]> git.rkrishnan.org Git - tahoe-lafs/tahoe-lafs.git/blobdiff - src/allmydata/immutable/checker.py
Add comments and a caveat in webapi.rst indicating that
[tahoe-lafs/tahoe-lafs.git] / src / allmydata / immutable / checker.py
index cbb8637d1102c30923e787a3bd53d27a1f25ab57..41000f7e0d14df68ae0a5bf0faedefeb017867aa 100644 (file)
+from zope.interface import implements
+from twisted.internet import defer
 from foolscap.api import DeadReferenceError, RemoteException
+from allmydata import hashtree, codec, uri
+from allmydata.interfaces import IValidatedThingProxy, IVerifierURI
 from allmydata.hashtree import IncompleteHashTree
 from allmydata.check_results import CheckResults
-from allmydata.immutable import download
 from allmydata.uri import CHKFileVerifierURI
 from allmydata.util.assertutil import precondition
-from allmydata.util import base32, idlib, deferredutil, dictutil, log
+from allmydata.util import base32, deferredutil, dictutil, log, mathutil
 from allmydata.util.hashutil import file_renewal_secret_hash, \
      file_cancel_secret_hash, bucket_renewal_secret_hash, \
-     bucket_cancel_secret_hash
+     bucket_cancel_secret_hash, uri_extension_hash, CRYPTO_VAL_SIZE, \
+     block_hash
 
