[temporary-2.9.1-branch-releng-only] lineinfile - use correct index value when inserting at the end (#63696)

(cherry picked from commit 92cd13a2cf)

Co-authored-by: Sam Doran <sdoran@redhat.com>

changelogs/fragments/lineinfile-use-correct-index-value.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - lineinfile - use correct index value when inserting a line at the end of a file (https://github.com/ansible/ansible/issues/63684)

lib/ansible/modules/files/lineinfile.py

@@ -409,8 +409,14 @@ def present(module, dest, regexp, line, insertafter, insertbefore, create,
 
     elif insertafter and index[1] != -1:
 
-        # Don't insert the line if it already matches at the index
-        if b_line != b_lines[index[1]].rstrip(b'\n\r'):
+        # Don't insert the line if it already matches at the index.
+        # If the line to insert after is at the end of the file use the appropriate index value.
+        if len(b_lines) == index[1]:
+            if b_lines[index[1] - 1].rstrip(b'\r\n') != b_line:
+                b_lines.append(b_line + b_linesep)
+                msg = 'line added'
+                changed = True
+        elif b_line != b_lines[index[1]].rstrip(b'\n\r'):
             b_lines.insert(index[1], b_line + b_linesep)
             msg = 'line added'
             changed = True

test/integration/targets/lineinfile/tasks/main.yml

@@ -1107,3 +1107,33 @@
       - insertbefore_test4_file.stat.checksum == '3c6630b9d44f561ea9ad999be56a7504cadc12f7'
       - insertbefore_test5 is not changed
       - insertbefore_test5_file.stat.checksum == '3c6630b9d44f561ea9ad999be56a7504cadc12f7'
+
+
+# Test inserting a line at the end of the file using regexp with insertafter
+# https://github.com/ansible/ansible/issues/63684
+- name: Create a file by inserting a line
+  lineinfile:
+    path: "{{ output_dir }}/testend.txt"
+    create: yes
+    line: testline
+  register: testend1
+
+- name: Insert a line at the end of the file
+  lineinfile:
+    path: "{{ output_dir }}/testend.txt"
+    insertafter: testline
+    regexp: line at the end
+    line: line at the end
+  register: testend2
+
+- name: Stat the file
+  stat:
+    path: "{{ output_dir }}/testend.txt"
+  register: testend_file
+
+- name: Assert inserting at the end gave the expected results.
+  assert:
+    that:
+      - testend1 is changed
+      - testend2 is changed
+      - testend_file.stat.checksum == 'ef36116966836ce04f6b249fd1837706acae4e19'