slapformat: Wrap partition IPv6 mapping

When the range available to allocate a single IPv6 to each partition is
too small, wrap around by allocating the same IPs instead of aborting.

The wrap around logic loops over the addresses in the range other than
0 and 1 which are reserved for the slapnode, except if the range is so
small that there are no other addresses.

slapos/util.py

@@ -38,6 +38,7 @@ import socket
 import sqlite3
 import struct
 import subprocess
+import sys
 import warnings
 
 import jsonschema
@@ -201,7 +202,7 @@ def ipv6FromBin(ip, suffix=''):
   if suffix_len > 0:
     ip += suffix.rjust(suffix_len, '0')
   elif suffix_len:
-    sys.exit("Prefix exceeds 128 bits")
+    sys.exit("Prefix %s exceeds 128 bits by %d bit" % (ip, -suffix_len))
   return socket.inet_ntop(socket.AF_INET6,
     struct.pack('>QQ', int(ip[:64], 2), int(ip[64:], 2)))
 
@@ -214,11 +215,23 @@ def getPartitionIpv6Addr(ipv6_range, partition_index):
   }
   returns the IPv6 addr
   addr::(partition_index+2) (address 1 is is used by re6st)
+  If the range is too small, wrap around
   """
   addr = ipv6_range['addr']
   prefixlen = ipv6_range['prefixlen']
   prefix = binFromIpv6(addr)[:prefixlen]
-  return dict(addr=ipv6FromBin(prefix + bin(partition_index+2)[2:].zfill(128 - prefixlen)), prefixlen=prefixlen)
+  remaining = 128 - prefixlen
+  suffix = bin(partition_index+2)[2:]
+  if len(suffix) > remaining:
+    if remaining >= 2:
+      # skip reserved addresses 0 and 1
+      suffix = bin((partition_index % ((1 << remaining) - 2)) + 2)[2:]
+    else:
+      # truncate, we have no other addresses than 0 and 1
+      suffix = suffix[len(suffix) - remaining:]
+  suffix = suffix.zfill(remaining)
+  bits = prefix + suffix
+  return dict(addr=ipv6FromBin(bits), prefixlen=prefixlen)
 
 def getIpv6RangeFactory(k, s):
   """