Skip if insertbefore BOF until later (#41767)

If a line match is found in the file and no regexp is specified, insertbefore would improperly try to add a line if set to BOF.

Add tests for this scenario.

lib/ansible/modules/files/lineinfile.py

@@ -291,8 +291,8 @@ def present(module, dest, regexp, line, insertafter, insertbefore, create,
 
     msg = ''
     changed = False
-    # Regexp matched a line in the file
     b_linesep = to_bytes(os.linesep, errors='surrogate_or_strict')
+    # Regexp matched a line in the file
     if index[0] != -1:
         if backrefs:
             b_new_line = m.expand(b_line)
@@ -303,13 +303,12 @@ def present(module, dest, regexp, line, insertafter, insertbefore, create,
         if not b_new_line.endswith(b_linesep):
             b_new_line += b_linesep
 
-        # If a regexp is specified and a match is found anywhere in the file, do
+        # If no regexp was given and a line match is found anywhere in the file,
-        # not insert the line before or after.
+        # insert the line appropriately if using insertbefore or insertafter
         if regexp is None and m:
 
             # Insert lines
             if insertafter and insertafter != 'EOF':
-
                 # Ensure there is a line separator after the found string
                 # at the end of the file.
                 if b_lines and not b_lines[-1][-1:] in (b('\n'), b('\r')):
@@ -327,7 +326,7 @@ def present(module, dest, regexp, line, insertafter, insertbefore, create,
                     msg = 'line added'
                     changed = True
 
-            elif insertbefore:
+            elif insertbefore and insertbefore != 'BOF':
                 # If the line to insert before is at the beginning of the file
                 # use the appropriate index value.
                 if index[1] == 0:

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

@@ -36,18 +36,27 @@
     line: "New line at the beginning"
     insertbefore: "BOF"
     backup: yes
-  register: result
+  register: result1
+
+- name: insert a line at the beginning of the file again
+  lineinfile:
+    dest: "{{ output_dir }}/test.txt"
+    state: present
+    line: "New line at the beginning"
+    insertbefore: "BOF"
+  register: result2
 
 - name: assert that the line was inserted at the head of the file
   assert:
     that:
-      - result is changed
+      - result1 is changed
-      - "result.msg == 'line added'"
+      - result2 is not changed
-      - "result.backup != ''"
+      - result1.msg == 'line added'
+      - result1.backup != ''
 
 - name: stat the backup file
   stat:
-    path: "{{ result.backup }}"
+    path: "{{ result1.backup }}"
   register: result
 
 - name: assert the backup file matches the previous hash