tests: extend readFile file length during read (#12835)

This commit adds some tests that demonstrate that Deno.readFile reads
the entire file, even if the read file is extended during read.

cli/tests/unit/read_file_test.ts

@@ -1,4 +1,5 @@
 // Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+import { writeAllSync } from "../../../test_util/std/io/util.ts";
 import {
   assert,
   assertEquals,
@@ -115,3 +116,42 @@ unitTest(
     });
   },
 );
+
+unitTest(
+  { permissions: { read: true, write: true } },
+  async function readFileExtendedDuringRead() {
+    // Write 128MB file
+    const filename = Deno.makeTempDirSync() + "/test.txt";
+    const data = new Uint8Array(1024 * 1024 * 128);
+    Deno.writeFileSync(filename, data);
+    const promise = Deno.readFile(filename);
+    queueMicrotask(() => {
+      // Append 128MB to file
+      const f = Deno.openSync(filename, { append: true });
+      writeAllSync(f, data);
+      f.close();
+    });
+    const read = await promise;
+    assertEquals(read.byteLength, data.byteLength * 2);
+  },
+);
+
+unitTest(
+  { permissions: { read: true, write: true } },
+  async function readFile0LengthExtendedDuringRead() {
+    // Write 0 byte file
+    const filename = Deno.makeTempDirSync() + "/test.txt";
+    const first = new Uint8Array(0);
+    const second = new Uint8Array(1024 * 1024 * 128);
+    Deno.writeFileSync(filename, first);
+    const promise = Deno.readFile(filename);
+    queueMicrotask(() => {
+      // Append 128MB to file
+      const f = Deno.openSync(filename, { append: true });
+      writeAllSync(f, second);
+      f.close();
+    });
+    const read = await promise;
+    assertEquals(read.byteLength, second.byteLength);
+  },
+);