Switch to random allocation on circuitIDs.

Fixes a possible root cause of 11553 by only making 64 attempts at
most to pick a circuitID.  Previously, we would test every possible
circuit ID until we found one or ran out.

This algorithm succeeds probabilistically. As the comment says:

  This potentially causes us to give up early if our circuit ID
  space is nearly full.  If we have N circuit IDs in use, then we
  will reject a new circuit with probability (N / max_range) ^
  MAX_CIRCID_ATTEMPTS.  This means that in practice, a few percent
  of our circuit ID capacity will go unused.

  The alternative here, though, is to do a linear search over the
  whole circuit ID space every time we extend a circuit, which is
  not so great either.

This makes new vs old clients distinguishable, so we should try to
batch it with other patches that do that, like 11438.

1 files changed, 0 insertions, 3 deletions

diff --git a/src/or/channel.c b/src/or/channel.c
index 1270eace7d..dfef703b1b 100644
--- a/src/or/channel.c
+++ b/src/or/channel.c
@@ -731,9 +731,6 @@ channel_init(channel_t *chan)
   /* Init timestamp */
   chan->timestamp_last_added_nonpadding = time(NULL);
 
-  /* Init next_circ_id */
-  chan->next_circ_id = crypto_rand_int(1 << 15);
-
   /* Initialize queues. */
   TOR_SIMPLEQ_INIT(&chan->incoming_queue);
   TOR_SIMPLEQ_INIT(&chan->outgoing_queue);