Patch #611760: read archives with comments.

Lib/zipfile.py

@@ -66,21 +66,51 @@ _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.
-
-    Will not accept a ZIP archive with an ending comment.
-    """
+    """Quickly see if file is a ZIP file by checking the magic number."""
     try:
         fpin = open(filename, "rb")
-        fpin.seek(-22, 2)               # Seek to end-of-file record
-        endrec = fpin.read()
+        endrec = _EndRecData(fpin)
         fpin.close()
-        if endrec[0:4] == "PK\005\006" and endrec[-2:] == "\000\000":
+        if endrec:
             return True                 # file has correct magic number
     except IOError:
         pass
     return False
 
+def _EndRecData(fpin):
+    """Return data from the "End of Central Directory" record, or None.
+
+    The data is a list of the nine items in the ZIP "End of central dir"
+    record followed by a tenth item, the file seek offset of this record."""
+    fpin.seek(-22, 2)               # Assume no archive comment.
+    filesize = fpin.tell() + 22     # Get file size
+    data = fpin.read()
+    if data[0:4] == stringEndArchive and data[-2:] == "\000\000":
+        endrec = struct.unpack(structEndArchive, data)
+        endrec = list(endrec)
+        endrec.append("")               # Append the archive comment
+        endrec.append(filesize - 22)    # Append the record start offset
+        return endrec
+    # Search the last END_BLOCK bytes of the file for the record signature.
+    # The comment is appended to the ZIP file and has a 16 bit length.
+    # So the comment may be up to 64K long.  We limit the search for the
+    # signature to a few Kbytes at the end of the file for efficiency.
+    # also, the signature must not appear in the comment.
+    END_BLOCK = min(filesize, 1024 * 4)
+    fpin.seek(filesize - END_BLOCK, 0)
+    data = fpin.read()
+    start = data.rfind(stringEndArchive)
+    if start >= 0:     # Correct signature string was found
+        endrec = struct.unpack(structEndArchive, data[start:start+22])
+        endrec = list(endrec)
+        comment = data[start+22:]
+        if endrec[7] == len(comment):     # Comment length checks out
+            # Append the archive comment and start offset
+            endrec.append(comment)
+            endrec.append(filesize - END_BLOCK + start)
+            return endrec
+    return      # Error, return None
+
 
 class ZipInfo:
     """Class with attributes describing each file in the ZIP archive."""
@@ -183,16 +213,12 @@ class ZipFile:
         elif key == 'w':
             pass
         elif key == 'a':
-            fp = self.fp
-            fp.seek(-22, 2)             # Seek to end-of-file record
-            endrec = fp.read()
-            if endrec[0:4] == stringEndArchive and \
-                       endrec[-2:] == "\000\000":
-                self._GetContents()     # file is a zip file
+            try:                        # See if file is a zip file
+                self._RealGetContents()
                 # seek to start of directory and overwrite
-                fp.seek(self.start_dir, 0)
-            else:               # file is not a zip file, just append
-                fp.seek(0, 2)
+                self.fp.seek(self.start_dir, 0)
+            except BadZipfile:          # file is not a zip file, just append
+                self.fp.seek(0, 2)
         else:
             if not self._filePassed:
                 self.fp.close()
@@ -213,17 +239,16 @@ class ZipFile:
     def _RealGetContents(self):
         """Read in the table of contents for the ZIP file."""
         fp = self.fp
-        fp.seek(-22, 2)         # Start of end-of-archive record
-        filesize = fp.tell() + 22       # Get file size
-        endrec = fp.read(22)    # Archive must not end with a comment!
-        if endrec[0:4] != stringEndArchive or endrec[-2:] != "\000\000":
-            raise BadZipfile, "File is not a zip file, or ends with a comment"
-        endrec = struct.unpack(structEndArchive, endrec)
+        endrec = _EndRecData(fp)
+        if not endrec:
+            raise BadZipfile, "File is not a zip file"
         if self.debug > 1:
             print endrec
         size_cd = endrec[5]             # bytes in central directory
         offset_cd = endrec[6]   # offset of central directory
-        x = filesize - 22 - size_cd
+        self.comment = endrec[8]        # archive comment
+        # endrec[9] is the offset of the "End of Central Dir" record
+        x = endrec[9] - size_cd
         # "concat" is zero, unless zip was concatenated to another file
         concat = x - offset_cd
         if self.debug > 2: