Fixed wheel pack duplicating WHEEL contents on build number change

Fixes #415.

docs/news.rst

@@ -1,6 +1,10 @@
 Release Notes
 =============
 
+**UNRELEASED**
+
+- Fixed ``wheel pack`` duplicating the ``WHEEL`` contents when the build number has changed (#415)
+
 **0.37.0 (2021-08-09)**
 
 - Added official Python 3.10 support

src/wheel/cli/pack.py

@@ -57,9 +57,12 @@ def pack(directory, dest_dir, build_number):
             replacement = ('Build: %s\r\n' % build_number).encode('ascii') if build_number else b''
             with open(wheel_file_path, 'rb+') as f:
                 wheel_file_content = f.read()
-                if not BUILD_NUM_RE.subn(replacement, wheel_file_content)[1]:
+                wheel_file_content, num_replaced = BUILD_NUM_RE.subn(replacement,
+                                                                     wheel_file_content)
+                if not num_replaced:
                     wheel_file_content += replacement
 
+                f.seek(0)
                 f.truncate()
                 f.write(wheel_file_content)
 

tests/cli/test_pack.py

@@ -1,4 +1,5 @@
 import os
+from textwrap import dedent
 from zipfile import ZipFile
 
 import pytest
@@ -47,7 +48,15 @@ def test_pack(tmpdir_factory, tmpdir, build_tag_arg, existing_build_tag, filenam
     assert new_record_lines == old_record_lines
 
     expected_build_num = build_tag_arg or existing_build_tag
+    expected_wheel_content = dedent("""\
+        Wheel-Version: 1.0
+        Generator: bdist_wheel (0.30.0)
+        Root-Is-Purelib: false
+        Tag: py2-none-any
+        Tag: py3-none-any
+    """.replace('\n', '\r\n'))
     if expected_build_num:
-        assert ('Build: %s\r\n' % expected_build_num).encode() in new_wheel_file_content
-    else:
-        assert b'Build: ' not in new_wheel_file_content
+        expected_wheel_content += 'Build: %s\r\n' % expected_build_num
+
+    expected_wheel_content = expected_wheel_content.encode('ascii')
+    assert new_wheel_file_content == expected_wheel_content