Strip out 0xd935 extra field record in zip entries

The extra field record was previously stripped out due to a bug in
Python's zipfile implementation. That was fixed by [1], which was
backported to 3.10 and 3.11, but not 3.9.

This commit explicitly strips out the 0xd935 record to ensure that the
checksums of the patched files are the same across all Python versions.
This is probably the correct thing to do anyway, since the record is
related to zip entry alignment [2], which we don't do.

Note that the Python bug can affect other extra field records too, but
we don't see that in practice.

[1] https://github.com/python/cpython/commit/62c0327487ce39ec6504919b7712e0f197075ef9
[2] https://android.googlesource.com/platform/build/+/android-13.0.0_r1/tools/signapk/src/com/android/signapk/SignApk.java#133

Fixes: #93

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

avbroot/main.py

@@ -4,6 +4,7 @@ import copy
 import io
 import os
 import shutil
+import struct
 import tempfile
 import time
 import zipfile
@@ -150,6 +151,25 @@ def patch_ota_payload(f_in, open_more_f_in, f_out, file_size, boot_partition,
         )
 
 
+def strip_alignment_extra_field(extra):
+    offset = 0
+    new_extra = bytearray()
+
+    while offset < len(extra):
+        record_sig, record_len = \
+            struct.unpack('<HH', extra[offset:offset + 4])
+
+        next_offset = offset + 4 + record_len
+
+        # ALIGNMENT_ZIP_EXTRA_DATA_FIELD_HEADER_ID
+        if record_sig != 0xd935:
+            new_extra.extend(extra[offset:next_offset])
+
+        offset = next_offset
+
+    return new_extra
+
+
 def patch_ota_zip(f_zip_in, f_zip_out, boot_partition, root_patch,
                   clear_vbmeta_flags, privkey_avb, passphrase_avb, privkey_ota,
                   passphrase_ota, cert_ota):
@@ -195,16 +215,17 @@ def patch_ota_zip(f_zip_in, f_zip_out, boot_partition, root_patch,
 
         for info in infolist:
             out_info = copy.copy(info)
+            out_info.extra = strip_alignment_extra_field(out_info.extra)
 
             # Ignore because the plain-text legacy metadata file is regenerated
             # from the new metadata
             if info.filename == PATH_METADATA:
-                metadata_info = info
+                metadata_info = out_info
                 continue
 
             # The existing metadata is needed to generate a new signed zip
             elif info.filename == PATH_METADATA_PB:
-                metadata_pb_info = info
+                metadata_pb_info = out_info
 
                 with z_in.open(info, 'r') as f_in:
                     metadata_pb_raw = f_in.read()

tests/tests.yaml

@@ -135,8 +135,8 @@ device:
         full: 929f892fbd70699cf7f118a119aac1ae1b86351e1ada17715666fa4401e63472
         stripped: 6d9013c29aff5dc07ec549d844c5888089fe65f4a5c624bbd0afa04d1503ec8a
       patched:
-        full: 25f14582186e0f544480c6126b6b6ff0b6616023b796372d39af909300fb2e3a
-        stripped: 8c82dcc4427320fd8df4e1026c6d50ff92d5caf96e66a5c8f8b698f9fbe8806d
+        full: 1c4294f2faa711eacf4647025041e73d10488d7b9ff3a1a3142fe84b195cd9e0
+        stripped: c6abc228354ee6f4e338fce6860884c0aa6d4d2a72920e411c491ff52278702d
       avb_images:
         boot.img: ec703e143c24e7eb5b2eb96beb1d4342c1dd2179b12612d41430b1b55e7dec0f
         recovery.img: 2ff62bc52a5900eb170cc44df0f0ee8062ee6dcb48ecf1899ac556145161f53c