fetch - add error check on calculated dest (#82970)

Add explicit error when the calculated dest path for fetch becomes a
local directory. The existing behaviour will not be checked unlike when
the path did not end with a trailing slash.

changelogs/fragments/82878-fetch-dest-is-dir.yml

@@ -0,0 +1,4 @@
+bugfixes:
+- >-
+    fetch - add error message when using ``dest`` with a trailing slash that becomes a local directory -
+    https://github.com/ansible/ansible/issues/82878

lib/ansible/plugins/action/fetch.py

@@ -149,6 +149,10 @@ class ActionModule(ActionBase):
                     # destination filename
                     base = os.path.basename(source_local)
                     dest = os.path.join(dest, base)
+
+                    if os.path.isdir(to_bytes(dest, errors='surrogate_or_strict')):
+                        raise AnsibleActionFail(
+                            f"calculated dest '{dest}' is an existing directory, use another path that does not point to an existing directory")
                 if not dest.startswith("/"):
                     # if dest does not start with "/", we'll assume a relative path
                     dest = self._loader.path_dwim(dest)

test/integration/targets/fetch/roles/fetch_tests/tasks/failures.yml

@@ -28,6 +28,28 @@
   register: failed_fetch_dest_dir
   ignore_errors: true
 
+- block:
+  - name: create local dir for test
+    file:
+      path: "{{ output_dir }}/test dir/orig"
+      state: directory
+    delegate_to: localhost
+
+  - name: Dest is a path that is calculated as an existing directory, should fail
+    fetch:
+      src: "{{ remote_tmp_dir }}/orig"
+      dest: "{{ output_dir }}/test dir/"
+      flat: true
+    register: failed_detch_dest_calc_dir
+    ignore_errors: true
+
+  always:
+  - name: remote local dir for test
+    file:
+      path: "{{ output_dir }}/test dir"
+      state: absent
+    delegate_to: localhost
+
 - name: Test unreachable
   fetch:
     src: "{{ remote_tmp_dir }}/orig"
@@ -48,4 +70,6 @@
       - failed_fetch_no_access.msg is search('file is not readable')
       - failed_fetch_dest_dir is failed
       - failed_fetch_dest_dir.msg is search('dest is an existing directory')
+      - failed_detch_dest_calc_dir is failed
+      - failed_detch_dest_calc_dir.msg is search("calculated dest '" ~ output_dir ~ "/test dir/orig' is an existing directory")
       - unreachable_fetch is unreachable