Issue #26293: Fixed writing ZIP files that starts not from the start of the

file.  Offsets in ZIP file now are relative to the start of the archive in
conforming to the specification.

Lib/test/test_zipfile.py

@@ -420,6 +420,49 @@ class StoredTestsWithSourceFile(AbstractTestsWithSourceFile,
             f.seek(len(data))
             with zipfile.ZipFile(f, "r") as zipfp:
                 self.assertEqual(zipfp.namelist(), [TESTFN])
+                self.assertEqual(zipfp.read(TESTFN), self.data)
+        with open(TESTFN2, 'rb') as f:
+            self.assertEqual(f.read(len(data)), data)
+            zipfiledata = f.read()
+        with io.BytesIO(zipfiledata) as bio, zipfile.ZipFile(bio) as zipfp:
+            self.assertEqual(zipfp.namelist(), [TESTFN])
+            self.assertEqual(zipfp.read(TESTFN), self.data)
+
+    def test_read_concatenated_zip_file(self):
+        with io.BytesIO() as bio:
+            with zipfile.ZipFile(bio, 'w', zipfile.ZIP_STORED) as zipfp:
+                zipfp.write(TESTFN, TESTFN)
+            zipfiledata = bio.getvalue()
+        data = b'I am not a ZipFile!'*10
+        with open(TESTFN2, 'wb') as f:
+            f.write(data)
+            f.write(zipfiledata)
+
+        with zipfile.ZipFile(TESTFN2) as zipfp:
+            self.assertEqual(zipfp.namelist(), [TESTFN])
+            self.assertEqual(zipfp.read(TESTFN), self.data)
+
+    def test_append_to_concatenated_zip_file(self):
+        with io.BytesIO() as bio:
+            with zipfile.ZipFile(bio, 'w', zipfile.ZIP_STORED) as zipfp:
+                zipfp.write(TESTFN, TESTFN)
+            zipfiledata = bio.getvalue()
+        data = b'I am not a ZipFile!'*1000000
+        with open(TESTFN2, 'wb') as f:
+            f.write(data)
+            f.write(zipfiledata)
+
+        with zipfile.ZipFile(TESTFN2, 'a') as zipfp:
+            self.assertEqual(zipfp.namelist(), [TESTFN])
+            zipfp.writestr('strfile', self.data)
+
+        with open(TESTFN2, 'rb') as f:
+            self.assertEqual(f.read(len(data)), data)
+            zipfiledata = f.read()
+        with io.BytesIO(zipfiledata) as bio, zipfile.ZipFile(bio) as zipfp:
+            self.assertEqual(zipfp.namelist(), [TESTFN, 'strfile'])
+            self.assertEqual(zipfp.read(TESTFN), self.data)
+            self.assertEqual(zipfp.read('strfile'), self.data)
 
     def test_ignores_newline_at_end(self):
         with zipfile.ZipFile(TESTFN2, "w", zipfile.ZIP_STORED) as zipfp:

Lib/zipfile.py

@@ -1103,10 +1103,10 @@ class ZipFile:
                 # even if no files are added to the archive
                 self._didModify = True
                 try:
-                    self.start_dir = self.fp.tell()
+                    self.start_dir = self._start_disk = self.fp.tell()
                 except (AttributeError, OSError):
                     self.fp = _Tellable(self.fp)
-                    self.start_dir = 0
+                    self.start_dir = self._start_disk = 0
                     self._seekable = False
                 else:
                     # Some file-like objects can provide tell() but not seek()
@@ -1127,7 +1127,7 @@ class ZipFile:
                     # set the modified flag so central directory gets written
                     # even if no files are added to the archive
                     self._didModify = True
-                    self.start_dir = self.fp.tell()
+                    self.start_dir = self._start_disk = self.fp.tell()
             else:
                 raise ValueError("Mode must be 'r', 'w', 'x', or 'a'")
         except:
@@ -1171,17 +1171,18 @@ class ZipFile:
         offset_cd = endrec[_ECD_OFFSET]         # offset of central directory
         self._comment = endrec[_ECD_COMMENT]    # archive comment
 
-        # "concat" is zero, unless zip was concatenated to another file
-        concat = endrec[_ECD_LOCATION] - size_cd - offset_cd
+        # self._start_disk:  Position of the start of ZIP archive
+        # It is zero, unless ZIP was concatenated to another file
+        self._start_disk = endrec[_ECD_LOCATION] - size_cd - offset_cd
         if endrec[_ECD_SIGNATURE] == stringEndArchive64:
             # If Zip64 extension structures are present, account for them
-            concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)
+            self._start_disk -= (sizeEndCentDir64 + sizeEndCentDir64Locator)
 
         if self.debug > 2:
-            inferred = concat + offset_cd
-            print("given, inferred, offset", offset_cd, inferred, concat)
+            inferred = self._start_disk + offset_cd
+            print("given, inferred, offset", offset_cd, inferred, self._start_disk)
         # self.start_dir:  Position of start of central directory
-        self.start_dir = offset_cd + concat
+        self.start_dir = offset_cd + self._start_disk
         fp.seek(self.start_dir, 0)
         data = fp.read(size_cd)
         fp = io.BytesIO(data)
@@ -1221,7 +1222,7 @@ class ZipFile:
                             t>>11, (t>>5)&0x3F, (t&0x1F) * 2 )
 
             x._decodeExtra()
-            x.header_offset = x.header_offset + concat
+            x.header_offset = x.header_offset + self._start_disk
             self.filelist.append(x)
             self.NameToInfo[x.filename] = x
 
@@ -1685,11 +1686,10 @@ class ZipFile:
                 file_size = zinfo.file_size
                 compress_size = zinfo.compress_size
 
-            if zinfo.header_offset > ZIP64_LIMIT:
-                extra.append(zinfo.header_offset)
+            header_offset = zinfo.header_offset - self._start_disk
+            if header_offset > ZIP64_LIMIT:
+                extra.append(header_offset)
                 header_offset = 0xffffffff
-            else:
-                header_offset = zinfo.header_offset
 
             extra_data = zinfo.extra
             min_version = 0
@@ -1736,7 +1736,7 @@ class ZipFile:
         # Write end-of-zip-archive record
         centDirCount = len(self.filelist)
         centDirSize = pos2 - self.start_dir
-        centDirOffset = self.start_dir
+        centDirOffset = self.start_dir - self._start_disk
         requires_zip64 = None
         if centDirCount > ZIP_FILECOUNT_LIMIT:
             requires_zip64 = "Files count"

Misc/NEWS

@@ -64,6 +64,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #26293: Fixed writing ZIP files that starts not from the start of the
+  file.  Offsets in ZIP file now are relative to the start of the archive in
+  conforming to the specification.
+
 - Issue #28380: unittest.mock Mock autospec functions now properly support
   assert_called, assert_not_called, and assert_called_once.