Bail on attempts to do an empty commit

mercurial/hg.py

@@ -1,892 +1,896 b''
 # hg.py - repository classes for mercurial
 #
 # Copyright 2005 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms
 # of the GNU General Public License, incorporated herein by reference.
 
 import sys, struct, sha, socket, os, time, re, urllib2
 import urllib
 from mercurial import byterange
 from mercurial.transaction import *
 from mercurial.revlog import *
 from difflib import SequenceMatcher
 
 class filelog(revlog):
     def __init__(self, opener, path):
         revlog.__init__(self, opener,
                         os.path.join("data", path + ".i"),
                         os.path.join("data", path + ".d"))
 
     def read(self, node):
         return self.revision(node)
     def add(self, text, transaction, link, p1=None, p2=None):
         return self.addrevision(text, transaction, link, p1, p2)
 
     def annotate(self, node):
         revs = []
         while node != nullid:
             revs.append(node)
             node = self.parents(node)[0]
         revs.reverse()
         prev = []
         annotate = []
         
         for node in revs:
             curr = self.read(node).splitlines(1)
             linkrev = self.linkrev(node)
             sm = SequenceMatcher(None, prev, curr)
             new = []
             for o, m, n, s, t in sm.get_opcodes():
                 if o == 'equal':
                     new += annotate[m:n]
                 else:
                     new += [(linkrev, l) for l in curr[s:t]]
             annotate, prev = new, curr
         return annotate
 
 class manifest(revlog):
     def __init__(self, opener):
         self.mapcache = None
         self.listcache = None
         self.addlist = None
         revlog.__init__(self, opener, "00manifest.i", "00manifest.d")
 
     def read(self, node):
         if self.mapcache and self.mapcache[0] == node:
             return self.mapcache[1].copy()
         text = self.revision(node)
         map = {}
         self.listcache = (text, text.splitlines(1))
         for l in self.listcache[1]:
             (f, n) = l.split('\0')
             map[f] = bin(n[:40])
         self.mapcache = (node, map)
         return map
 
     def diff(self, a, b):
         # this is sneaky, as we're not actually using a and b
         if self.listcache and self.addlist and self.listcache[0] == a:
             d = mdiff.diff(self.listcache[1], self.addlist, 1)
             if mdiff.patch(a, d) != b:
                 sys.stderr.write("*** sortdiff failed, falling back ***\n")
                 return mdiff.textdiff(a, b)
             return d
         else:
             return mdiff.textdiff(a, b)
 
     def add(self, map, transaction, link, p1=None, p2=None):
         files = map.keys()
         files.sort()
 
         self.addlist = ["%s\000%s\n" % (f, hex(map[f])) for f in files]
         text = "".join(self.addlist)
 
         n = self.addrevision(text, transaction, link, p1, p2)
         self.mapcache = (n, map)
         self.listcache = (text, self.addlist)
         self.addlist = None
 
         return n
 
 class changelog(revlog):
     def __init__(self, opener):
         revlog.__init__(self, opener, "00changelog.i", "00changelog.d")
 
     def extract(self, text):
         if not text:
             return (nullid, "", "0", [], "")
         last = text.index("\n\n")
         desc = text[last + 2:]
         l = text[:last].splitlines()
         manifest = bin(l[0])
         user = l[1]
         date = l[2]
         files = l[3:]
         return (manifest, user, date, files, desc)
 
     def read(self, node):
         return self.extract(self.revision(node))
 
     def add(self, manifest, list, desc, transaction, p1=None, p2=None):
         user = (os.environ.get("HGUSER") or
                 os.environ.get("EMAIL") or
                 os.environ.get("LOGNAME", "unknown") + '@' + socket.getfqdn())
         date = "%d %d" % (time.time(), time.timezone)
         list.sort()
         l = [hex(manifest), user, date] + list + ["", desc]
         text = "\n".join(l)
         return self.addrevision(text, transaction, self.count(), p1, p2)
 
 class dircache:
     def __init__(self, opener, ui):
         self.opener = opener
         self.dirty = 0
         self.ui = ui
         self.map = None
     def __del__(self):
         if self.dirty: self.write()
     def __getitem__(self, key):
         try:
             return self.map[key]
         except TypeError:
             self.read()
             return self[key]
         
