fix: fall back to an absolute path when relpath crosses drives

samcli/commands/_utils/template.py

@@ -399,11 +399,19 @@ def _resolve_relative_to(path, original_root, new_root):
         return None
 
     # Value is definitely a relative path. Change it relative to the destination directory
-    return os.path.relpath(
-        # Resolve the paths to take care of symlinks
-        os.path.normpath(os.path.join(pathlib.Path(original_root).resolve(), path)),
-        pathlib.Path(new_root).resolve(),  # Absolute original path w.r.t ``original_root``
-    )  # Resolve the original path with respect to ``new_root``
+    # Resolve the paths to take care of symlinks
+    absolute_path = os.path.normpath(os.path.join(pathlib.Path(original_root).resolve(), path))
+    try:
+        return os.path.relpath(
+            absolute_path,
+            pathlib.Path(new_root).resolve(),  # Absolute original path w.r.t ``original_root``
+        )  # Resolve the original path with respect to ``new_root``
+    except ValueError:
+        # os.path.relpath raises ValueError when the two paths are on different drives or
+        # UNC mounts, which happens on Windows when the template and the build directory
+        # are not on the same drive. A relative path cannot express that, so fall back to
+        # the absolute path rather than letting the exception escape.
+        return absolute_path
 
 
 def get_template_parameters(template_file):

tests/unit/commands/_utils/test_template.py

@@ -1,7 +1,9 @@
 import copy
 import os
+import pathlib
+import platform
 import tempfile
-from unittest import TestCase
+from unittest import TestCase, skipIf
 from unittest.mock import patch, mock_open, MagicMock
 import shutil
 
@@ -803,6 +805,19 @@ def test_must_resolve_relative_to_symlinked_original_root_and_new_root(self):
 
         self.assertEqual(result, expected_result)
 
+    @skipIf(platform.system() != "Windows", "Different drives only exist on Windows")
+    def test_must_resolve_relative_to_across_different_drives(self):
+        # os.path.relpath raises ValueError when the two paths are on different drives,
+        # so a template on one drive and a --build-dir on another cannot be expressed
+        # relatively. Fall back to the absolute path rather than crashing.
+        original_root = os.path.join("C:" + os.sep, "src")
+        new_root = os.path.join("D:" + os.sep, "destination")
+        expected_result = os.path.normpath(os.path.join(pathlib.Path(original_root).resolve(), self.curpath))
+
+        result = _resolve_relative_to(self.curpath, original_root, new_root)
+
+        self.assertEqual(result, expected_result)
+
     def create_symlink(self, src, dest):
         os.makedirs(src)
         os.symlink(src, dest)