Keep a symlinked config.json linked when Paseo saves it (#6023)

* fix(server): keep a symlinked config.json linked when saving

Paseo saved config.json by renaming a temporary file over its path, which
replaced a symlink with a regular file and left the linked file unchanged.
Private file writes now resolve the symlink and replace the file it points at.

Closes #5932

* test(server): check a symlinked config.json stays private

---------

Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>

packages/server/src/server/persisted-config.test.ts

@@ -1,4 +1,14 @@
-import { chmodSync, mkdtempSync, rmSync, statSync, writeFileSync } from "node:fs";
+import {
+  chmodSync,
+  lstatSync,
+  mkdirSync,
+  mkdtempSync,
+  readFileSync,
+  rmSync,
+  statSync,
+  symlinkSync,
+  writeFileSync,
+} from "node:fs";
 import { tmpdir } from "node:os";
 import path from "node:path";
 import { describe, expect, test } from "vitest";
@@ -843,3 +853,35 @@ describe.skipIf(process.platform === "win32")("persisted config file permissions
     }
   });
 });
+
+describe.skipIf(process.platform === "win32")(
+  "config.json symlinked from another directory",
+  () => {
+    test("saving writes through the symlink and leaves it in place", () => {
+      const parent = createTempHome();
+      const home = path.join(parent, "home");
+      const dotfiles = path.join(parent, "dotfiles");
+      const linkedConfigPath = path.join(dotfiles, "config.json");
+      const configPath = path.join(home, "config.json");
+      try {
+        mkdirSync(home);
+        mkdirSync(dotfiles, { mode: 0o755 });
+        chmodSync(dotfiles, 0o755);
+        writeFileSync(linkedConfigPath, '{ "version": 1 }\n');
+        symlinkSync(linkedConfigPath, configPath);
+
+        savePersistedConfig(home, { version: 1, daemon: { relay: { enabled: false } } });
+
+        expect(lstatSync(configPath).isSymbolicLink()).toBe(true);
+        expect(JSON.parse(readFileSync(linkedConfigPath, "utf8"))).toEqual({
+          version: 1,
+          daemon: { relay: { enabled: false } },
+        });
+        expect(modeOf(dotfiles)).toBe(0o755);
+        expect(modeOf(linkedConfigPath)).toBe(PRIVATE_FILE_MODE);
+      } finally {
+        rmSync(parent, { recursive: true, force: true });
+      }
+    });
+  },
+);

packages/server/src/server/private-files.ts

@@ -1,4 +1,4 @@
-import { chmodSync, mkdirSync, renameSync, rmSync, writeFileSync } from "node:fs";
+import { chmodSync, mkdirSync, realpathSync, renameSync, rmSync, writeFileSync } from "node:fs";
 import { randomUUID } from "node:crypto";
 import path from "node:path";
 
@@ -28,14 +28,27 @@ export function writePrivateFileAtomicSync(
   data: string | NodeJS.ArrayBufferView,
 ): void {
   ensurePrivateDirectory(path.dirname(filePath));
-  const parent = path.dirname(filePath);
-  const temporary = path.join(parent, `.${path.basename(filePath)}.${process.pid}.${randomUUID()}`);
+  const destination = resolveSymlinkedFile(filePath);
+  const temporary = path.join(
+    path.dirname(destination),
+    `.${path.basename(destination)}.${process.pid}.${randomUUID()}`,
+  );
   try {
     writeFileSync(temporary, data, { mode: PRIVATE_FILE_MODE });
-    renameSync(temporary, filePath);
-    ensurePrivateFile(filePath);
+    renameSync(temporary, destination);
+    ensurePrivateFile(destination);
   } catch (error) {
     rmSync(temporary, { force: true });
     throw error;
   }
 }
+
+// Renaming onto a symlink replaces the link, so write to the file it points at.
+function resolveSymlinkedFile(filePath: string): string {
+  try {
+    return realpathSync(filePath);
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code === "ENOENT") return filePath;
+    throw error;
+  }
+}