     def read(self):
         if self.map is not None: return self.map
 
         self.map = {}
         try:
             st = self.opener("dircache").read()
         except: return
 
         pos = 0
         while pos < len(st):
             e = struct.unpack(">llll", st[pos:pos+16])
             l = e[3]
             pos += 16
             f = st[pos:pos + l]
             self.map[f] = e[:3]
             pos += l
         
     def update(self, files):
         if not files: return
         self.read()
         self.dirty = 1
         for f in files:
             try:
                 s = os.stat(f)
                 self.map[f] = (s.st_mode, s.st_size, s.st_mtime)
             except IOError:
                 self.remove(f)
 
     def taint(self, files):
         if not files: return
         self.read()
         self.dirty = 1
         for f in files:
             self.map[f] = (0, -1, 0)
 
     def remove(self, files):
         if not files: return
         self.read()
         self.dirty = 1
         for f in files:
             try:
                 del self.map[f]
             except KeyError:
                 self.ui.warn("Not in dircache: %s\n" % f)
                 pass
 
     def clear(self):
         self.map = {}
         self.dirty = 1
 
     def write(self):
         st = self.opener("dircache", "w")
         for f, e in self.map.items():
             e = struct.pack(">llll", e[0], e[1], e[2], len(f))
             st.write(e + f)
         self.dirty = 0
 
     def copy(self):
         self.read()
         return self.map.copy()
 
 # used to avoid circular references so destructors work
 def opener(base):
     p = base
     def o(path, mode="r"):
         if p[:7] == "http://":
             f = os.path.join(p, urllib.quote(path))
             return httprangereader(f)
 
         f = os.path.join(p, path)
 
         if mode != "r":
             try:
                 s = os.stat(f)
             except OSError:
                 d = os.path.dirname(f)
                 if not os.path.isdir(d):
                     os.makedirs(d)
             else:
                 if s.st_nlink > 1:
                     file(f + ".tmp", "w").write(file(f).read())
                     os.rename(f+".tmp", f)
 
         return file(f, mode)
 
     return o
 
 class localrepository:
     def __init__(self, ui, path=None, create=0):
         self.remote = 0
         if path and path[:7] == "http://":
             self.remote = 1
             self.path = path
         else:
             if not path:
                 p = os.getcwd()
                 while not os.path.isdir(os.path.join(p, ".hg")):
                     p = os.path.dirname(p)
                     if p == "/": raise "No repo found"
                 path = p
             self.path = os.path.join(path, ".hg")
 
         self.root = path
         self.ui = ui
 
         if create:
             os.mkdir(self.path)  
             os.mkdir(self.join("data"))
 
         self.opener = opener(self.path)
         self.manifest = manifest(self.opener)
         self.changelog = changelog(self.opener)
         self.ignorelist = None
         self.tags = None
 
         if not self.remote:
             self.dircache = dircache(self.opener, ui)
             try:
                 self.current = bin(self.opener("current").read())
             except IOError:
                 self.current = None
 
     def setcurrent(self, node):
         self.current = node
         self.opener("current", "w").write(hex(node))
       
     def ignore(self, f):
         if self.ignorelist is None:
             self.ignorelist = []
             try:
                 l = open(os.path.join(self.root, ".hgignore"))
                 for pat in l:
                     if pat != "\n":
                         self.ignorelist.append(re.compile(pat[:-1]))
             except IOError: pass
         for pat in self.ignorelist:
             if pat.search(f): return True
         return False
 
     def lookup(self, key):
         if self.tags is None:
             self.tags = {}
             try:
                 fl = self.file(".hgtags")
                 for l in fl.revision(fl.tip()).splitlines():
                     if l:
                         n, k = l.split(" ")
                         self.tags[k] = bin(n)
             except KeyError: pass
         try:
             return self.tags[key]
         except KeyError:
             return self.changelog.lookup(key)
 
     def join(self, f):
         return os.path.join(self.path, f)
 
     def file(self, f):
         return filelog(self.opener, f)
 
     def transaction(self):
         return transaction(self.opener, self.join("journal"),
                            self.join("undo"))
 
     def commit(self, parent, update = None, text = ""):
-        tr = self.transaction()
-        
         try:
             remove = [ l[:-1] for l in self.opener("to-remove") ]
             os.unlink(self.join("to-remove"))
 
         except IOError:
             remove = []
 
         if update == None:
             update = self.diffdir(self.root, parent)[0]
 
