals,
+  assertStringIncludes,
+} from "../../testing/asserts.ts";
 import { exists, existsSync } from "./_fs_exists.ts";
 
 Deno.test("existsFile", async function () {
@@ -23,3 +27,32 @@ Deno.test("existsSyncFile", function () {
   Deno.removeSync(tmpFilePath);
   assertEquals(existsSync("./notAvailable.txt"), false);
 });
+
+Deno.test("[std/node/fs] exists callback isn't called twice if error is thrown", async () => {
+  // This doesn't use `assertCallbackErrorUncaught()` because `exists()` doesn't return a standard node callback, which is what it expects.
+  const tempFile = await Deno.makeTempFile();
+  const importUrl = new URL("./_fs_exists.ts", import.meta.url);
+  const p = Deno.run({
+    cmd: [
+      Deno.execPath(),
+      "eval",
+      "--no-check",
+      `
+      import { exists } from ${JSON.stringify(importUrl)};
+
+      exists(${JSON.stringify(tempFile)}, (exists) => {
+        // If the bug is present and the callback is called again with false (meaning an error occured),
+        // don't throw another error, so if the subprocess fails we know it had the correct behaviour.
+        if (exists) throw new Error("success");
+      });`,
+    ],
+    stderr: "piped",
+  });
+  const status = await p.status();
+  const stderr = new TextDecoder().decode(await Deno.readAll(p.stderr));
+  p.close();
+  p.stderr.close();
+  await Deno.remove(tempFile);
+  assert(!status.success);
+  assertStringIncludes(stderr, "Error: success");
+});