 from allmydata.immutable import layout
 
+class IntegrityCheckReject(Exception):
+    pass
+class BadURIExtension(IntegrityCheckReject):
+    pass
+class BadURIExtensionHashValue(IntegrityCheckReject):
+    pass
+class BadOrMissingHash(IntegrityCheckReject):
+    pass
+class UnsupportedErasureCodec(BadURIExtension):
+    pass
+
+class ValidatedExtendedURIProxy:
+    implements(IValidatedThingProxy)
+    """ I am a front-end for a remote UEB (using a local ReadBucketProxy),
+    responsible for retrieving and validating the elements from the UEB."""
+
+    def __init__(self, readbucketproxy, verifycap, fetch_failures=None):
+        # fetch_failures is for debugging -- see test_encode.py
+        self._fetch_failures = fetch_failures
+        self._readbucketproxy = readbucketproxy
+        precondition(IVerifierURI.providedBy(verifycap), verifycap)
+        self._verifycap = verifycap
+
+        # required
+        self.segment_size = None
+        self.crypttext_root_hash = None
+        self.share_root_hash = None
+
+        # computed
+        self.block_size = None
+        self.share_size = None
+        self.num_segments = None
+        self.tail_data_size = None
+        self.tail_segment_size = None
+
+        # optional
+        self.crypttext_hash = None
+
+    def __str__(self):
+        return "<%s %s>" % (self.__class__.__name__, self._verifycap.to_string())
+
+    def _check_integrity(self, data):
+        h = uri_extension_hash(data)
+        if h != self._verifycap.uri_extension_hash:
+            msg = ("The copy of uri_extension we received from %s was bad: wanted %s, got %s" %
+                   (self._readbucketproxy,
+                    base32.b2a(self._verifycap.uri_extension_hash),
+                    base32.b2a(h)))
+            if self._fetch_failures is not None:
+                self._fetch_failures["uri_extension"] += 1
+            raise BadURIExtensionHashValue(msg)
+        else:
+            return data
+
+    def _parse_and_validate(self, data):
+        self.share_size = mathutil.div_ceil(self._verifycap.size,
+                                            self._verifycap.needed_shares)
+
+        d = uri.unpack_extension(data)
+
+        # There are several kinds of things that can be found in a UEB.
+        # First, things that we really need to learn from the UEB in order to
+        # do this download. Next: things which are optional but not redundant
+        # -- if they are present in the UEB they will get used. Next, things
+        # that are optional and redundant. These things are required to be
+        # consistent: they don't have to be in the UEB, but if they are in
+        # the UEB then they will be checked for consistency with the
+        # already-known facts, and if they are inconsistent then an exception
+        # will be raised. These things aren't actually used -- they are just
+        # tested for consistency and ignored. Finally: things which are
+        # deprecated -- they ought not be in the UEB at all, and if they are
+        # present then a warning will be logged but they are otherwise
+        # ignored.
+
+        # First, things that we really need to learn from the UEB:
+        # segment_size, crypttext_root_hash, and share_root_hash.
+        self.segment_size = d['segment_size']
+
+        self.block_size = mathutil.div_ceil(self.segment_size,
+                                            self._verifycap.needed_shares)
+        self.num_segments = mathutil.div_ceil(self._verifycap.size,
+                                              self.segment_size)
+
+        self.tail_data_size = self._verifycap.size % self.segment_size
+        if not self.tail_data_size:
+            self.tail_data_size = self.segment_size
+        # padding for erasure code
+        self.tail_segment_size = mathutil.next_multiple(self.tail_data_size,
+                                                        self._verifycap.needed_shares)
+
+        # Ciphertext hash tree root is mandatory, so that there is at most
+        # one ciphertext that matches this read-cap or verify-cap. The
+        # integrity check on the shares is not sufficient to prevent the
+        # original encoder from creating some shares of file A and other
+        # shares of file B.
+        self.crypttext_root_hash = d['crypttext_root_hash']
+
+        self.share_root_hash = d['share_root_hash']
+
+
+        # Next: things that are optional and not redundant: crypttext_hash
+        if d.has_key('crypttext_hash'):
+            self.crypttext_hash = d['crypttext_hash']
+            if len(self.crypttext_hash) != CRYPTO_VAL_SIZE:
+                raise BadURIExtension('crypttext_hash is required to be hashutil.CRYPTO_VAL_SIZE bytes, not %s bytes' % (len(self.crypttext_hash),))
+
+
+        # Next: things that are optional, redundant, and required to be
+        # consistent: codec_name, codec_params, tail_codec_params,
+        # num_segments, size, needed_shares, total_shares
+        if d.has_key('codec_name'):
+            if d['codec_name'] != "crs":
+                raise UnsupportedErasureCodec(d['codec_name'])
+
+        if d.has_key('codec_params'):
+            ucpss, ucpns, ucpts = codec.parse_params(d['codec_params'])
+            if ucpss != self.segment_size:
+                raise BadURIExtension("inconsistent erasure code params: "
+                                      "ucpss: %s != self.segment_size: %s" %
+                                      (ucpss, self.segment_size))
+            if ucpns != self._verifycap.needed_shares:
+                raise BadURIExtension("inconsistent erasure code params: ucpns: %s != "
+                                      "self._verifycap.needed_shares: %s" %
+                                      (ucpns, self._verifycap.needed_shares))
+            if ucpts != self._verifycap.total_shares:
+                raise BadURIExtension("inconsistent erasure code params: ucpts: %s != "
+                                      "self._verifycap.total_shares: %s" %
+                                      (ucpts, self._verifycap.total_shares))
+
+        if d.has_key('tail_codec_params'):
+            utcpss, utcpns, utcpts = codec.parse_params(d['tail_codec_params'])
+            if utcpss != self.tail_segment_size:
+                raise BadURIExtension("inconsistent erasure code params: utcpss: %s != "
+                                      "self.tail_segment_size: %s, self._verifycap.size: %s, "
+                                      "self.segment_size: %s, self._verifycap.needed_shares: %s"
+                                      % (utcpss, self.tail_segment_size, self._verifycap.size,
+                                         self.segment_size, self._verifycap.needed_shares))
+            if utcpns != self._verifycap.needed_shares:
+                raise BadURIExtension("inconsistent erasure code params: utcpns: %s != "
+                                      "self._verifycap.needed_shares: %s" % (utcpns,
+                                                                             self._verifycap.needed_shares))
+            if utcpts != self._verifycap.total_shares:
+                raise BadURIExtension("inconsistent erasure code params: utcpts: %s != "
