Use the doubly-linked list for the free conn pool as well.

conn.c

@@ -8,15 +8,15 @@
 #include "util.h"
 #include "prot.h"
 
-/* Singly-linked list of free connections (the prev pointer isn't used here). */
-static conn pool_front = NULL, pool_rear = NULL;
+/* Doubly-linked list of free connections. */
+static struct conn pool = { &pool, &pool, 0 };
 
 int cur_conn_ct = 0, cur_worker_ct = 0, cur_producer_ct = 0;
 
 static int
 pool_conn_p()
 {
- return !!pool_front;
+ return conn_list_empty_p(&pool);
 }
 
 static conn
@@ -27,9 +27,8 @@ conn_alloc()
  if (!pool_conn_p()) return malloc(sizeof(struct conn));
 
  /* remove it from the list */
- c = pool_front;
- pool_front = c->next;
- c->prev = c->next = NULL;
+ c = pool.next;
+ conn_remove(c);
 
  return c;
 }
@@ -38,13 +37,7 @@ static void
 conn_free(conn c)
 {
  c->fd = 0;
- c->prev = c->next = NULL;
- if (pool_conn_p()) {
- pool_rear->next = c;
- } else {
- pool_front = c;
- }
- pool_rear = c;
+ conn_insert(&pool, c);
 }
 
 conn
@@ -132,6 +125,12 @@ conn_update_evq(conn c, const int events)
  return conn_set_evq(c, events, c->evq.ev_callback);
 }
 
+int
+conn_list_empty_p(conn head)
+{
+ return head->next != head;
+}
+
 void
 conn_remove(conn c)
 {

conn.h

@@ -34,6 +34,7 @@
 typedef struct conn *conn;
 
 struct conn {
+ conn prev, next; /* linked list of connections */
  int fd;
  char state;
  char type;
@@ -54,7 +55,6 @@ struct conn {
  job out_job;
  int out_job_sent;
  job reserved_job;
- conn prev, next; /* linked list of connections */
 };
 
 void conn_init();
@@ -66,6 +66,7 @@ int conn_update_evq(conn c, const int flags);
 
 void conn_close(conn c);
 
+int conn_list_empty_p(conn head);
 void conn_remove(conn c);
 void conn_insert(conn head, conn c);
 

prot.c

@@ -12,12 +12,12 @@
 static pq ready_q;
 
 /* Doubly-linked list of waiting connections. */
-static struct conn wait_queue; /* sentinel */
+static struct conn wait_queue = { &wait_queue, &wait_queue, 0 };
 
 static int
 waiting_conn_p()
 {
- return wait_queue.next != &wait_queue;
+ return conn_list_empty_p(&wait_queue);
 }
 
 void
@@ -110,5 +110,4 @@ void
 prot_init()
 {
  ready_q = make_pq(HEAP_SIZE);
- wait_queue.next = wait_queue.prev = &wait_queue;
 }