[2.7] bpo-34341: Fix appending to ZIP archives with the ZIP64 extension. (GH-8683). (GH-9400)

(cherry picked from commit 9bdb7be4)

Lib/test/test_zipfile.py

@@ -812,6 +812,20 @@ class TestZip64InSmallFiles(unittest.TestCase):
             self.assertEqual(content, "%d" % (i**3 % 57))
         zipf2.close()
 
+    def test_append(self):
+        # Test that appending to the Zip64 archive doesn't change
+        # extra fields of existing entries.
+        with zipfile.ZipFile(TESTFN2, "w", allowZip64=True) as zipfp:
+            zipfp.writestr("strfile", self.data)
+        with zipfile.ZipFile(TESTFN2, "r", allowZip64=True) as zipfp:
+            zinfo = zipfp.getinfo("strfile")
+            extra = zinfo.extra
+        with zipfile.ZipFile(TESTFN2, "a", allowZip64=True) as zipfp:
+            zipfp.writestr("strfile2", self.data)
+        with zipfile.ZipFile(TESTFN2, "r", allowZip64=True) as zipfp:
+            zinfo = zipfp.getinfo("strfile")
+            self.assertEqual(zinfo.extra, extra)
+
     def tearDown(self):
         zipfile.ZIP64_LIMIT = self._limit
         zipfile.ZIP_FILECOUNT_LIMIT = self._filecount_limit

Lib/zipfile.py

@@ -131,6 +131,27 @@ _CD64_NUMBER_ENTRIES_TOTAL = 7
 _CD64_DIRECTORY_SIZE = 8
 _CD64_OFFSET_START_CENTDIR = 9
 
+_EXTRA_FIELD_STRUCT = struct.Struct('<HH')
+
+def _strip_extra(extra, xids):
+    # Remove Extra Fields with specified IDs.
+    unpack = _EXTRA_FIELD_STRUCT.unpack
+    modified = False
+    buffer = []
+    start = i = 0
+    while i + 4 <= len(extra):
+        xid, xlen = unpack(extra[i : i + 4])
+        j = i + 4 + xlen
+        if xid in xids:
+            if i != start:
+                buffer.append(extra[start : i])
+            start = j
+            modified = True
+        i = j
+    if not modified:
+        return extra
+    return b''.join(buffer)
+
 def _check_zipfile(fp):
     try:
         if _EndRecData(fp):
@@ -1293,6 +1314,7 @@ class ZipFile(object):
                     extra_data = zinfo.extra
                     if extra:
                         # Append a ZIP64 field to the extra's
+                        extra_data = _strip_extra(extra_data, (1,))
                         extra_data = struct.pack(
                                 '<HH' + 'Q'*len(extra),
                                 1, 8*len(extra), *extra) + extra_data

Misc/NEWS.d/next/Library/2018-08-06-11-01-18.bpo-34341.E0b9p2.rst

+Appending to the ZIP archive with the ZIP64 extension no longer grows the
+size of extra fields of existing entries.