+                                      "self._verifycap.total_shares: %s" % (utcpts,
+                                                                            self._verifycap.total_shares))
+
+        if d.has_key('num_segments'):
+            if d['num_segments'] != self.num_segments:
+                raise BadURIExtension("inconsistent num_segments: size: %s, "
+                                      "segment_size: %s, computed_num_segments: %s, "
+                                      "ueb_num_segments: %s" % (self._verifycap.size,
+                                                                self.segment_size,
+                                                                self.num_segments, d['num_segments']))
+
+        if d.has_key('size'):
+            if d['size'] != self._verifycap.size:
+                raise BadURIExtension("inconsistent size: URI size: %s, UEB size: %s" %
+                                      (self._verifycap.size, d['size']))
+
+        if d.has_key('needed_shares'):
+            if d['needed_shares'] != self._verifycap.needed_shares:
+                raise BadURIExtension("inconsistent needed shares: URI needed shares: %s, UEB "
+                                      "needed shares: %s" % (self._verifycap.total_shares,
+                                                             d['needed_shares']))
+
+        if d.has_key('total_shares'):
+            if d['total_shares'] != self._verifycap.total_shares:
+                raise BadURIExtension("inconsistent total shares: URI total shares: %s, UEB "
+                                      "total shares: %s" % (self._verifycap.total_shares,
+                                                            d['total_shares']))
+
+        # Finally, things that are deprecated and ignored: plaintext_hash,
+        # plaintext_root_hash
+        if d.get('plaintext_hash'):
+            log.msg("Found plaintext_hash in UEB. This field is deprecated for security reasons "
+                    "and is no longer used.  Ignoring.  %s" % (self,))
+        if d.get('plaintext_root_hash'):
+            log.msg("Found plaintext_root_hash in UEB. This field is deprecated for security "
+                    "reasons and is no longer used.  Ignoring.  %s" % (self,))
+
+        return self
+
+    def start(self):
+        """Fetch the UEB from bucket, compare its hash to the hash from
+        verifycap, then parse it. Returns a deferred which is called back
+        with self once the fetch is successful, or is erred back if it
+        fails."""
+        d = self._readbucketproxy.get_uri_extension()
+        d.addCallback(self._check_integrity)
+        d.addCallback(self._parse_and_validate)
+        return d
+
+class ValidatedReadBucketProxy(log.PrefixingLogMixin):
+    """I am a front-end for a remote storage bucket, responsible for
+    retrieving and validating data from that bucket.
+
+    My get_block() method is used by BlockDownloaders.
+    """
+
+    def __init__(self, sharenum, bucket, share_hash_tree, num_blocks,
+                 block_size, share_size):
+        """ share_hash_tree is required to have already been initialized with
+        the root hash (the number-0 hash), using the share_root_hash from the
+        UEB"""
+        precondition(share_hash_tree[0] is not None, share_hash_tree)
+        prefix = "%d-%s-%s" % (sharenum, bucket,
+                               base32.b2a_l(share_hash_tree[0][:8], 60))
+        log.PrefixingLogMixin.__init__(self,
+                                       facility="tahoe.immutable.download",
+                                       prefix=prefix)
+        self.sharenum = sharenum
+        self.bucket = bucket
+        self.share_hash_tree = share_hash_tree
+        self.num_blocks = num_blocks
+        self.block_size = block_size
+        self.share_size = share_size
+        self.block_hash_tree = hashtree.IncompleteHashTree(self.num_blocks)
+
+    def get_all_sharehashes(self):
+        """Retrieve and validate all the share-hash-tree nodes that are
+        included in this share, regardless of whether we need them to
+        validate the share or not. Each share contains a minimal Merkle tree
+        chain, but there is lots of overlap, so usually we'll be using hashes
+        from other shares and not reading every single hash from this share.
+        The Verifier uses this function to read and validate every single
+        hash from this share.
+
+        Call this (and wait for the Deferred it returns to fire) before
+        calling get_block() for the first time: this lets us check that the
+        share share contains enough hashes to validate its own data, and
+        avoids downloading any share hash twice.
+
+        I return a Deferred which errbacks upon failure, probably with
+        BadOrMissingHash."""
+
+        d = self.bucket.get_share_hashes()
+        def _got_share_hashes(sh):
+            sharehashes = dict(sh)
+            try:
+                self.share_hash_tree.set_hashes(sharehashes)
+            except IndexError, le:
+                raise BadOrMissingHash(le)
+            except (hashtree.BadHashError, hashtree.NotEnoughHashesError), le:
+                raise BadOrMissingHash(le)
+        d.addCallback(_got_share_hashes)
+        return d
+
+    def get_all_blockhashes(self):
+        """Retrieve and validate all the block-hash-tree nodes that are
+        included in this share. Each share contains a full Merkle tree, but
+        we usually only fetch the minimal subset necessary for any particular
+        block. This function fetches everything at once. The Verifier uses
+        this function to validate the block hash tree.
+
+        Call this (and wait for the Deferred it returns to fire) after
+        calling get_all_sharehashes() and before calling get_block() for the
+        first time: this lets us check that the share contains all block
+        hashes and avoids downloading them multiple times.
+
+        I return a Deferred which errbacks upon failure, probably with
+        BadOrMissingHash.
+        """
+
+        # get_block_hashes(anything) currently always returns everything
+        needed = list(range(len(self.block_hash_tree)))
+        d = self.bucket.get_block_hashes(needed)
+        def _got_block_hashes(blockhashes):
+            if len(blockhashes) < len(self.block_hash_tree):
+                raise BadOrMissingHash()
+            bh = dict(enumerate(blockhashes))
+
+            try:
+                self.block_hash_tree.set_hashes(bh)
+            except IndexError, le:
+                raise BadOrMissingHash(le)
+            except (hashtree.BadHashError, hashtree.NotEnoughHashesError), le:
+                raise BadOrMissingHash(le)
+        d.addCallback(_got_block_hashes)
+        return d
+
+    def get_all_crypttext_hashes(self, crypttext_hash_tree):
+        """Retrieve and validate all the crypttext-hash-tree nodes that are
+        in this share. Normally we don't look at these at all: the download
+        process fetches them incrementally as needed to validate each segment
+        of ciphertext. But this is a convenient place to give the Verifier a
+        function to validate all of these at once.
+
