Skip filter_strip_marker for symlinks during shipit mirror

Summary:
The `oss-mcrouter-darwin-getdeps` (T262442347) and `oss-mcrouter-windows-getdeps`
sandcastle jobs have been failing continuously since D93801016 landed
(2026-02-27) with:

```
FileNotFoundError: [Errno 2] No such file or directory:
'/data/sandcastle/temp/skycastle/fbcode_builder_getdeps/shipit/mcrouter/mcrouter/scripts/order_centos-7.2/15_fbthrift'
```

Root cause: mcrouter's `scripts/order_centos-7.2/` directory contains relative
symlinks like `15_fbthrift -> ../recipes/fbthrift.sh`. During
`ShipitPathMap.mirror()`, `os.walk` visits `order_centos-7.2/` before
`recipes/` (alphabetical order). For each file `copy_if_different()` reproduces
symlinks via `os.symlink(target, dest)` — which can be dangling because the
target hasn't been mirrored yet. The new `filter_strip_marker()` call added by
D93801016 then tries to `open(dest_name)`, follows the dangling symlink, and
crashes.

Fix: skip `filter_strip_marker` for symlinks. The actual target file gets its
markers stripped when `os.walk` visits it directly, so behavior on real files
is unchanged.

This unblocks external builds of mcrouter on macOS and Windows, which have
been broken for ~2 months. No production impact — internal mcrouter/uCache
builds use Buck2 and never go through this shipit/getdeps path.

Two earlier attempts addressed the Windows variant of the same bug but did not
land: D98509103 (abandoned) and D98518095 (unpublished, same approach as this
diff). This diff supersedes both and ties to the Darwin task.

#minion-generated-fix

Reviewed By: r-barnes

Differential Revision: D102330897

fbshipit-source-id: 72ca83526b66d088c4094235d8e8727414330d15

Author: egorovcharenko

build/fbcode_builder/getdeps/test/strip_marker_test.py

@@ -10,7 +10,7 @@
 import tempfile
 import unittest
 
-from ..fetcher import filter_strip_marker
+from ..fetcher import filter_strip_marker, ShipitPathMap
 from ..manifest import ManifestParser
 
 
@@ -157,3 +157,43 @@ def test_binary_file_skipped(self) -> None:
                 self.assertEqual(f.read(), binary_content)
         finally:
             os.unlink(path)
+
+
+class ShipitMirrorSymlinkTest(unittest.TestCase):
+    """Regression test for mcrouter OSS getdeps build (T262442347).
+
+    mcrouter's scripts/order_centos-7.2/15_fbthrift is a relative symlink to
+    ../recipes/fbthrift.sh. os.walk visits order_centos-7.2/ before recipes/
+    alphabetically, so when mirror() copied the symlink first, filter_strip_marker
+    followed the dangling link and crashed with FileNotFoundError. mirror() must
+    skip filter_strip_marker for symlinks; the actual target file gets filtered
+    when os.walk reaches it on its own.
+    """
+
+    def test_mirror_skips_filter_for_dangling_symlinks(self) -> None:
+        with tempfile.TemporaryDirectory() as src_root, tempfile.TemporaryDirectory() as dest_root:
+            project_dir = os.path.join(src_root, "proj")
+            order_dir = os.path.join(project_dir, "scripts", "order_centos-7.2")
+            recipes_dir = os.path.join(project_dir, "scripts", "recipes")
+            os.makedirs(order_dir)
+            os.makedirs(recipes_dir)
+
+            target_file = os.path.join(recipes_dir, "fbthrift.sh")
+            with open(target_file, "w") as f:
+                f.write("#!/bin/bash\necho hello\n")
+
+            symlink_path = os.path.join(order_dir, "15_fbthrift")
+            os.symlink("../recipes/fbthrift.sh", symlink_path)
+
+            mapping = ShipitPathMap()
+            mapping.add_mapping("proj", "proj")
+            mapping.mirror(src_root, dest_root)
+
+            mirrored_symlink = os.path.join(
+                dest_root, "proj", "scripts", "order_centos-7.2", "15_fbthrift"
+            )
+            mirrored_target = os.path.join(
+                dest_root, "proj", "scripts", "recipes", "fbthrift.sh"
+            )
+            self.assertTrue(os.path.islink(mirrored_symlink))
+            self.assertTrue(os.path.isfile(mirrored_target))