~bzr-pqm/bzr/bzr.dev : contents of bzrlib/revfile.py at revision 254

~bzr-pqm/bzr/bzr.dev : (revision 254)
#! /usr/bin/env python

# (C) 2005 Canonical Ltd

# based on an idea by Matt Mackall
# modified to squish into bzr by Martin Pool

# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.

# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.

# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA


"""Packed file revision storage.

A Revfile holds the text history of a particular source file, such
as Makefile.  It can represent a tree of text versions for that
file, allowing for microbranches within a single repository.

This is stored on disk as two files: an index file, and a data file.
The index file is short and always read completely into memory; the
data file is much longer and only the relevant bits of it,
identified by the index file, need to be read.

Each text version is identified by the SHA-1 of the full text of
that version.  It also has a sequence number within the file.

The index file has a short header and then a sequence of fixed-length
records:

* byte[20]    SHA-1 of text (as binary, not hex)
* uint32      sequence number this is based on, or -1 for full text
* uint32      flags: 1=zlib compressed
* uint32      offset in text file of start
* uint32      length of compressed delta in text file
* uint32[3]   reserved

total 48 bytes.

The header is also 48 bytes for tidyness and easy calculation.

Both the index and the text are only ever appended to; a consequence
is that sequence numbers are stable references.  But not every
repository in the world will assign the same sequence numbers,
therefore the SHA-1 is the only universally unique reference.

This is meant to scale to hold 100,000 revisions of a single file, by
which time the index file will be ~4.8MB and a bit big to read
sequentially.

Some of the reserved fields could be used to implement a (semi?)
balanced tree indexed by SHA1 so we can much more efficiently find the
index associated with a particular hash.  For 100,000 revs we would be
able to find it in about 17 random reads, which is not too bad.

This performs pretty well except when trying to calculate deltas of
really large files.  For that the main thing would be to plug in
something faster than difflib, which is after all pure Python.
Another approach is to just store the gzipped full text of big files,
though perhaps that's too perverse?

The iter method here will generally read through the whole index file
in one go.  With readahead in the kernel and python/libc (typically
128kB) this means that there should be no seeks and often only one
read() call to get everything into memory.
"""
 

# TODO: Something like pread() would make this slightly simpler and
# perhaps more efficient.

# TODO: Could also try to mmap things...  Might be faster for the
# index in particular?

# TODO: Some kind of faster lookup of SHAs?  The bad thing is that probably means
# rewriting existing records, which is not so nice.

# TODO: Something to check that regions identified in the index file
# completely butt up and do not overlap.  Strictly it's not a problem
# if there are gaps and that can happen if we're interrupted while
# writing to the datafile.  Overlapping would be very bad though.



import sys, zlib, struct, mdiff, stat, os, sha
from binascii import hexlify, unhexlify

factor = 10

_RECORDSIZE = 48

_HEADER = "bzr revfile v1\n"
_HEADER = _HEADER + ('\xff' * (_RECORDSIZE - len(_HEADER)))
_NO_RECORD = 0xFFFFFFFFL

# fields in the index record
I_SHA = 0
I_BASE = 1
I_FLAGS = 2
I_OFFSET = 3
I_LEN = 4

FL_GZIP = 1

# maximum number of patches in a row before recording a whole text.
CHAIN_LIMIT = 50


class RevfileError(Exception):
    pass

class LimitHitException(Exception):
    pass

class Revfile:
    def __init__(self, basename, mode):
        # TODO: Lock file  while open

        # TODO: advise of random access

        self.basename = basename

        if mode not in ['r', 'w']:
            raise RevfileError("invalid open mode %r" % mode)
        self.mode = mode
        
        idxname = basename + '.irev'
        dataname = basename + '.drev'

        idx_exists = os.path.exists(idxname)
        data_exists = os.path.exists(dataname)

        if idx_exists != data_exists:
            raise RevfileError("half-assed revfile")
        
        if not idx_exists:
            if mode == 'r':
                raise RevfileError("Revfile %r does not exist" % basename)
            
            self.idxfile = open(idxname, 'w+b')
            self.datafile = open(dataname, 'w+b')
            
            print 'init empty file'
            self.idxfile.write(_HEADER)
            self.idxfile.flush()
        else:
            if mode == 'r':
                diskmode = 'rb'
            else:
                diskmode = 'r+b'
                
            self.idxfile = open(idxname, diskmode)
            self.datafile = open(dataname, diskmode)
            
            h = self.idxfile.read(_RECORDSIZE)
            if h != _HEADER:
                raise RevfileError("bad header %r in index of %r"
                                   % (h, self.basename))


    def _check_index(self, idx):
        if idx < 0 or idx > len(self):
            raise RevfileError("invalid index %r" % idx)

    def _check_write(self):
        if self.mode != 'w':
            raise RevfileError("%r is open readonly" % self.basename)