+        Call this with a new hashtree object for each share, initialized with
+        the crypttext hash tree root. I return a Deferred which errbacks upon
+        failure, probably with BadOrMissingHash.
+        """
+
+        # get_crypttext_hashes() always returns everything
+        d = self.bucket.get_crypttext_hashes()
+        def _got_crypttext_hashes(hashes):
+            if len(hashes) < len(crypttext_hash_tree):
+                raise BadOrMissingHash()
+            ct_hashes = dict(enumerate(hashes))
+            try:
+                crypttext_hash_tree.set_hashes(ct_hashes)
+            except IndexError, le:
+                raise BadOrMissingHash(le)
+            except (hashtree.BadHashError, hashtree.NotEnoughHashesError), le:
+                raise BadOrMissingHash(le)
+        d.addCallback(_got_crypttext_hashes)
+        return d
+
+    def get_block(self, blocknum):
+        # the first time we use this bucket, we need to fetch enough elements
+        # of the share hash tree to validate it from our share hash up to the
+        # hashroot.
+        if self.share_hash_tree.needed_hashes(self.sharenum):
+            d1 = self.bucket.get_share_hashes()
+        else:
+            d1 = defer.succeed([])
+
+        # We might need to grab some elements of our block hash tree, to
+        # validate the requested block up to the share hash.
+        blockhashesneeded = self.block_hash_tree.needed_hashes(blocknum, include_leaf=True)
+        # We don't need the root of the block hash tree, as that comes in the
+        # share tree.
+        blockhashesneeded.discard(0)
+        d2 = self.bucket.get_block_hashes(blockhashesneeded)
+
+        if blocknum < self.num_blocks-1:
+            thisblocksize = self.block_size
+        else:
+            thisblocksize = self.share_size % self.block_size
+            if thisblocksize == 0:
+                thisblocksize = self.block_size
+        d3 = self.bucket.get_block_data(blocknum,
+                                        self.block_size, thisblocksize)
+
+        dl = deferredutil.gatherResults([d1, d2, d3])
+        dl.addCallback(self._got_data, blocknum)
+        return dl
+
+    def _got_data(self, results, blocknum):
+        precondition(blocknum < self.num_blocks,
+                     self, blocknum, self.num_blocks)
+        sharehashes, blockhashes, blockdata = results
+        try:
+            sharehashes = dict(sharehashes)
+        except ValueError, le:
+            le.args = tuple(le.args + (sharehashes,))
+            raise
+        blockhashes = dict(enumerate(blockhashes))
+
+        candidate_share_hash = None # in case we log it in the except block below
+        blockhash = None # in case we log it in the except block below
+
+        try:
+            if self.share_hash_tree.needed_hashes(self.sharenum):
+                # This will raise exception if the values being passed do not
+                # match the root node of self.share_hash_tree.
+                try:
+                    self.share_hash_tree.set_hashes(sharehashes)
+                except IndexError, le:
+                    # Weird -- sharehashes contained index numbers outside of
+                    # the range that fit into this hash tree.
+                    raise BadOrMissingHash(le)
+
+            # To validate a block we need the root of the block hash tree,
+            # which is also one of the leafs of the share hash tree, and is
+            # called "the share hash".
+            if not self.block_hash_tree[0]: # empty -- no root node yet
+                # Get the share hash from the share hash tree.
+                share_hash = self.share_hash_tree.get_leaf(self.sharenum)
+                if not share_hash:
+                    # No root node in block_hash_tree and also the share hash
+                    # wasn't sent by the server.
+                    raise hashtree.NotEnoughHashesError
+                self.block_hash_tree.set_hashes({0: share_hash})
+
+            if self.block_hash_tree.needed_hashes(blocknum):
+                self.block_hash_tree.set_hashes(blockhashes)
+
+            blockhash = block_hash(blockdata)
+            self.block_hash_tree.set_hashes(leaves={blocknum: blockhash})
+            #self.log("checking block_hash(shareid=%d, blocknum=%d) len=%d "
+            #        "%r .. %r: %s" %
+            #        (self.sharenum, blocknum, len(blockdata),
+            #         blockdata[:50], blockdata[-50:], base32.b2a(blockhash)))
+
+        except (hashtree.BadHashError, hashtree.NotEnoughHashesError), le:
+            # log.WEIRD: indicates undetected disk/network error, or more
+            # likely a programming error
+            self.log("hash failure in block=%d, shnum=%d on %s" %
+                    (blocknum, self.sharenum, self.bucket))
+            if self.block_hash_tree.needed_hashes(blocknum):
+                self.log(""" failure occurred when checking the block_hash_tree.
+                This suggests that either the block data was bad, or that the
+                block hashes we received along with it were bad.""")
+            else:
+                self.log(""" the failure probably occurred when checking the
+                share_hash_tree, which suggests that the share hashes we
+                received from the remote peer were bad.""")
+            self.log(" have candidate_share_hash: %s" % bool(candidate_share_hash))
+            self.log(" block length: %d" % len(blockdata))
+            self.log(" block hash: %s" % base32.b2a_or_none(blockhash))
+            if len(blockdata) < 100:
+                self.log(" block data: %r" % (blockdata,))
+            else:
+                self.log(" block data start/end: %r .. %r" %
+                        (blockdata[:50], blockdata[-50:]))
+            self.log(" share hash tree:\n" + self.share_hash_tree.dump())
+            self.log(" block hash tree:\n" + self.block_hash_tree.dump())
+            lines = []
+            for i,h in sorted(sharehashes.items()):
+                lines.append("%3d: %s" % (i, base32.b2a_or_none(h)))
+            self.log(" sharehashes:\n" + "\n".join(lines) + "\n")
+            lines = []
+            for i,h in blockhashes.items():
+                lines.append("%3d: %s" % (i, base32.b2a_or_none(h)))
+            log.msg(" blockhashes:\n" + "\n".join(lines) + "\n")
+            raise BadOrMissingHash(le)
+
+        # If we made it here, the block is good. If the hash trees didn't
+        # like what they saw, they would have raised a BadHashError, causing
+        # our caller to see a Failure and thus ignore this block (as well as
+        # dropping this bucket).
+        return blockdata
+
+
 class Checker(log.PrefixingLogMixin):
     """I query all servers to see if M uniquely-numbered shares are
     available.
@@ -35,11 +463,8 @@ class Checker(log.PrefixingLogMixin):
     def __init__(self, verifycap, servers, verify, add_lease, secret_holder,
                  monitor):
         assert precondition(isinstance(verifycap, CHKFileVerifierURI), verifycap, type(verifycap))
-        assert precondition(isinstance(servers, (set, frozenset)), servers)
-        for (serverid, serverrref) in servers:
-            assert precondition(isinstance(serverid, str))
 
