This program is bzip2recover.py, a program to parse and extract bz2 blocks. This may be used to salvage a damaged bz2 file. Ported from C to python.

bz2recover.py

import struct, sys, os

#-----------------------------------------------------------
#--- Block recoverer program for bzip2                    --
#---                                      bzip2recover.py --
#-----------------------------------------------------------

#  This program is bzip2recover, a program to attempt data 
#  salvage from damaged files.

#  Copyright (C) 1996-2002 Julian R Seward.
#  (C) 2014 Tim Sheerman-Chase. All rights reserved.

#  Redistribution and use in source and binary forms, with or without
#  modification, are permitted provided that the following conditions
#  are met:

#  1. Redistributions of source code must retain the above copyright
#	  notice, this list of conditions and the following disclaimer.

#  2. The origin of this software must not be misrepresented; you must 
#	  not claim that you wrote the original software.  If you use this 
#	  software in a product, an acknowledgment in the product 
#	  documentation would be appreciated but is not required.

#  3. Altered source versions must be plainly marked as such, and must
#	  not be misrepresented as being the original software.

#  4. The name of the author may not be used to endorse or promote 
#	  products derived from this software without specific prior written 
#	  permission.

#  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
#  OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
#  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
#  ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
#  DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
#  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
#  GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
#  WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
#  NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
#  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#  Julian Seward, Cambridge, UK.
#  jseward@acm.org
#  bzip2/libbzip2 version 1.0 of 21 March 2000

#  This program is a complete hack and should be rewritten
#  properly.  It isn't very complicated.

#---------------------------------------------------
#--- Header bytes                                ---
#---------------------------------------------------

BZ_HDR_B = "\x42"								 # 'B' 
BZ_HDR_Z = "\x5a"								 # 'Z' 
BZ_HDR_h = "\x68"								 # 'h' 
BZ_HDR_0 = "\x30"								 # '0'

#---------------------------------------------------
#--- Bit stream I/O                              ---
#---------------------------------------------------

class BitStream(object):

	def __init__(self, stream, mode = 'r'):
		self.handle = stream
		self.buffer = 0
		self.buffLive = 0
		self.mode = mode
		self.bytesOut = 0

	def __del__(self):
		if self.mode == 'w':
			while self.buffLive < 8:
				self.buffLive += 1
				self.buffer = self.buffer << 1

			self.handle.write(chr(self.buffer))
			self.bytesOut += 1
			self.handle.flush()

			self.handle.close()
 
	def putBit (self, bit):

		if self.buffLive == 8:
			self.handle.write(chr(self.buffer))
			self.bytesOut += 1
			self.buffLive = 1
			self.buffer = bool(bit) & 0x1
	 	else:
			self.buffer = (self.buffer << 1) | (bool(bit) & 0x1)
			self.buffLive += 1

	def getBit (self):

		if self.buffLive > 0:
			self.buffLive -= 1
			return ((self.buffer) >> (self.buffLive)) & 0x1
		else:
			retVal = self.handle.read(1)
			if len(retVal)==0:
				return 2

			self.buffLive = 7
			self.buffer = ord(retVal[0])
			return bool(((self.buffer) >> 7) & 0x1)

	def putUChar(self, c):
		for i in range(7, -1, -1):
			b = (ord(c) >> i) & 0x1
			self.putBit(b)

	def putUInt32(self, c):
		cbin = struct.pack(">I", c)
		for ch in cbin:
			self.putUChar(ch)

#---------------------------------------------------
#---                                             ---
#---------------------------------------------------

BLOCK_HEADER_HI = 0x00003141
BLOCK_HEADER_LO = 0x59265359

BLOCK_ENDMARK_HI = 0x00001772
BLOCK_ENDMARK_LO = 0x45385090

def main():

	inFileName = None
	outFileName = None
	progName = None
	progName = sys.argv[0]

	bStart = []
	bEnd = []
	rbStart = []
	rbEnd = []

	print "{0} 1.0.0: extracts blocks from damaged .bz2 files.".format(progName)

	if len(sys.argv) != 2:
		print "{0}: usage is `{1} damaged_file_name'.".format(progName, progName)
		exit(1)

	inFileName = sys.argv[1]
	inFile = open(inFileName, "rb")
	bsIn = BitStream(inFile)
	print "{0}: searching for block boundaries ...".format(progName)