    def find_sha(self, s):
        assert isinstance(s, str)
        assert len(s) == 20
        
        for idx, idxrec in enumerate(self):
            if idxrec[I_SHA] == s:
                return idx
        else:
            return _NO_RECORD



    def _add_compressed(self, text_sha, data, base, compress):
        # well, maybe compress
        flags = 0
        if compress:
            data_len = len(data)
            if data_len > 50:
                # don't do compression if it's too small; it's unlikely to win
                # enough to be worthwhile
                compr_data = zlib.compress(data)
                compr_len = len(compr_data)
                if compr_len < data_len:
                    data = compr_data
                    flags = FL_GZIP
                    ##print '- compressed %d -> %d, %.1f%%' \
                    ##      % (data_len, compr_len, float(compr_len)/float(data_len) * 100.0)
        return self._add_raw(text_sha, data, base, flags)
        


    def _add_raw(self, text_sha, data, base, flags):
        """Add pre-processed data, can be either full text or delta.

        This does the compression if that makes sense."""
        idx = len(self)
        self.datafile.seek(0, 2)        # to end
        self.idxfile.seek(0, 2)
        assert self.idxfile.tell() == _RECORDSIZE * (idx + 1)
        data_offset = self.datafile.tell()

        assert isinstance(data, str) # not unicode or anything weird

        self.datafile.write(data)
        self.datafile.flush()

        assert isinstance(text_sha, str)
        entry = text_sha
        entry += struct.pack(">IIII12x", base, flags, data_offset, len(data))
        assert len(entry) == _RECORDSIZE

        self.idxfile.write(entry)
        self.idxfile.flush()

        return idx
        


    def _add_full_text(self, text, text_sha, compress):
        """Add a full text to the file.

        This is not compressed against any reference version.

        Returns the index for that text."""
        return self._add_compressed(text_sha, text, _NO_RECORD, compress)


    def _add_delta(self, text, text_sha, base, compress):
        """Add a text stored relative to a previous text."""
        self._check_index(base)
        
        try:
            base_text = self.get(base, recursion_limit=CHAIN_LIMIT)
        except LimitHitException:
            return self._add_full_text(text, text_sha, compress)
        
        data = mdiff.bdiff(base_text, text)
        
        # If the delta is larger than the text, we might as well just
        # store the text.  (OK, the delta might be more compressible,
        # but the overhead of applying it probably still makes it
        # bad, and I don't want to compress both of them to find out.)
        if len(data) >= len(text):
            return self._add_full_text(text, text_sha, compress)
        else:
            return self._add_compressed(text_sha, data, base, compress)


    def add(self, text, base=_NO_RECORD, compress=True):
        """Add a new text to the revfile.

        If the text is already present them its existing id is
        returned and the file is not changed.

        If compress is true then gzip compression will be used if it
        reduces the size.

        If a base index is specified, that text *may* be used for
        delta compression of the new text.  Delta compression will
        only be used if it would be a size win and if the existing
        base is not at too long of a delta chain already.
        """
        self._check_write()
        
        text_sha = sha.new(text).digest()

        idx = self.find_sha(text_sha)
        if idx != _NO_RECORD:
            # TODO: Optional paranoid mode where we read out that record and make sure
            # it's the same, in case someone ever breaks SHA-1.
            return idx                  # already present
        
        if base == _NO_RECORD:
            return self._add_full_text(text, text_sha, compress)
        else:
            return self._add_delta(text, text_sha, base, compress)



    def get(self, idx, recursion_limit=None):
        """Retrieve text of a previous revision.

        If recursion_limit is an integer then walk back at most that
        many revisions and then raise LimitHitException, indicating
        that we ought to record a new file text instead of another
        delta.  Don't use this when trying to get out an existing
        revision."""
        
        idxrec = self[idx]
        base = idxrec[I_BASE]
        if base == _NO_RECORD:
            text = self._get_full_text(idx, idxrec)
        else:
            text = self._get_patched(idx, idxrec, recursion_limit)

        if sha.new(text).digest() != idxrec[I_SHA]:
            raise RevfileError("corrupt SHA-1 digest on record %d"
                               % idx)

        return text



    def _get_raw(self, idx, idxrec):
        flags = idxrec[I_FLAGS]
        if flags & ~FL_GZIP:
            raise RevfileError("unsupported index flags %#x on index %d"
                               % (flags, idx))
        
        l = idxrec[I_LEN]
        if l == 0:
            return ''

        self.datafile.seek(idxrec[I_OFFSET])

        data = self.datafile.read(l)
        if len(data) != l:
            raise RevfileError("short read %d of %d "
                               "getting text for record %d in %r"
                               % (len(data), l, idx, self.basename))

        if flags & FL_GZIP:
            data = zlib.decompress(data)

        return data
        

    def _get_full_text(self, idx, idxrec):
        assert idxrec[I_BASE] == _NO_RECORD

        text = self._get_raw(idx, idxrec)

        return text


    def _get_patched(self, idx, idxrec, recursion_limit):
        base = idxrec[I_BASE]
        assert base >= 0
        assert base < idx    # no loops!