-        prefix = "%s" % base32.b2a_l(verifycap.storage_index[:8], 60)
+        prefix = "%s" % base32.b2a_l(verifycap.get_storage_index()[:8], 60)
         log.PrefixingLogMixin.__init__(self, facility="tahoe.immutable.checker", prefix=prefix)
 
         self._verifycap = verifycap
@@ -50,18 +475,18 @@ class Checker(log.PrefixingLogMixin):
         self._add_lease = add_lease
 
         frs = file_renewal_secret_hash(secret_holder.get_renewal_secret(),
-                                       self._verifycap.storage_index)
+                                       self._verifycap.get_storage_index())
         self.file_renewal_secret = frs
         fcs = file_cancel_secret_hash(secret_holder.get_cancel_secret(),
-                                      self._verifycap.storage_index)
+                                      self._verifycap.get_storage_index())
         self.file_cancel_secret = fcs
 
-    def _get_renewal_secret(self, peerid):
-        return bucket_renewal_secret_hash(self.file_renewal_secret, peerid)
-    def _get_cancel_secret(self, peerid):
-        return bucket_cancel_secret_hash(self.file_cancel_secret, peerid)
+    def _get_renewal_secret(self, seed):
+        return bucket_renewal_secret_hash(self.file_renewal_secret, seed)
+    def _get_cancel_secret(self, seed):
+        return bucket_cancel_secret_hash(self.file_cancel_secret, seed)
 
-    def _get_buckets(self, server, storageindex, serverid):
+    def _get_buckets(self, s, storageindex):
         """Return a deferred that eventually fires with ({sharenum: bucket},
         serverid, success). In case the server is disconnected or returns a
         Failure then it fires with ({}, serverid, False) (A server
@@ -70,28 +495,32 @@ class Checker(log.PrefixingLogMixin):
         that we want to track and report whether or not each server
         responded.)"""
 