	bitsRead = 0;
	buffHi, buffLo = 0, 0
	currBlock = 0
	bStart.append(0)
	rbCtr = 0

	while True:
		b = bsIn.getBit()
		bitsRead += 1
		if b == 2:
			if bitsRead >= bStart[currBlock] and (bitsRead - bStart[currBlock]) >= 40:
				bEnd[currBlock] = bitsRead-1
				if currBlock > 0:
					print "block {0} runs from {1} to {2} (incomplete)".format(\
						currBlock, bStart[currBlock], bEnd[currBlock])
			else:
				currBlock -= 1
			break
		
		buffHi = (buffHi << 1) | (buffLo >> 31)
		buffHi = buffHi & 0xffffffff
		buffLo = (buffLo << 1) | (b & 1)
		buffLo = buffLo & 0xffffffff

		if ( (buffHi & 0x0000ffff) == BLOCK_HEADER_HI 
				 and buffLo == BLOCK_HEADER_LO) or \
			  ( (buffHi & 0x0000ffff) == BLOCK_ENDMARK_HI 
				 and buffLo == BLOCK_ENDMARK_LO):		

			if bitsRead > 49:
				bEnd.append(bitsRead-49)
			else:
				bEnd.append(0)
			
			if currBlock > 0 and (bEnd[currBlock] - bStart[currBlock]) >= 130:
				print " block {0} runs from {1} to {2}".format(
							 rbCtr+1,  bStart[currBlock], bEnd[currBlock])
				rbStart.append(bStart[currBlock])
				rbEnd.append(bEnd[currBlock])
				rbCtr += 1
			
			currBlock += 1

			bStart.append(bitsRead)

	del bsIn

	#-- identified blocks run from 1 to rbCtr inclusive. --
	if rbCtr < 1:
		print "{0}: sorry, I couldn't find any block boundaries.".format(progName)
		exit(1)

	print "{0}: splitting into blocks".format(progName)

	inFile = open(inFileName,"rb")
	bsIn = BitStream(inFile)

	#-- placate gcc's dataflow analyser
	blockCRC = 0
	bsWr = None

	bitsRead = 0
	outFile = None
	wrBlock = 0
	while True:
		b = bsIn.getBit()
		if b == 2: break
		if wrBlock >= len(rbStart): break

		buffHi = (buffHi << 1) | (buffLo >> 31)
		buffHi = buffHi & 0xffffffff
		buffLo = (buffLo << 1) | (b & 1)
		buffLo = buffLo & 0xffffffff

		if bitsRead == 47+rbStart[wrBlock]:
			blockCRC = (buffHi << 16) | (buffLo >> 16)

		if bsWr != None and bitsRead >= rbStart[wrBlock] and bitsRead <= rbEnd[wrBlock]:
			bsWr.putBit(b)

		bitsRead += 1

		if bitsRead == rbEnd[wrBlock]+1:
			if outFile != None:
				bsWr.putUChar('\x17'); bsWr.putUChar ('\x72')
				bsWr.putUChar('\x45'); bsWr.putUChar ('\x38')
				bsWr.putUChar('\x50'); bsWr.putUChar ('\x90')
				bsWr.putUInt32 (blockCRC & 0xffffffff)
				del bsWr
				bsWr = None
			
			if wrBlock >= rbCtr: break
			wrBlock += 1

		elif bitsRead == rbStart[wrBlock]:

			baseName = os.path.split(inFileName)[1]
			baseName = os.path.splitext(baseName)[0]
			outFileName = "rec{0:05d}{1}".format(wrBlock+1, baseName)
			if outFileName[-4:] != ".bz2":
				outFileName += ".bz2"

			outFile = open(outFileName, "wb")
			bsWr = BitStream(outFile,"w")
			bsWr.putUChar(BZ_HDR_B)
			bsWr.putUChar(BZ_HDR_Z)
			bsWr.putUChar(BZ_HDR_h)
			bsWr.putUChar(chr(ord(BZ_HDR_0) + 9))
			bsWr.putUChar('\x31'); bsWr.putUChar('\x41')
			bsWr.putUChar('\x59'); bsWr.putUChar('\x26')
			bsWr.putUChar('\x53'); bsWr.putUChar('\x59')

	print "{0}: finished".format(progName)

if __name__ == "__main__":
	main()

#-----------------------------------------------------------
#--- end                                 bzip2recover.py ---
#-----------------------------------------------------------