From fb0ba014ff4df76411cdd426a15764695a33c59e Mon Sep 17 00:00:00 2001
From: Alard <alard@example.com>
Date: Fri, 12 Oct 2012 17:41:24 +0200
Subject: [PATCH] Fixed gzip checking.

---
 megawarc | 101 ++++++++++++++++++++++++++++++-------------------------
 1 file changed, 55 insertions(+), 46 deletions(-)

diff --git a/megawarc b/megawarc
index 86cd763..f442389 100755
--- a/megawarc
+++ b/megawarc
@@ -63,7 +63,6 @@ import os.path
 import re
 import sys
 import tarfile
-import zlib
 
 from optparse import OptionParser
 try:
@@ -99,6 +98,59 @@ def copy_to_stream(stream, input_filename, offset, size):
   stream.flush()
 
 
+# part of a stream as a file
+# (seek relative to an offset)
+class RangeFile(object):
+  def __init__(self, stream, offset, size):
+    self._stream = stream
+    self._offset = offset
+    self._size = size
+
+    self._current_rel_offset = 0
+
+  def tell(self):
+    return self._current_rel_offset
+
+  def seek(self, pos, whence=os.SEEK_SET):
+    if whence == os.SEEK_SET:
+      self._current_rel_offset = pos
+    elif whence == os.SEEK_CUR:
+      self._current_rel_offset += pos
+    elif whence == os.SEEK_END:
+      self._current_rel_offset = self._size + pos
+    else:
+      raise Exception("Unknown whence: %d." % whence)
+    if self._current_rel_offset < 0 or self._current_rel_offset > self._size:
+      raise Exception("Seek outside file: %d." % self._current_rel_offset)
+    self._stream.seek(self._offset + self._current_rel_offset)
+
+  def read(self, size):
+    size = min(self._size - self._current_rel_offset, size)
+    self._current_rel_offset += size
+    buf = self._stream.read(size)
+    if len(buf) < size:
+      raise Exception("Expected to read %d but received %d." % (size, len(buf)))
+    return buf
+
+
+# check for gzip errors
+def test_gz(filename, offset, size, verbose=False):
+  with open(filename, "r") as f_stream:
+    f = RangeFile(f_stream, offset, size)
+    try:
+      gz = gzip.GzipFile(fileobj=f, mode="rb")
+      while True:
+        buf = gz.read(4096)
+        if len(buf) == 0:
+          break
+    except (IOError, ValueError) as e:
+      if verbose:
+        print >>sys.stderr, e
+      return False
+
+  return True
+
+
 # converting a .tar with warcs to megawarc tar+warc+json
 class MegawarcBuilder(object):
   def __init__(self, input_filename):
@@ -121,28 +173,6 @@ class MegawarcBuilder(object):
             if padding > 0:
               tar_out.write("\0" * padding)
 
-  def test_gz(self, offset, size):
-    with open(self.input_filename, "r") as f:
-      z = zlib.decompressobj(15 + 32)
-      f.seek(offset)
-
-      to_read = size
-      while to_read > 0:
-        buf_size = min(to_read, 4096)
-        buf = f.read(buf_size)
-        if len(buf) < buf_size:
-          # end of file, not a valid gz
-          return False
-        else:
-          z.decompress(buf)
-          to_read -= len(buf)
-
-      if z.flush()!="":
-        # remaining uncompressed data
-        return False
-
-    return True
-
   def process_entry(self, entry, warc_out, tar_out, json_out):
     # calculate position of tar entry
     block_size = (tarfile.BLOCKSIZE +  # header
@@ -161,7 +191,7 @@ class MegawarcBuilder(object):
     if entry.isfile() and re.search(r"\.warc\.gz", entry.name):
       if self.verbose:
         print >>sys.stderr, "Checking %s" % entry.name
-      valid_warc_gz = self.test_gz(data_offset, entry.size)
+      valid_warc_gz = test_gz(self.input_filename, data_offset, entry.size, self.verbose)
       if not valid_warc_gz:
         if self.verbose:
           print >>sys.stderr, "Invalid gzip %s" % entry.name
@@ -234,27 +264,6 @@ class MegawarcPacker(object):
           if padding > 0:
             tar_out.write("\0" * padding)
 
-  def test_gz(self, filename, size):
-    with open(filename, "r") as f:
-      z = zlib.decompressobj(15 + 32)
-
-      to_read = size
-      while to_read > 0:
-        buf_size = min(to_read, 4096)
-        buf = f.read(buf_size)
-        if len(buf) < buf_size:
-          # end of file, not a valid gz
-          return False
-        else:
-          z.decompress(buf)
-          to_read -= len(buf)
-
-      if z.flush()!="":
-        # remaining uncompressed data
-        return False
-
-    return True
-
   def process_file(self, filename, warc_out, tar_out, json_out):
     # make tar header
     arcname = filename
@@ -294,7 +303,7 @@ class MegawarcPacker(object):
     if re.search(r"\.warc\.gz", filename):
       if self.verbose:
         print >>sys.stderr, "Checking %s" % filename
-      valid_warc_gz = self.test_gz(filename, entry.size)
+      valid_warc_gz = test_gz(filename, 0, entry.size, self.verbose)
       if not valid_warc_gz:
         if self.verbose:
           print >>sys.stderr, "Invalid gzip %s" % filename