fix(shared): keep LAMBDA_TASK_ROOT POSIX-shaped off Linux

abs_lambda_path joined LAMBDA_TASK_ROOT with pathlib.Path, which is WindowsPath
off Linux, so str(Path("/var/task", "")) returned "\var\task". The docstring
promises the environment variable is used as given, and the repo's own
test_abs_lambda_path_empty_envvar fails on an unmodified Windows checkout. CI is
ubuntu-latest only, so it never surfaced there.

The Lambda runtime is always Linux, so use PurePosixPath when LAMBDA_TASK_ROOT is
set and keep the existing Path behaviour for the unset local case. Behaviour on
Linux is unchanged.

test_abs_lambda_path_w_filename_envvar built its expected value with the same
platform-dependent Path call, so it passed either way and masked the bug; it now
asserts the POSIX result directly.

Author: MohammedAlkindi

aws_lambda_powertools/shared/functions.py

@@ -7,7 +7,7 @@
 import re
 import warnings
 from binascii import Error as BinAsciiError
-from pathlib import Path
+from pathlib import Path, PurePosixPath
 from typing import TYPE_CHECKING, Any, TypeGuard, overload
 
 from aws_lambda_powertools.shared import constants
@@ -280,10 +280,17 @@ def abs_lambda_path(relative_path: str = "") -> str:
         Otherwise, it will use the current working directory.
         If the path is empty, it will return the current working directory.
     """
-    # Retrieve the LAMBDA_TASK_ROOT environment variable or default to an empty string
-    current_working_directory = os.environ.get("LAMBDA_TASK_ROOT", "") or str(Path.cwd())
-
-    return str(Path(current_working_directory, relative_path))
+    # The Lambda runtime is Linux, so LAMBDA_TASK_ROOT is always a POSIX path. Joining it with
+    # pathlib.Path rewrites it with the separators of whatever platform this code runs on, so a
+    # developer running the suite on Windows would get "\var\task" instead of "/var/task".
+    # PurePosixPath keeps the value the runtime gave us intact on every platform.
+    lambda_task_root = os.environ.get("LAMBDA_TASK_ROOT", "")
+    if lambda_task_root:
+        return str(PurePosixPath(lambda_task_root, relative_path))
+
+    # Off Lambda there is no task root, so fall back to the current working directory and let
+    # pathlib use the local platform's separators.
+    return str(Path(Path.cwd(), relative_path))
 
 
 def sanitize_xray_segment_name(name: str) -> str:

tests/unit/test_shared_functions.py

@@ -177,8 +177,10 @@ def test_abs_lambda_path_w_filename_envvar(default_lambda_path):
     # Given Env is set and relative_path provided
     relative_path = "cert/pub.cert"
     os.environ["LAMBDA_TASK_ROOT"] = default_lambda_path
-    # Then path = env + relative_path
-    assert abs_lambda_path(relative_path="cert/pub.cert") == str(Path(os.environ["LAMBDA_TASK_ROOT"], relative_path))
+    # Then path = env + relative_path, joined with POSIX separators on every platform because the
+    # Lambda runtime is Linux. Building the expectation with Path() would hide a native-separator
+    # rewrite, since both sides would be rewritten the same way.
+    assert abs_lambda_path(relative_path=relative_path) == f"{default_lambda_path}/{relative_path}"
 
 
 def test_sanitize_xray_segment_name():