From 05933cd275c880a64e3ed18dac85cce6cf5db4f1 Mon Sep 17 00:00:00 2001
From: Rob Hogan <rob@expo.dev>
Date: Wed, 16 Sep 2026 13:06:32 +0100
Subject: [PATCH] metro-file-map: Identify TreeFS directory nodes by shape, not
 realm

Summary:
`TreeFS` told directory nodes apart from file nodes with `node instanceof Map`, which holds only when the tree was built in the same realm as the code reading it. A tree restored through `TreeFS.fromDeserializedSnapshot` is used as-is, so a snapshot deserialized in another realm traverses wrongly: every directory below the root reads as a symlink and lookups through it fail with `Expected symlink target to be populated`, while `hierarchicalLookup` treats the root as a non-directory and returns null without probing it.

Nothing in a Metro process crosses a realm, but Jest does: each test file runs in its own `vm` context while `node:v8` is a host module, so a file map cache read back within a test holds `Map`s from another realm. The `metro` tests that build two `DependencyGraph`s on one config have been traversing such a tree, and pass only because the walks they make happen to fail in the direction their expectations need.

File nodes are metadata tuples, so `isDirectory` checks `!Array.isArray(node)` instead, which reads an internal slot and is realm-independent. The new test round-trips a snapshot through `v8.serialize`/`deserialize` and looks up a nested file in the result.

Changelog: [Internal]
---
 packages/metro-file-map/src/lib/TreeFS.js          |  2 +-
 .../src/lib/__tests__/TreeFS-test.js               | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/packages/metro-file-map/src/lib/TreeFS.js b/packages/metro-file-map/src/lib/TreeFS.js
index 3dc71cb7ff..5f78836665 100644
--- a/packages/metro-file-map/src/lib/TreeFS.js
+++ b/packages/metro-file-map/src/lib/TreeFS.js
@@ -30,7 +30,7 @@ type FileNode = FileMetadata;
 type MixedNode = FileNode | DirectoryNode;
 
 function isDirectory(node: ?MixedNode): node is DirectoryNode {
-  return node instanceof Map;
+  return node != null && !Array.isArray(node);
 }
 
 function isRegularFile(node: FileNode): boolean {
diff --git a/packages/metro-file-map/src/lib/__tests__/TreeFS-test.js b/packages/metro-file-map/src/lib/__tests__/TreeFS-test.js
index a94efce125..152ca14933 100644
--- a/packages/metro-file-map/src/lib/__tests__/TreeFS-test.js
+++ b/packages/metro-file-map/src/lib/__tests__/TreeFS-test.js
@@ -90,6 +90,20 @@ describe.each([['win32'], ['posix']])('TreeFS on %s', platform => {
     expect(tfs.exists(p('/project/link-to-nowhere'))).toBe(false);
   });
 
+  test('traverses a snapshot deserialized in another realm', () => {
+    // Under Jest, `node:v8` is a host module, so `deserialize` returns `Map`s
+    // whose prototype is not the sandbox's `Map.prototype`.
+    const {deserialize, serialize} = require('node:v8');
+    const deserialized = TreeFS.fromDeserializedSnapshot({
+      rootDir: p('/project'),
+      fileSystemData: deserialize(serialize(tfs.getSerializableSnapshot())),
+      processFile: () => {
+        throw new Error('Not implemented');
+      },
+    });
+    expect(deserialized.exists(p('/project/foo/another.js'))).toBe(true);
+  });
+
   test('implements linkStats()', () => {
     expect(tfs.linkStats(p('/project/link-to-foo/another.js'))).toEqual({
       fileType: 'f',