fix permission errors are swallowed by fs.exists (#3493)

std/fs/exists_test.ts

@@ -46,3 +46,96 @@ test(async function existsLink(): Promise<void> {
   // in repository
   assertEquals(await exists(path.join(testdataDir, "0-link.ts")), true);
 });
+
+test(async function existsPermission(): Promise<void> {
+  interface Scenes {
+    read: boolean; // --allow-read
+    async: boolean;
+    output: string;
+    file: string; // target file to run
+  }
+
+  const scenes: Scenes[] = [
+    // 1
+    {
+      read: false,
+      async: true,
+      output: "run again with the --allow-read flag",
+      file: "0.ts"
+    },
+    {
+      read: false,
+      async: false,
+      output: "run again with the --allow-read flag",
+      file: "0.ts"
+    },
+    // 2
+    {
+      read: true,
+      async: true,
+      output: "exist",
+      file: "0.ts"
+    },
+    {
+      read: true,
+      async: false,
+      output: "exist",
+      file: "0.ts"
+    },
+    // 3
+    {
+      read: false,
+      async: true,
+      output: "run again with the --allow-read flag",
+      file: "no_exist_file_for_test.ts"
+    },
+    {
+      read: false,
+      async: false,
+      output: "run again with the --allow-read flag",
+      file: "no_exist_file_for_test.ts"
+    },
+    // 4
+    {
+      read: true,
+      async: true,
+      output: "not exist",
+      file: "no_exist_file_for_test.ts"
+    },
+    {
+      read: true,
+      async: false,
+      output: "not exist",
+      file: "no_exist_file_for_test.ts"
+    }
+  ];
+
+  for (const s of scenes) {
+    console.log(
+      `test ${s.async ? "exists" : "existsSync"}("testdata/${s.file}") ${
+        s.read ? "with" : "without"
+      } --allow-read`
+    );
+
+    const args = [Deno.execPath(), "run"];
+
+    if (s.read) {
+      args.push("--allow-read");
+    }
+
+    args.push(path.join(testdataDir, s.async ? "exists.ts" : "exists_sync.ts"));
+    args.push(s.file);
+
+    const { stdout } = Deno.run({
+      stdout: "piped",
+      cwd: testdataDir,
+      args: args
+    });
+
+    const output = await Deno.readAll(stdout);
+
+    assertEquals(new TextDecoder().decode(output), s.output);
+  }
+
+  // done
+});