Fix `archive` truncating archived file names based on prefix length (#3124)

When archiving multiple files, the full length of the calculated
`arcroot` would be removed from the beginning of all file names. If
there was no arcroot, the first two characters of all files would be
removed, so `file.txt` would become `le.txt`.

This patch uses the regular expressions substitution anchored to the
start of the string, so the arcroot is only removed if it is actually
present.

lib/ansible/modules/extras/files/archive.py

@@ -245,6 +245,7 @@ def main():
                     elif format == 'tar':
                         arcfile = tarfile.open(dest, 'w')
 
+                    match_root = re.compile('^%s' % re.escape(arcroot))
                     for path in archive_paths:
                         if os.path.isdir(path):
                             # Recurse into directories
@@ -254,7 +255,7 @@ def main():
 
                                 for dirname in dirnames:
                                     fullpath = dirpath + dirname
-                                    arcname = fullpath[len(arcroot):]
+                                    arcname = match_root.sub('', fullpath)
 
                                     try:
                                         if format == 'zip':
@@ -268,7 +269,7 @@ def main():
 
                                 for filename in filenames:
                                     fullpath = dirpath + filename
-                                    arcname = fullpath[len(arcroot):]
+                                    arcname = match_root.sub('', fullpath)
 
                                     if not filecmp.cmp(fullpath, dest):
                                         try:
@@ -283,9 +284,9 @@ def main():
                                             errors.append('Adding %s: %s' % (path, str(e)))
                         else:
                             if format == 'zip':
-                                arcfile.write(path, path[len(arcroot):])
+                                arcfile.write(path, match_root.sub('', path))
                             else:
-                                arcfile.add(path, path[len(arcroot):], recursive=False)
+                                arcfile.add(path, match_root.sub('', path), recursive=False)
 
                             successes.append(path)