+        if not update:
+            self.ui.status("nothing changed\n")
+            return
+
+        tr = self.transaction()
+
         # check in files
         new = {}
         linkrev = self.changelog.count()
         update.sort()
         for f in update:
             self.ui.note(f + "\n")
             try:
                 t = file(f).read()
             except IOError:
                 remove.append(f)
                 continue
             r = self.file(f)
             new[f] = r.add(t, tr, linkrev)
 
         # update manifest
         mmap = self.manifest.read(self.manifest.tip())
         mmap.update(new)
         for f in remove:
             del mmap[f]
         mnode = self.manifest.add(mmap, tr, linkrev)
 
         # add changeset
         new = new.keys()
         new.sort()
 
         edittext = text + "\n"+"".join(["HG: changed %s\n" % f for f in new])
         edittext += "".join(["HG: removed %s\n" % f for f in remove])
         edittext = self.ui.edit(edittext)
 
         n = self.changelog.add(mnode, new, edittext, tr)
         tr.close()
 
         self.setcurrent(n)
         self.dircache.update(new)
         self.dircache.remove(remove)
 
     def checkout(self, node):
         # checkout is really dumb at the moment
         # it ought to basically merge
         change = self.changelog.read(node)
         l = self.manifest.read(change[0]).items()
         l.sort()
 
         for f,n in l:
             if f[0] == "/": continue
             self.ui.note(f, "\n")
             t = self.file(f).revision(n)
             try:
                 file(f, "w").write(t)
             except IOError:
                 os.makedirs(os.path.dirname(f))
                 file(f, "w").write(t)
 
         self.setcurrent(node)
         self.dircache.clear()
         self.dircache.update([f for f,n in l])
 
     def diffdir(self, path, changeset):
         changed = []
         mf = {}
         added = []
 
         if changeset:
             change = self.changelog.read(changeset)
             mf = self.manifest.read(change[0])
 
         if changeset == self.current:
             dc = self.dircache.copy()
         else:
             dc = dict.fromkeys(mf)
 
         def fcmp(fn):
             t1 = file(os.path.join(self.root, fn)).read()
             t2 = self.file(fn).revision(mf[fn])
             return cmp(t1, t2)
 
         for dir, subdirs, files in os.walk(self.root):
             d = dir[len(self.root)+1:]
             if ".hg" in subdirs: subdirs.remove(".hg")
             
             for f in files:
                 fn = os.path.join(d, f)
                 try: s = os.stat(os.path.join(self.root, fn))
                 except: continue
                 if fn in dc:
                     c = dc[fn]
                     del dc[fn]
                     if not c:
                         if fcmp(fn):
                             changed.append(fn)
                     elif c[1] != s.st_size:
                         changed.append(fn)
                     elif c[0] != s.st_mode or c[2] != s.st_mtime:
                         if fcmp(fn):
                             changed.append(fn)
                 else:
                     if self.ignore(fn): continue
                     added.append(fn)
 
         deleted = dc.keys()
         deleted.sort()
 
         return (changed, added, deleted)
 
     def diffrevs(self, node1, node2):
         changed, added = [], []
 
         change = self.changelog.read(node1)
         mf1 = self.manifest.read(change[0])
         change = self.changelog.read(node2)
         mf2 = self.manifest.read(change[0])
 
         for fn in mf2:
             if mf1.has_key(fn):
                 if mf1[fn] != mf2[fn]:
                     changed.append(fn)
                 del mf1[fn]
             else:
                 added.append(fn)
                 
         deleted = mf1.keys()
         deleted.sort()
     
         return (changed, added, deleted)
 
     def add(self, list):
         self.dircache.taint(list)
 
     def remove(self, list):
         dl = self.opener("to-remove", "a")
         for f in list:
             dl.write(f + "\n")
 
     def branches(self, nodes):
         if not nodes: nodes = [self.changelog.tip()]
         b = []
         for n in nodes:
             t = n
             while n:
                 p = self.changelog.parents(n)
                 if p[1] != nullid or p[0] == nullid:
                     b.append((t, n, p[0], p[1]))
                     break
                 n = p[0]
         return b
 
     def between(self, pairs):
         r = []
 
         for top, bottom in pairs:
             n, l, i = top, [], 0
             f = 1
 
             while n != bottom:
                 p = self.changelog.parents(n)[0]
                 if i == f:
                     l.append(n) 
                     f = f * 2
                 n = p
                 i += 1
 
             r.append(l)
 
         return r
 
     def newer(self, nodes):
         m = {}
         nl = []
         pm = {}
         cl = self.changelog
         t = l = cl.count()
 
