storage: remove the storage server's "remote_cancel_lease" function

We're removing this function because it is currently unused, because it is dangerous, and because the bug described in #1528 leaks the cancellation secret, which allows anyone who knows a file's storage index to abuse this function to delete shares of that file.
fixes #1528 (there are two patches that are each a sufficient fix to #1528 and this is one of them)

docs/garbage-collection.rst

@@ -18,7 +18,7 @@ making room for other shares. Tahoe currently uses a garbage collection
 one or more "leases", which are managed by clients who want the
 file/directory to be retained. The storage server accepts each share for a
 pre-defined period of time, and is allowed to delete the share if all of the
-leases are cancelled or allowed to expire.
+leases expire.
 
 Garbage collection is not enabled by default: storage servers will not delete
 shares without being explicitly configured to do so. When GC is enabled,
@@ -266,18 +266,12 @@ replace the previous set.
 The GC mechanism is also not immediate: a client which deletes a file will
 nevertheless be consuming extra disk space (and might be charged or otherwise
 held accountable for it) until the ex-file's leases finally expire on their
-own. If the client is certain that they've removed their last reference to
-the file, they could accelerate the GC process by cancelling their lease. The
-current storage server API provides a method to cancel a lease, but the
-client must be careful to coordinate with anyone else who might be
-referencing the same lease (perhaps a second directory in the same virtual
-drive), otherwise they might accidentally remove a lease that should have
-been retained.
+own.
 
 In the current release, these leases are each associated with a single "node
 secret" (stored in $BASEDIR/private/secret), which is used to generate
-renewal- and cancel- secrets for each lease. Two nodes with different secrets
-will produce separate leases, and will not be able to renew or cancel each
+renewal-secrets for each lease. Two nodes with different secrets
+will produce separate leases, and will not be able to renew each
 others' leases.
 
 Once the Accounting project is in place, leases will be scoped by a

src/allmydata/interfaces.py

@@ -148,21 +148,6 @@ class RIStorageServer(RemoteInterface):
         """
         return Any()
 
-    def cancel_lease(storage_index=StorageIndex,
-                     cancel_secret=LeaseCancelSecret):
-        """
-        Cancel the lease on a given bucket. If this was the last lease on the
-        bucket, the bucket will be deleted. If there is no bucket for the
-        given storage_index, IndexError will be raised.
-
-        For mutable shares, if the given cancel_secret does not match an
-        existing lease, IndexError will be raised with a note listing the
-        server-nodeids on the existing leases, so leases on migrated shares
-        can be renewed or cancelled. For immutable shares, IndexError
-        (without the note) will be raised.
-        """
-        return Any()
-
     def get_buckets(storage_index=StorageIndex):
         return DictOf(int, RIBucketReader, maxKeys=MAX_BUCKETS)
 

src/allmydata/storage/server.py

@@ -345,34 +345,6 @@ class StorageServer(service.MultiService, Referenceable):
         if not found_buckets:
             raise IndexError("no such lease to renew")
 
-    def remote_cancel_lease(self, storage_index, cancel_secret):
-        start = time.time()
-        self.count("cancel")
-
-        total_space_freed = 0
-        found_buckets = False
-        for sf in self._iter_share_files(storage_index):
-            # note: if we can't find a lease on one share, we won't bother
-            # looking in the others. Unless something broke internally
-            # (perhaps we ran out of disk space while adding a lease), the
-            # leases on all shares will be identical.
-            found_buckets = True
-            # this raises IndexError if the lease wasn't present XXXX
-            total_space_freed += sf.cancel_lease(cancel_secret)
-
-        if found_buckets:
-            storagedir = os.path.join(self.sharedir,
-                                      storage_index_to_dir(storage_index))
-            if not os.listdir(storagedir):
-                os.rmdir(storagedir)
-
-        if self.stats_provider:
-            self.stats_provider.count('storage_server.bytes_freed',
-                                      total_space_freed)
-        self.add_latency("cancel", time.time() - start)
-        if not found_buckets:
-            raise IndexError("no such storage index")
-
     def bucket_writer_closed(self, bw, consumed_size):
         if self.stats_provider:
             self.stats_provider.count('storage_server.bytes_added', consumed_size)