Provide stats count of current "using" per tube.

conn.c

@@ -42,7 +42,6 @@ static void
 conn_free(conn c)
 {
  c->fd = 0;
- TUBE_ASSIGN(c->use, NULL);
  conn_insert(&pool, c);
 }
 
@@ -77,6 +76,7 @@ make_conn(int fd, char start_state, tube use, tube watch)
 
  c->use = NULL; /* initialize */
  TUBE_ASSIGN(c->use, use);
+ use->using_ct++;
 
  c->fd = fd;
  c->state = start_state;
@@ -251,6 +251,8 @@ conn_close(conn c)
  if (has_reserved_job(c)) enqueue_reserved_jobs(c);
 
  ms_clear(&c->watch);
+ c->use->using_ct--;
+ TUBE_ASSIGN(c->use, NULL);
 
  conn_free(c);
 }

prot.c

@@ -149,6 +149,7 @@
  "current-jobs-reserved: %u\n" \
  "current-jobs-buried: %u\n" \
  "total-jobs: %llu\n" \
+ "current-using: %u\n" \
  "current-watching: %u\n" \
  "current-waiting: %u\n" \
  "\r\n"
@@ -892,6 +893,7 @@ fmt_stats_tube(char *buf, size_t size, tube t)
  t->stat.reserved_ct,
  t->stat.buried_ct,
  t->stat.total_jobs_ct,
+ t->using_ct,
  t->watching_ct,
  t->stat.waiting_ct);
 }
@@ -1214,8 +1216,10 @@ dispatch_cmd(conn c)
  TUBE_ASSIGN(t, find_or_make_tube(name));
  if (!t) return reply_serr(c, MSG_OUT_OF_MEMORY);
 
+ c->use->using_ct--;
  TUBE_ASSIGN(c->use, t);
  TUBE_ASSIGN(t, NULL);
+ c->use->using_ct++;
 
  use_ct++;
  reply_line(c, STATE_SENDWORD, "USING %s\r\n", c->use->name);

tube.c

@@ -43,7 +43,7 @@ make_tube(const char *name)
  ms_init(&t->waiting, NULL, NULL);
 
  t->stat = (struct stats) {0, 0, 0, 0};
- t->watching_ct = 0;
+ t->using_ct = t->watching_ct = 0;
 
  return t;
 }

tube.h

@@ -35,6 +35,7 @@ struct tube {
  struct job buried;
  struct ms waiting; /* set of conns */
  struct stats stat;
+ unsigned int using_ct;
  unsigned int watching_ct;
 };