+        rref = s.get_rref()
+        lease_seed = s.get_lease_seed()
         if self._add_lease:
-            renew_secret = self._get_renewal_secret(serverid)
-            cancel_secret = self._get_cancel_secret(serverid)
-            d2 = server.callRemote("add_lease", storageindex,
-                                   renew_secret, cancel_secret)
-            d2.addErrback(self._add_lease_failed, serverid, storageindex)
+            renew_secret = self._get_renewal_secret(lease_seed)
+            cancel_secret = self._get_cancel_secret(lease_seed)
+            d2 = rref.callRemote("add_lease", storageindex,
+                                 renew_secret, cancel_secret)
+            d2.addErrback(self._add_lease_failed, s.get_name(), storageindex)
 
-        d = server.callRemote("get_buckets", storageindex)
+        d = rref.callRemote("get_buckets", storageindex)
         def _wrap_results(res):
-            return (res, serverid, True)
+            return (res, True)
 
         def _trap_errs(f):
             level = log.WEIRD
             if f.check(DeadReferenceError):
                 level = log.UNUSUAL
-            self.log("failure from server on 'get_buckets' the REMOTE failure was:", facility="tahoe.immutable.checker", failure=f, level=level, umid="3uuBUQ")
-            return ({}, serverid, False)
+            self.log("failure from server on 'get_buckets' the REMOTE failure was:",
+                     facility="tahoe.immutable.checker",
+                     failure=f, level=level, umid="AX7wZQ")
+            return ({}, False)
 
         d.addCallbacks(_wrap_results, _trap_errs)
         return d
 
-    def _add_lease_failed(self, f, peerid, storage_index):
+    def _add_lease_failed(self, f, server_name, storage_index):
         # Older versions of Tahoe didn't handle the add-lease message very
         # well: <=1.1.0 throws a NameError because it doesn't implement
         # remote_add_lease(), 1.2.0/1.3.0 throw IndexError on unknown buckets
@@ -111,21 +540,21 @@ class Checker(log.PrefixingLogMixin):
                 # this may ignore a bit too much, but that only hurts us
                 # during debugging
                 return
-            self.log(format="error in add_lease from [%(peerid)s]: %(f_value)s",
-                     peerid=idlib.shortnodeid_b2a(peerid),
+            self.log(format="error in add_lease from [%(name)s]: %(f_value)s",
+                     name=server_name,
                      f_value=str(f.value),
                      failure=f,
                      level=log.WEIRD, umid="atbAxw")
             return
         # local errors are cause for alarm
-        log.err(format="local error in add_lease to [%(peerid)s]: %(f_value)s",
-                peerid=idlib.shortnodeid_b2a(peerid),
+        log.err(f,
+                format="local error in add_lease to [%(name)s]: %(f_value)s",
+                name=server_name,
                 f_value=str(f.value),
-                failure=f,
                 level=log.WEIRD, umid="hEGuQg")
 
 
-    def _download_and_verify(self, serverid, sharenum, bucket):
+    def _download_and_verify(self, server, sharenum, bucket):
         """Start an attempt to download and verify every block in this bucket
         and return a deferred that will eventually fire once the attempt
         completes.
@@ -145,19 +574,19 @@ class Checker(log.PrefixingLogMixin):
         results."""
 
         vcap = self._verifycap
-        b = layout.ReadBucketProxy(bucket, serverid, vcap.storage_index)
-        veup = download.ValidatedExtendedURIProxy(b, vcap)
+        b = layout.ReadBucketProxy(bucket, server, vcap.get_storage_index())
+        veup = ValidatedExtendedURIProxy(b, vcap)
         d = veup.start()
 
         def _got_ueb(vup):
             share_hash_tree = IncompleteHashTree(vcap.total_shares)
             share_hash_tree.set_hashes({0: vup.share_root_hash})
 
-            vrbp = download.ValidatedReadBucketProxy(sharenum, b,
-                                                     share_hash_tree,
-                                                     vup.num_segments,
-                                                     vup.block_size,
-                                                     vup.share_size)
+            vrbp = ValidatedReadBucketProxy(sharenum, b,
+                                            share_hash_tree,
+                                            vup.num_segments,
+                                            vup.block_size,
+                                            vup.share_size)
 
             # note: normal download doesn't use get_all_sharehashes(),
             # because it gets more data than necessary. We've discussed the
@@ -186,15 +615,22 @@ class Checker(log.PrefixingLogMixin):
             assert isinstance(r, str), r
             # to free up the RAM
             return None
+
         def _get_blocks(vrbp):
-            ds = []
-            for blocknum in range(veup.num_segments):
+            def _get_block(ign, blocknum):
                 db = vrbp.get_block(blocknum)
                 db.addCallback(_discard_result)
-                ds.append(db)
-            # this gatherResults will fire once every block of this share has
-            # been downloaded and verified, or else it will errback.
-            return deferredutil.gatherResults(ds)
+                return db
+
+            dbs = defer.succeed(None)
+            for blocknum in range(veup.num_segments):
+                dbs.addCallback(_get_block, blocknum)
+
+            # The Deferred we return will fire after every block of this
+            # share has been downloaded and verified successfully, or else it
+            # will errback as soon as the first error is observed.
+            return dbs
+
         d.addCallback(_get_blocks)
 
