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Fix off by one error in SRV result shuffling

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This commit is contained in:
Erik Johnston 2019-08-27 13:56:42 +01:00
parent fbb758a7ce
commit 91caa5b430
1 changed files with 13 additions and 8 deletions
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21
synapse/http/federation/srv_resolver.py
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@ -66,17 +66,18 @@ def _sort_server_list(server_list):
for priority in sorted(priority_map):
servers = priority_map[priority]
# This algorithms follows the algorithm described in RFC2782.
# This algorithms roughly follows the algorithm described in RFC2782,
# changed to remove an off-by-one error.
#
# N.B. Weights can be zero, which means that you should pick that server
# last *or* that its the only server in this priority.
# We sort to ensure zero weighted items are first.
servers.sort(key=lambda s: s.weight)
# N.B. Weights can be zero, which means that they should be picked
# rarely.
total_weight = sum(s.weight for s in servers)
while servers:
target_weight = random.randint(0, total_weight)
# Total weight can become zero if there are only zero weight servers
# left, which we handle by just shuffling and appending to the results.
while servers and total_weight:
target_weight = random.randint(1, total_weight)
for s in servers:
target_weight -= s.weight
@ -88,6 +89,10 @@ def _sort_server_list(server_list):
servers.remove(s)
total_weight -= s.weight
if servers:
random.shuffle(servers)
results.extend(servers)
return results