         # find the lowest numbered node
         for n in nodes:
             l = min(l, cl.rev(n))
             m[n] = 1
 
         for i in xrange(l, t):
             n = cl.node(i)
             if n in m: # explicitly listed
                 pm[n] = 1
                 nl.append(n)
                 continue
             for p in cl.parents(n):
                 if p in pm: # parent listed
                     pm[n] = 1
                     nl.append(n)
                     break
 
         return nl
 
     def getchangegroup(self, remote):
         tip = remote.branches([])[0]
         self.ui.debug("remote tip branch is %s:%s\n" %
                       (short(tip[0]), short(tip[1])))
         m = self.changelog.nodemap
         unknown = [tip]
         search = []
         fetch = []
         seen = {}
         seenbranch = {}
 
         if tip[0] in m:
             self.ui.note("nothing to do!\n")
             return None
 
         while unknown:
             n = unknown.pop(0)
             seen[n[0]] = 1
             
             self.ui.debug("examining %s:%s\n" % (short(n[0]), short(n[1])))
             if n == nullid: break
             if n in seenbranch:
                 self.ui.debug("branch already found\n")
                 continue
             if n[1] and n[1] in m: # do we know the base?
                 self.ui.debug("found incomplete branch %s:%s\n"
                               % (short(n[0]), short(n[1])))
                 search.append(n) # schedule branch range for scanning
                 seenbranch[n] = 1
             else:
                 if n[2] in m and n[3] in m:
                     if n[1] not in fetch:
                         self.ui.debug("found new changeset %s\n" %
                                       short(n[1]))
                         fetch.append(n[1]) # earliest unknown
                         continue
 
                 r = []
                 for a in n[2:4]:
                     if a not in seen: r.append(a)
                     
                 if r:
                     self.ui.debug("requesting %s\n" %
                                 " ".join(map(short, r)))
                     for b in remote.branches(r):
                         self.ui.debug("received %s:%s\n" %
                                       (short(b[0]), short(b[1])))
                         if b[0] not in m and b[0] not in seen:
                             unknown.append(b)
   
         while search:
             n = search.pop(0)
             l = remote.between([(n[0], n[1])])[0]
             p = n[0]
             f = 1
             for i in l + [n[1]]:
                 if i in m:
                     if f <= 2:
                         self.ui.debug("found new branch changeset %s\n" %
                                           short(p))
                         fetch.append(p)
                     else:
                         self.ui.debug("narrowed branch search to %s:%s\n"
                                       % (short(p), short(i)))
                         search.append((p, i))
                     break
                 p, f = i, f * 2
 
         for f in fetch:
             if f in m:
                 raise "already have", short(f[:4])
 
         self.ui.note("adding new changesets starting at " +
                      " ".join([short(f) for f in fetch]) + "\n")
 
         return remote.changegroup(fetch)
     
     def changegroup(self, basenodes):
         nodes = self.newer(basenodes)
 
         # construct the link map
         linkmap = {}
         for n in nodes:
             linkmap[self.changelog.rev(n)] = n
 
         # construct a list of all changed files
         changed = {}
         for n in nodes:
             c = self.changelog.read(n)
             for f in c[3]:
                 changed[f] = 1
         changed = changed.keys()
         changed.sort()
 
         # the changegroup is changesets + manifests + all file revs
         revs = [ self.changelog.rev(n) for n in nodes ]
 
         yield self.changelog.group(linkmap)
         yield self.manifest.group(linkmap)
 
         for f in changed:
             g = self.file(f).group(linkmap)
             if not g: raise "couldn't find change to %s" % f
             l = struct.pack(">l", len(f))
             yield "".join([l, f, g])
 
     def addchangegroup(self, generator):
         class genread:
             def __init__(self, generator):
                 self.g = generator
                 self.buf = ""
             def read(self, l):
                 while l > len(self.buf):
                     try:
                         self.buf += self.g.next()
                     except StopIteration:
                         break
                 d, self.buf = self.buf[:l], self.buf[l:]
                 return d
                 
