Update client target faster

Adjust client's target faster to prevent DDoS when hashrate is unknown at the
beginning and start difficulty is low or hashrate changed significantly later.
Add option retarget-ratio to control safe boundaries for client hashrate.

pool.conf

@@ -12,6 +12,7 @@ pool-fee = 0.01
 payment-threshold = 0.33
 share-mul = 2.0
 retarget-time = 120
+retarget-ratio = 0.55
 log-level = 5
 log-file =
 block-notified = 0

src/pool.c

@@ -134,6 +134,7 @@ typedef struct config_t
     uint64_t pool_start_diff;
     double share_mul;
     uint32_t retarget_time;
+    double retarget_ratio;
     double pool_fee;
     double payment_threshold;
     uint32_t pool_port;
@@ -842,8 +843,8 @@ template_recycle(void *item)
     }
 }
 
-static void
-retarget(client_t *client, job_t *job)
+static uint64_t
+client_target(client_t *client, job_t *job)
 {
     uint64_t bd = 0xFFFFFFFFFFFFFFFF;
     if (job->block_template)
@@ -852,6 +853,20 @@ retarget(client_t *client, job_t *job)
     uint8_t retarget_time = client->is_xnp ? 5 : config.retarget_time;
     uint64_t target = fmin(fmax((double)client->hashes /
             duration * retarget_time, config.pool_start_diff), bd);
+    return target;
+}
+
+static bool
+retarget_required(client_t *client, job_t *job)
+{
+    return ((double)job->target / client_target(client, job)
+            < config.retarget_ratio);
+}
+
+static void
+retarget(client_t *client, job_t *job)
+{
+    uint64_t target = client_target(client, job);
     job->target = target;
     log_debug("Client %.32s target now %"PRIu64, client->client_id, target);
 }
@@ -2531,6 +2546,12 @@ client_on_submit(json_object *message, client_t *client)
         stratum_get_status_body(body, client->json_id, "OK");
         evbuffer_add(output, body, strlen(body));
     }
+    if (retarget_required(client, job))
+    {
+        log_debug("Sending an early job as this was less than %u%% of"
+                " potential", (unsigned)(100.*config.retarget_ratio));
+        client_send_job(client, false);
+    }
 }
 
 static void
@@ -2695,6 +2716,7 @@ read_config(const char *config_file)
     config.pool_start_diff = 100;
     config.share_mul = 2.0;
     config.retarget_time = 120;
+    config.retarget_ratio = 0.55;
     config.pool_fee = 0.01;
     config.payment_threshold = 0.33;
     config.pool_port = 4242;
@@ -2807,6 +2829,10 @@ read_config(const char *config_file)
         {
             config.retarget_time = atoi(val);
         }
+        else if (strcmp(key, "retarget-ratio") == 0)
+        {
+            config.retarget_ratio = atof(val);
+        }
         else if (strcmp(key, "log-level") == 0)
         {
             config.log_level = atoi(val);
@@ -2854,6 +2880,13 @@ read_config(const char *config_file)
                 "Aborting.");
         exit(-1);
     }
+    if (config.retarget_ratio < 0 || config.retarget_ratio > 1)
+    {
+        log_fatal("Set retarget-ratio to any rational value within range "
+                "[0, 1]. Clients will receive new jobs earlier if their latest"
+                " work is less than retarget-ratio percentage of potential.");
+        exit(-1);
+    }
 
 }
 static void print_config()
@@ -2873,6 +2906,7 @@ static void print_config()
         "  payment-threshold = %.2f\n"
         "  share-mul = %.2f\n"
         "  retarget-time = %u\n"
+        "  retarget-ratio = %.2f\n"
         "  log-level = %u\n"
         "  log-file = %s\n"
         "  block-notified = %u\n"
@@ -2894,6 +2928,7 @@ static void print_config()
         config.payment_threshold,
         config.share_mul,
         config.retarget_time,
+        config.retarget_ratio,
         config.log_level,
         config.log_file,
         config.block_notified,