         # if none of those errbacked, the blocks (and the hashes above them)
@@ -216,8 +652,8 @@ class Checker(log.PrefixingLogMixin):
                 return (False, sharenum, 'incompatible')
             elif f.check(layout.LayoutInvalid,
                          layout.RidiculouslyLargeURIExtensionBlock,
-                         download.BadOrMissingHash,
-                         download.BadURIExtensionHashValue):
+                         BadOrMissingHash,
+                         BadURIExtensionHashValue):
                 return (False, sharenum, 'corrupt')
 
             # if it wasn't one of those reasons, re-raise the error
@@ -226,9 +662,9 @@ class Checker(log.PrefixingLogMixin):
 
         return d
 
-    def _verify_server_shares(self, serverid, ss):
+    def _verify_server_shares(self, s):
         """ Return a deferred which eventually fires with a tuple of
-        (set(sharenum), serverid, set(corruptsharenum),
+        (set(sharenum), server, set(corruptsharenum),
         set(incompatiblesharenum), success) showing all the shares verified
         to be served by this server, and all the corrupt shares served by the
         server, and all the incompatible shares served by the server. In case
@@ -249,14 +685,14 @@ class Checker(log.PrefixingLogMixin):
         then disconnected and ceased responding, or returned a failure, it is
         still marked with the True flag for 'success'.
         """
-        d = self._get_buckets(ss, self._verifycap.storage_index, serverid)
+        d = self._get_buckets(s, self._verifycap.get_storage_index())
 
         def _got_buckets(result):
-            bucketdict, serverid, success = result
+            bucketdict, success = result
 
             shareverds = []
             for (sharenum, bucket) in bucketdict.items():
-                d = self._download_and_verify(serverid, sharenum, bucket)
+                d = self._download_and_verify(s, sharenum, bucket)
                 shareverds.append(d)
 
             dl = deferredutil.gatherResults(shareverds)
@@ -273,101 +709,108 @@ class Checker(log.PrefixingLogMixin):
                             corrupt.add(sharenum)
                         elif whynot == 'incompatible':
                             incompatible.add(sharenum)
-                return (verified, serverid, corrupt, incompatible, success)
+                return (verified, s, corrupt, incompatible, success)
 
             dl.addCallback(collect)
             return dl
 
         def _err(f):
             f.trap(RemoteException, DeadReferenceError)
-            return (set(), serverid, set(), set(), False)
+            return (set(), s, set(), set(), False)
 
         d.addCallbacks(_got_buckets, _err)
         return d
 
-    def _check_server_shares(self, serverid, ss):
+    def _check_server_shares(self, s):
         """Return a deferred which eventually fires with a tuple of
-        (set(sharenum), serverid, set(), set(), responded) showing all the
+        (set(sharenum), server, set(), set(), responded) showing all the
         shares claimed to be served by this server. In case the server is
-        disconnected then it fires with (set() serverid, set(), set(), False)
+        disconnected then it fires with (set(), server, set(), set(), False)
         (a server disconnecting when we ask it for buckets is the same, for
         our purposes, as a server that says it has none, except that we want
         to track and report whether or not each server responded.)"""
         def _curry_empty_corrupted(res):
-            buckets, serverid, responded = res
-            return (set(buckets), serverid, set(), set(), responded)
-        d = self._get_buckets(ss, self._verifycap.storage_index, serverid)
+            buckets, responded = res
+            return (set(buckets), s, set(), set(), responded)
+        d = self._get_buckets(s, self._verifycap.get_storage_index())
         d.addCallback(_curry_empty_corrupted)
         return d
 
     def _format_results(self, results):
-        cr = CheckResults(self._verifycap, self._verifycap.storage_index)
-        d = {}
-        d['count-shares-needed'] = self._verifycap.needed_shares
-        d['count-shares-expected'] = self._verifycap.total_shares
-
-        verifiedshares = dictutil.DictOfSets() # {sharenum: set(serverid)}
-        servers = {} # {serverid: set(sharenums)}
-        corruptsharelocators = [] # (serverid, storageindex, sharenum)
-        incompatiblesharelocators = [] # (serverid, storageindex, sharenum)
-
-        for theseverifiedshares, thisserverid, thesecorruptshares, theseincompatibleshares, thisresponded in results:
-            servers.setdefault(thisserverid, set()).update(theseverifiedshares)
-            for sharenum in theseverifiedshares:
-                verifiedshares.setdefault(sharenum, set()).add(thisserverid)
-            for sharenum in thesecorruptshares:
-                corruptsharelocators.append((thisserverid, self._verifycap.storage_index, sharenum))
-            for sharenum in theseincompatibleshares:
-                incompatiblesharelocators.append((thisserverid, self._verifycap.storage_index, sharenum))
-
-        d['count-shares-good'] = len(verifiedshares)
-        d['count-good-share-hosts'] = len([s for s in servers.keys() if servers[s]])
+        SI = self._verifycap.get_storage_index()
+
+        verifiedshares = dictutil.DictOfSets() # {sharenum: set(server)}
+        servers = {} # {server: set(sharenums)}
+        corruptshare_locators = [] # (server, storageindex, sharenum)
+        incompatibleshare_locators = [] # (server, storageindex, sharenum)
+        servers_responding = set() # server
+
+        for verified, server, corrupt, incompatible, responded in results:
+            servers.setdefault(server, set()).update(verified)
+            for sharenum in verified:
+                verifiedshares.setdefault(sharenum, set()).add(server)
+            for sharenum in corrupt:
+                corruptshare_locators.append((server, SI, sharenum))
+            for sharenum in incompatible:
+                incompatibleshare_locators.append((server, SI, sharenum))
+            if responded:
+                servers_responding.add(server)
+
+        good_share_hosts = len([s for s in servers.keys() if servers[s]])
 