        if recursion_limit == None:
            sub_limit = None
        else:
            sub_limit = recursion_limit - 1
            if sub_limit < 0:
                raise LimitHitException()
            
        base_text = self.get(base, sub_limit)
        patch = self._get_raw(idx, idxrec)

        text = mdiff.bpatch(base_text, patch)

        return text



    def __len__(self):
        """Return number of revisions."""
        l = os.fstat(self.idxfile.fileno())[stat.ST_SIZE]
        if l % _RECORDSIZE:
            raise RevfileError("bad length %d on index of %r" % (l, self.basename))
        if l < _RECORDSIZE:
            raise RevfileError("no header present in index of %r" % (self.basename))
        return int(l / _RECORDSIZE) - 1


    def __getitem__(self, idx):
        """Index by sequence id returns the index field"""
        ## TODO: Can avoid seek if we just moved there...
        self._seek_index(idx)
        idxrec = self._read_next_index()
        if idxrec == None:
            raise IndexError()
        else:
            return idxrec


    def _seek_index(self, idx):
        if idx < 0:
            raise RevfileError("invalid index %r" % idx)
        self.idxfile.seek((idx + 1) * _RECORDSIZE)



    def __iter__(self):
        """Read back all index records.

        Do not seek the index file while this is underway!"""
        sys.stderr.write(" ** iter called ** \n")
        self._seek_index(0)
        while True:
            idxrec = self._read_next_index()
            if not idxrec:
                break
            yield idxrec
        

    def _read_next_index(self):
        rec = self.idxfile.read(_RECORDSIZE)
        if not rec:
            return None
        elif len(rec) != _RECORDSIZE:
            raise RevfileError("short read of %d bytes getting index %d from %r"
                               % (len(rec), idx, self.basename))
        
        return struct.unpack(">20sIIII12x", rec)

        
    def dump(self, f=sys.stdout):
        f.write('%-8s %-40s %-8s %-8s %-8s %-8s\n' 
                % tuple('idx sha1 base flags offset len'.split()))
        f.write('-------- ---------------------------------------- ')
        f.write('-------- -------- -------- --------\n')

        for i, rec in enumerate(self):
            f.write("#%-7d %40s " % (i, hexlify(rec[0])))
            if rec[1] == _NO_RECORD:
                f.write("(none)   ")
            else:
                f.write("#%-7d " % rec[1])
                
            f.write("%8x %8d %8d\n" % (rec[2], rec[3], rec[4]))


    def total_text_size(self):
        """Return the sum of sizes of all file texts.

        This is how much space they would occupy if they were stored without
        delta and gzip compression.

        As a side effect this completely validates the Revfile, checking that all
        texts can be reproduced with the correct SHA-1."""
        t = 0L
        for idx in range(len(self)):
            t += len(self.get(idx))
        return t
        


def main(argv):
    try:
        cmd = argv[1]
    except IndexError:
        sys.stderr.write("usage: revfile dump\n"
                         "       revfile add\n"
                         "       revfile add-delta BASE\n"
                         "       revfile get IDX\n"
                         "       revfile find-sha HEX\n"
                         "       revfile total-text-size\n"
                         "       revfile last\n")
        return 1

    def rw():
        return Revfile('testrev', 'w')

    def ro():
        return Revfile('testrev', 'r')

    if cmd == 'add':
        print rw().add(sys.stdin.read())
    elif cmd == 'add-delta':
        print rw().add(sys.stdin.read(), int(argv[2]))
    elif cmd == 'dump':
        ro().dump()
    elif cmd == 'get':
        try:
            idx = int(argv[2])
        except IndexError:
            sys.stderr.write("usage: revfile get IDX\n")
            return 1

        if idx < 0 or idx >= len(r):
            sys.stderr.write("invalid index %r\n" % idx)
            return 1

        sys.stdout.write(ro().get(idx))
    elif cmd == 'find-sha':
        try:
            s = unhexlify(argv[2])
        except IndexError:
            sys.stderr.write("usage: revfile find-sha HEX\n")
            return 1

        idx = ro().find_sha(s)
        if idx == _NO_RECORD:
            sys.stderr.write("no such record\n")
            return 1
        else:
            print idx
    elif cmd == 'total-text-size':
        print ro().total_text_size()
    elif cmd == 'last':
        print len(ro())-1
    else:
        sys.stderr.write("unknown command %r\n" % cmd)
        return 1
    

if __name__ == '__main__':
    import sys
    sys.exit(main(sys.argv) or 0)