         if not generator: return
         source = genread(generator)
 
         def getchunk(add = 0):
             d = source.read(4)
             if not d: return ""
             l = struct.unpack(">l", d)[0]
             return source.read(l - 4 + add)
 
         tr = self.transaction()
         simple = True
         need = {}
 
         self.ui.status("adding changesets\n")
         # pull off the changeset group
         def report(x):
             self.ui.debug("add changeset %s\n" % short(x))
             return self.changelog.count()
             
         csg = getchunk()
         co = self.changelog.tip()
         cn = self.changelog.addgroup(csg, report, tr)
 
         self.ui.status("adding manifests\n")
         # pull off the manifest group
         mfg = getchunk()
         mm = self.manifest.tip()
         mo = self.manifest.addgroup(mfg, lambda x: self.changelog.rev(x), tr)
 
         # do we need a resolve?
         if self.changelog.ancestor(co, cn) != co:
             simple = False
             resolverev = self.changelog.count()
 
             # resolve the manifest to determine which files
             # we care about merging
             self.ui.status("resolving manifests\n")
             ma = self.manifest.ancestor(mm, mo)
             omap = self.manifest.read(mo) # other
             amap = self.manifest.read(ma) # ancestor
             mmap = self.manifest.read(mm) # mine
             nmap = {}
 
             self.ui.debug(" ancestor %s local %s remote %s\n" %
                           (short(ma), short(mm), short(mo)))
 
             for f, mid in mmap.iteritems():
                 if f in omap:
                     if mid != omap[f]:
                         self.ui.debug(" %s versions differ, do resolve\n" % f)
                         need[f] = mid # use merged version or local version
                     else:
                         nmap[f] = mid # keep ours
                     del omap[f]
                 elif f in amap:
                     if mid != amap[f]:
                         r = self.ui.prompt(
                             (" local changed %s which remote deleted\n" % f) +
                             "(k)eep or (d)elete?", "[kd]", "k")
                         if r == "k": nmap[f] = mid
                     else:
                         self.ui.debug("other deleted %s\n" % f)
                         pass # other deleted it
                 else:
                     self.ui.debug("local created %s\n" %f)
                     nmap[f] = mid # we created it
 
             del mmap
 
             for f, oid in omap.iteritems():
                 if f in amap:
                     if oid != amap[f]:
                         r = self.ui.prompt(
                             ("remote changed %s which local deleted\n" % f) +
                             "(k)eep or (d)elete?", "[kd]", "k")
                         if r == "k": nmap[f] = oid
                     else:
                         pass # probably safe
                 else:
                     self.ui.debug("remote created %s, do resolve\n" % f)
                     need[f] = oid
 
             del omap
             del amap
 
         new = need.keys()
         new.sort()
 
         # process the files
         self.ui.status("adding files\n")
         while 1:
             f = getchunk(4)
             if not f: break
             fg = getchunk()
             self.ui.debug("adding %s revisions\n" % f)
             fl = self.file(f)
             o = fl.tip()
             n = fl.addgroup(fg, lambda x: self.changelog.rev(x), tr)
             if f in need:
                 del need[f]
                 # manifest resolve determined we need to merge the tips
                 nmap[f] = self.merge3(fl, f, o, n, tr, resolverev)
 
         if need:
             # we need to do trivial merges on local files
             for f in new:
                 if f not in need: continue
                 fl = self.file(f)
                 nmap[f] = self.merge3(fl, f, need[f], fl.tip(), tr, resolverev)
 
         # For simple merges, we don't need to resolve manifests or changesets
         if simple:
             self.ui.debug("simple merge, skipping resolve\n")
             tr.close()
             return
 
         node = self.manifest.add(nmap, tr, resolverev, mm, mo)
 
