From 1c0c0022d8199a9b998a0c4e13a85a382abf7fb0 Mon Sep 17 00:00:00 2001
From: Andrea Shepard <andrea@torproject.org>
Date: Thu, 30 Jun 2016 10:26:13 +0000
Subject: [PATCH] Compute thresholds for OOS from ConnLimit_ when setting
 options

---
 src/or/config.c | 26 ++++++++++++++++++++++++++
 src/or/or.h     |  4 ++++
 2 files changed, 30 insertions(+)

diff --git a/src/or/config.c b/src/or/config.c
index 3fab3b3ec8..33c80e52b0 100644
--- a/src/or/config.c
+++ b/src/or/config.c
@@ -1334,6 +1334,32 @@ options_act_reversible(const or_options_t *old_options, char **msg)
       connection_mark_for_close(conn);
     }
   });
+
+  if (set_conn_limit) {
+    /*
+     * If we adjusted the conn limit, recompute the OOS threshold too
+     *
+     * How many possible sockets to keep in reserve?  If we have lots of
+     * possible sockets, keep this below a limit and set ConnLimit_high_thresh
+     * very close to ConnLimit_, but if ConnLimit_ is low, shrink it in
+     * proportion.
+     *
+     * Somewhat arbitrarily, set socks_in_reserve to 5% of ConnLimit_, but
+     * cap it at 64.
+     */
+    int socks_in_reserve = options->ConnLimit_ / 20;
+    if (socks_in_reserve > 64) socks_in_reserve = 64;
+
+    options->ConnLimit_high_thresh = options->ConnLimit_ - socks_in_reserve;
+    options->ConnLimit_low_thresh = (options->ConnLimit_ / 4) * 3;
+    log_info(LD_GENERAL,
+             "Recomputed OOS thresholds: ConnLimit %d, ConnLimit_ %d, "
+             "ConnLimit_high_thresh %d, ConnLimit_low_thresh %d",
+             options->ConnLimit, options->ConnLimit_,
+             options->ConnLimit_high_thresh,
+             options->ConnLimit_low_thresh);
+  }
+
   goto done;
 
  rollback:
diff --git a/src/or/or.h b/src/or/or.h
index 1bac43c20a..262085ea62 100644
--- a/src/or/or.h
+++ b/src/or/or.h
@@ -3699,6 +3699,10 @@ typedef struct {
 
   int ConnLimit; /**< Demanded minimum number of simultaneous connections. */
   int ConnLimit_; /**< Maximum allowed number of simultaneous connections. */
+  int ConnLimit_high_thresh; /**< start trying to lower socket usage if we
+                              *   have this many. */
+  int ConnLimit_low_thresh; /**< try to get down to here after socket
+                             *   exhaustion. */
   int RunAsDaemon; /**< If true, run in the background. (Unix only) */
   int FascistFirewall; /**< Whether to prefer ORs reachable on open ports. */
   smartlist_t *FirewallPorts; /**< Which ports our firewall allows