         assert len(verifiedshares) <= self._verifycap.total_shares, (verifiedshares.keys(), self._verifycap.total_shares)
         if len(verifiedshares) == self._verifycap.total_shares:
-            cr.set_healthy(True)
-            cr.set_summary("Healthy")
+            healthy = True
+            summary = "Healthy"
         else:
-            cr.set_healthy(False)
-            cr.set_summary("Not Healthy: %d shares (enc %d-of-%d)" %
-                           (len(verifiedshares),
-                            self._verifycap.needed_shares,
-                            self._verifycap.total_shares))
+            healthy = False
+            summary = ("Not Healthy: %d shares (enc %d-of-%d)" %
+                       (len(verifiedshares),
+                        self._verifycap.needed_shares,
+                        self._verifycap.total_shares))
         if len(verifiedshares) >= self._verifycap.needed_shares:
-            cr.set_recoverable(True)
-            d['count-recoverable-versions'] = 1
-            d['count-unrecoverable-versions'] = 0
+            recoverable = 1
+            unrecoverable = 0
         else:
-            cr.set_recoverable(False)
-            d['count-recoverable-versions'] = 0
-            d['count-unrecoverable-versions'] = 1
-
-        d['servers-responding'] = list(servers)
-        d['sharemap'] = verifiedshares
-        # no such thing as wrong shares of an immutable file
-        d['count-wrong-shares'] = 0
-        d['list-corrupt-shares'] = corruptsharelocators
-        d['count-corrupt-shares'] = len(corruptsharelocators)
-        d['list-incompatible-shares'] = incompatiblesharelocators
-        d['count-incompatible-shares'] = len(incompatiblesharelocators)
-
+            recoverable = 0
+            unrecoverable = 1
 
         # The file needs rebalancing if the set of servers that have at least
         # one share is less than the number of uniquely-numbered shares
         # available.
-        cr.set_needs_rebalancing(d['count-good-share-hosts'] < d['count-shares-good'])
-
-        cr.set_data(d)
+        # TODO: this may be wrong, see ticket #1115 comment:27 and ticket #1784.
+        needs_rebalancing = bool(good_share_hosts < len(verifiedshares))
+
+        cr = CheckResults(self._verifycap, SI,
+                          healthy=healthy, recoverable=bool(recoverable),
+                          needs_rebalancing=needs_rebalancing,
+                          count_shares_needed=self._verifycap.needed_shares,
+                          count_shares_expected=self._verifycap.total_shares,
+                          count_shares_good=len(verifiedshares),
+                          count_good_share_hosts=good_share_hosts,
+                          count_recoverable_versions=recoverable,
+                          count_unrecoverable_versions=unrecoverable,
+                          servers_responding=list(servers_responding),
+                          sharemap=verifiedshares,
+                          count_wrong_shares=0, # no such thing, for immutable
+                          list_corrupt_shares=corruptshare_locators,
+                          count_corrupt_shares=len(corruptshare_locators),
+                          list_incompatible_shares=incompatibleshare_locators,
+                          count_incompatible_shares=len(incompatibleshare_locators),
+                          summary=summary,
+                          report=[],
+                          share_problems=[],
+                          servermap=None)
 
         return cr
 
     def start(self):
         ds = []
         if self._verify:
-            for (serverid, ss) in self._servers:
-                ds.append(self._verify_server_shares(serverid, ss))
+            for s in self._servers:
+                ds.append(self._verify_server_shares(s))
         else:
-            for (serverid, ss) in self._servers:
-                ds.append(self._check_server_shares(serverid, ss))
+            for s in self._servers:
+                ds.append(self._check_server_shares(s))
 
         return deferredutil.gatherResults(ds).addCallback(self._format_results)