Define lots of constants for indexes into the structures for the file

header and central directory structures, and use them as appropriate.
The point being to make it easier to tell what is getting pulled out
where; magic numbers are evil!

Change the computation of the ZipInfo.file_offset field to use the
length of the relevant "extra" field -- there are two different ones,
and the wrong one had been used.  ;-(

This closes SF tracker patch #403276, but more verbosely than the
proposed patch.

Lib/zipfile.py

@@ -27,6 +27,41 @@ stringCentralDir = "PK\001\002"   # magic number for central directory
 structFileHeader = "<4s2B4H3l2H"  # 12 items, file header record, 30 bytes
 stringFileHeader = "PK\003\004"   # magic number for file header
 
+# indexes of entries in the central directory structure
+_CD_SIGNATURE = 0
+_CD_CREATE_VERSION = 1
+_CD_CREATE_SYSTEM = 2
+_CD_EXTRACT_VERSION = 3
+_CD_EXTRACT_SYSTEM = 4                  # is this meaningful?
+_CD_FLAG_BITS = 5
+_CD_COMPRESS_TYPE = 6
+_CD_TIME = 7
+_CD_DATE = 8
+_CD_CRC = 9
+_CD_COMPRESSED_SIZE = 10
+_CD_UNCOMPRESSED_SIZE = 11
+_CD_FILENAME_LENGTH = 12
+_CD_EXTRA_FIELD_LENGTH = 13
+_CD_COMMENT_LENGTH = 14
+_CD_DISK_NUMBER_START = 15
+_CD_INTERNAL_FILE_ATTRIBUTES = 16
+_CD_EXTERNAL_FILE_ATTRIBUTES = 17
+_CD_LOCAL_HEADER_OFFSET = 18
+
+# indexes of entries in the local file header structure
+_FH_SIGNATURE = 0
+_FH_EXTRACT_VERSION = 1
+_FH_EXTRACT_SYSTEM = 2                  # is this meaningful?
+_FH_GENERAL_PURPOSE_FLAG_BITS = 3
+_FH_COMPRESSION_METHOD = 4
+_FH_LAST_MOD_TIME = 5
+_FH_LAST_MOD_DATE = 6
+_FH_CRC = 7
+_FH_COMPRESSED_SIZE = 8
+_FH_UNCOMPRESSED_SIZE = 9
+_FH_FILENAME_LENGTH = 10
+_FH_EXTRA_FIELD_LENGTH = 11
+
 
 def is_zipfile(filename):
     """Quickly see if file is a ZIP file by checking the magic number.
@@ -159,14 +194,16 @@ class ZipFile:
             centdir = struct.unpack(structCentralDir, centdir)
             if self.debug > 2:
                 print centdir
-            filename = fp.read(centdir[12])
+            filename = fp.read(centdir[_CD_FILENAME_LENGTH])
             # Create ZipInfo instance to store file information
             x = ZipInfo(filename)
-            x.extra = fp.read(centdir[13])
-            x.comment = fp.read(centdir[14])
-            total = total + centdir[12] + centdir[13] + centdir[14]
-            x.header_offset = centdir[18] + concat
-            x.file_offset = x.header_offset + 30 + centdir[12] + centdir[13]
+            x.extra = fp.read(centdir[_CD_EXTRA_FIELD_LENGTH])
+            x.comment = fp.read(centdir[_CD_COMMENT_LENGTH])
+            total = (total + centdir[_CD_FILENAME_LENGTH]
+                     + centdir[_CD_EXTRA_FIELD_LENGTH]
+                     + centdir[_CD_COMMENT_LENGTH])
+            x.header_offset = centdir[_CD_LOCAL_HEADER_OFFSET] + concat
+            # file_offset must be computed below...
             (x.create_version, x.create_system, x.extract_version, x.reserved,
                 x.flag_bits, x.compress_type, t, d,
                 x.CRC, x.compress_size, x.file_size) = centdir[1:12]
@@ -184,7 +221,14 @@ class ZipFile:
             if fheader[0:4] != stringFileHeader:
                 raise BadZipfile, "Bad magic number for file header"
             fheader = struct.unpack(structFileHeader, fheader)
-            fname = fp.read(fheader[10])
+            # file_offset is computed here, since the extra field for
+            # the central directory and for the local file header
+            # refer to different fields, and they can have different
+            # lengths
+            data.file_offset = (data.header_offset + 30
+                                + fheader[_FH_FILENAME_LENGTH]
+                                + fheader[_FH_EXTRA_FIELD_LENGTH])
+            fname = fp.read(fheader[_FH_FILENAME_LENGTH])
             if fname != data.filename:
                 raise RuntimeError, \
                       'File name in directory "%s" and header "%s" differ.' % (