feat(fs/unstable): add readLink and readLinkSync (#6373)

_tools/node_test_runner/run_test.mjs

@@ -51,6 +51,7 @@ import "../../collections/without_all_test.ts";
 import "../../collections/zip_test.ts";
 import "../../fs/unstable_link_test.ts";
 import "../../fs/unstable_read_dir_test.ts";
+import "../../fs/unstable_read_link_test.ts";
 import "../../fs/unstable_real_path_test.ts";
 import "../../fs/unstable_stat_test.ts";
 import "../../fs/unstable_symlink_test.ts";

fs/deno.json

@@ -17,6 +17,7 @@
     "./unstable-link": "./unstable_link.ts",
     "./unstable-lstat": "./unstable_lstat.ts",
     "./unstable-read-dir": "./unstable_read_dir.ts",
+    "./unstable-read-link": "./unstable_read_link.ts",
     "./unstable-real-path": "./unstable_real_path.ts",
     "./unstable-stat": "./unstable_stat.ts",
     "./unstable-symlink": "./unstable_symlink.ts",

fs/unstable_read_link.ts

@@ -0,0 +1,69 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+
+import { getNodeFs, isDeno } from "./_utils.ts";
+import { mapError } from "./_map_error.ts";
+
+/**
+ * Resolves to the path destination of the named symbolic link.
+ *
+ * Throws Error if called with a hard link.
+ *
+ * Requires `allow-read` permission.
+ *
+ * @example Usage
+ * ```ts ignore
+ * import { readLink } from "@std/fs/unstable-read-link";
+ * import { symlink } from "@std/fs/unstable-symlink";
+ * await symlink("./test.txt", "./test_link.txt");
+ * const target = await readLink("./test_link.txt"); // full path of ./test.txt
+ * ```
+ *
+ * @tags allow-read
+ *
+ * @param path The path of the symbolic link.
+ * @returns A promise that resolves to the file path pointed by the symbolic
+ * link.
+ */
+export async function readLink(path: string | URL): Promise<string> {
+  if (isDeno) {
+    return Deno.readLink(path);
+  } else {
+    try {
+      return await getNodeFs().promises.readlink(path);
+    } catch (error) {
+      throw mapError(error);
+    }
+  }
+}
+
+/**
+ * Synchronously returns the path destination of the named symbolic link.
+ *
+ * Throws Error if called with a hard link.
+ *
+ * Requires `allow-read` permission.
+ *
+ * @example Usage
+ * ```ts ignore
+ * import { readLinkSync } from "@std/fs/unstable-read-link";
+ * import { symlinkSync } from "@std/fs/unstable-symlink";
+ * symlinkSync("./test.txt", "./test_link.txt");
+ * const target = readLinkSync("./test_link.txt"); // full path of ./test.txt
+ * ```
+ *
+ * @tags allow-read
+ *
+ * @param path The path of the symbolic link.
+ * @returns The file path pointed by the symbolic link.
+ */
+export function readLinkSync(path: string | URL): string {
+  if (isDeno) {
+    return Deno.readLinkSync(path);
+  } else {
+    try {
+      return getNodeFs().readlinkSync(path);
+    } catch (error) {
+      throw mapError(error);
+    }
+  }
+}

fs/unstable_read_link_test.ts

@@ -0,0 +1,85 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+
+import { assertEquals, assertRejects, assertThrows } from "@std/assert";
+import { readLink, readLinkSync } from "./unstable_read_link.ts";
+import { NotFound } from "./unstable_errors.js";
+import {
+  linkSync,
+  mkdtempSync,
+  rmSync,
+  symlinkSync,
+  writeFileSync,
+} from "node:fs";
+import { link, mkdtemp, rm, symlink, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join, resolve } from "node:path";
+
+Deno.test("readLink() can read through symlink", async () => {
+  const tempDirPath = await mkdtemp(resolve(tmpdir(), "readLink_"));
+  const testFile = join(tempDirPath, "testFile.txt");
+  const symlinkFile = join(tempDirPath, "testFile.txt.link");
+
+  await writeFile(testFile, "Hello, Standard Library");
+  await symlink(testFile, symlinkFile);
+
+  const realFile = await readLink(symlinkFile);
+  assertEquals(testFile, realFile);
+
+  await rm(tempDirPath, { recursive: true, force: true });
+});
+
+Deno.test("readLink() rejects with Error when reading from a hard link", async () => {
+  const tempDirPath = await mkdtemp(resolve(tmpdir(), "readLink_"));
+  const testFile = join(tempDirPath, "testFile.txt");
+  const linkFile = join(tempDirPath, "testFile.txt.hlink");
+
+  await writeFile(testFile, "Hello, Standard Library");
+  await link(testFile, linkFile);
+
+  await assertRejects(async () => {
+    await readLink(linkFile);
+  }, Error);
+
+  await rm(tempDirPath, { recursive: true, force: true });
+});
+
+Deno.test("readLink() rejects with NotFound when reading through a non-existent file", async () => {
+  await assertRejects(async () => {
+    await readLink("non-existent-file.txt.link");
+  }, NotFound);
+});
+
+Deno.test("readLinkSync() can read through symlink", () => {
+  const tempDirPath = mkdtempSync(resolve(tmpdir(), "readLink_"));
+  const testFile = join(tempDirPath, "testFile.txt");
+  const symlinkFile = join(tempDirPath, "testFile.txt.link");
+
+  writeFileSync(testFile, "Hello, Standard Library");
+  symlinkSync(testFile, symlinkFile);
+
+  const realFile = readLinkSync(symlinkFile);
+  assertEquals(testFile, realFile);
+
+  rmSync(tempDirPath, { recursive: true, force: true });
+});
+
+Deno.test("readLinkSync() throws Error when reading from a hard link", () => {
+  const tempDirPath = mkdtempSync(resolve(tmpdir(), "readLinkSync_"));
+  const testFile = join(tempDirPath, "testFile.txt");
+  const linkFile = join(tempDirPath, "testFile.txt.hlink");
+
+  writeFileSync(testFile, "Hello, Standard Library!");
+  linkSync(testFile, linkFile);
+
+  assertThrows(() => {
+    readLinkSync(linkFile);
+  }, Error);
+
+  rmSync(tempDirPath, { recursive: true, force: true });
+});
+
+Deno.test("readLinkSync() throws NotFound when reading through a non-existent file", () => {
+  assertThrows(() => {
+    readLinkSync("non-existent-file.txt.hlink");
+  }, NotFound);
+});