Log the number and size of DNS cache entries on SIGUSR1.

Specifically if you send SIGUSR1, it will add two lines to the log file:
May 22 07:41:59.576 [notice] Our DNS cache has 3364 entries.
May 22 07:41:59.576 [notice] Our DNS cache size is approximately 1022656
bytes.

[tweaked a bit by nickm]

1 files changed, 25 insertions, 0 deletions

diff --git a/src/or/dns.c b/src/or/dns.c
index 1742e3dd4b..88d08a8ffe 100644
--- a/src/or/dns.c
+++ b/src/or/dns.c
@@ -1612,6 +1612,31 @@ assert_resolve_ok(cached_resolve_t *resolve)
   }
 }
 
+/** Return the number of DNS cache entries as an int */
+static int
+dns_cache_entry_count(void)
+{
+   return HT_SIZE(&cache_root);
+}
+
+/** Log memory information about our internal DNS cache at level 'severity'. */
+void
+dump_dns_mem_usage(int severity)
+{
+  /* This should never be larger than INT_MAX. */
+  int hash_count = dns_cache_entry_count();
+  size_t hash_mem = sizeof(struct cached_resolve_t) * hash_count;
+  hash_mem += HT_MEM_USAGE(&cache_root);
+
+  /* Print out the count and estimated size of our &cache_root.  It undercounts
+     hostnames in cached reverse resolves.
+   */
+  log(severity, LD_MM, "Our DNS cache has %d entries.", hash_count);
+  log(severity, LD_MM, "Our DNS cache size is approximately %u bytes.",
+      (unsigned)hash_mem);
+}
+
+
 #ifdef DEBUG_DNS_CACHE
 /** Exit with an assertion if the DNS cache is corrupt. */
 static void