         # Now all files and manifests are merged, we add the changed files
         # and manifest id to the changelog
         self.ui.status("committing merge changeset\n")
         if co == cn: cn = -1
 
         edittext = "\nHG: merge resolve\n" + \
                    "".join(["HG: changed %s\n" % f for f in new])
         edittext = self.ui.edit(edittext)
         n = self.changelog.add(node, new, edittext, tr, co, cn)
 
         tr.close()
 
     def merge3(self, fl, fn, my, other, transaction, link):
         """perform a 3-way merge and append the result"""
         
         def temp(prefix, node):
             pre = "%s~%s." % (os.path.basename(fn), prefix)
             (fd, name) = tempfile.mkstemp("", pre)
             f = os.fdopen(fd, "w")
             f.write(fl.revision(node))
             f.close()
             return name
 
         base = fl.ancestor(my, other)
         self.ui.note("resolving %s\n" % fn)
         self.ui.debug("local %s remote %s ancestor %s\n" %
                               (short(my), short(other), short(base)))
 
         if my == base: 
             text = fl.revision(other)
         else:
             a = temp("local", my)
             b = temp("remote", other)
             c = temp("parent", base)
 
             cmd = os.environ["HGMERGE"]
             self.ui.debug("invoking merge with %s\n" % cmd)
             r = os.system("%s %s %s %s %s" % (cmd, a, b, c, fn))
             if r:
                 raise "Merge failed!"
 
             text = open(a).read()
             os.unlink(a)
             os.unlink(b)
             os.unlink(c)
             
         return fl.addrevision(text, transaction, link, my, other)
 
 class remoterepository:
     def __init__(self, ui, path):
         self.url = path.replace("hg://", "http://", 1)
         self.ui = ui
 
     def do_cmd(self, cmd, **args):
         self.ui.debug("sending %s command\n" % cmd)
         q = {"cmd": cmd}
         q.update(args)
         qs = urllib.urlencode(q)
         cu = "%s?%s" % (self.url, qs)
         return urllib.urlopen(cu)
 
     def branches(self, nodes):
         n = " ".join(map(hex, nodes))
         d = self.do_cmd("branches", nodes=n).read()
         br = [ tuple(map(bin, b.split(" "))) for b in d.splitlines() ]
         return br
 
     def between(self, pairs):
         n = "\n".join(["-".join(map(hex, p)) for p in pairs])
         d = self.do_cmd("between", pairs=n).read()
         p = [ map(bin, l.split(" ")) for l in d.splitlines() ]
         return p
 
     def changegroup(self, nodes):
         n = " ".join(map(hex, nodes))
         zd = zlib.decompressobj()
         f = self.do_cmd("changegroup", roots=n)
         while 1:
             d = f.read(4096)
             if not d:
                 yield zd.flush()
                 break
             yield zd.decompress(d)
 
 def repository(ui, path=None, create=0):
     if path and path[:5] == "hg://":
         return remoterepository(ui, path)
     else:
         return localrepository(ui, path, create)
 
 class ui:
     def __init__(self, verbose=False, debug=False, quiet=False,
                  interactive=True):
         self.quiet = quiet and not verbose and not debug
         self.verbose = verbose or debug
         self.debugflag = debug
         self.interactive = interactive
     def write(self, *args):
         for a in args:
             sys.stdout.write(str(a))
     def readline(self):
         return sys.stdin.readline()[:-1]
     def prompt(self, msg, pat, default = "y"):
         if not self.interactive: return default
         while 1:
             self.write(msg, " ")
             r = self.readline()
             if re.match(pat, r):
                 return r
             else:
                 self.write("unrecognized response\n")
     def status(self, *msg):
         if not self.quiet: self.write(*msg)
     def warn(self, msg):
         self.write(*msg)
     def note(self, *msg):
         if self.verbose: self.write(*msg)
     def debug(self, *msg):
         if self.debugflag: self.write(*msg)
     def edit(self, text):
         (fd, name) = tempfile.mkstemp("hg")
         f = os.fdopen(fd, "w")
         f.write(text)
         f.close()
 
         editor = os.environ.get("EDITOR", "vi")
         r = os.system("%s %s" % (editor, name))
         if r:
             raise "Edit failed!"
 
         t = open(name).read()
         t = re.sub("(?m)^HG:.*\n", "", t)
 
         return t
 
     
 class httprangereader:
     def __init__(self, url):
         self.url = url
         self.pos = 0
     def seek(self, pos):
         self.pos = pos
     def read(self, bytes=None):
         opener = urllib2.build_opener(byterange.HTTPRangeHandler())
         urllib2.install_opener(opener)
         req = urllib2.Request(self.url)
         end = ''
         if bytes: end = self.pos + bytes
         req.add_header('Range', 'bytes=%d-%s' % (self.pos, end))
         f = urllib2.urlopen(req)
         return f.read()