dns: optimize tx list walks

The detection engine and log engines can walk the tx list indirectly,
by looping AppLayerParserGetTx. This would lead to new list walks in
the DNS tx list though. Leading to bad performance.

This patch stores the last returned tx and uses that to determine if
the next tx is what we need next. If so, we can return that w/o list
walk.

src/app-layer-dns-common.c

@@ -173,15 +173,31 @@ void *DNSGetTx(void *alstate, uint64_t tx_id)
     DNSState *dns_state = (DNSState *)alstate;
     DNSTransaction *tx = NULL;
 
+    /* fast track: try the current tx */
     if (dns_state->curr && dns_state->curr->tx_num == tx_id + 1)
         return dns_state->curr;
 
+    /* fast track:
+     * if the prev tx_id is equal to the stored tx ptr, we can
+     * use this shortcut to get to the next. */
+    if (dns_state->iter) {
+        if (tx_id == dns_state->iter->tx_num) {
+            tx = TAILQ_NEXT(dns_state->iter, next);
+            if (tx && tx->tx_num == tx_id + 1) {
+                dns_state->iter = tx;
+                return tx;
+            }
+        }
+    }
+
+    /* no luck with the fast tracks, do the full list walk */
     TAILQ_FOREACH(tx, &dns_state->tx_list, next) {
         SCLogDebug("tx->tx_num %u, tx_id %"PRIu64, tx->tx_num, (tx_id+1));
         if ((tx_id+1) != tx->tx_num)
             continue;
 
         SCLogDebug("returning tx %p", tx);
+        dns_state->iter = tx;
         return tx;
     }
 
@@ -280,6 +296,9 @@ static void DNSTransactionFree(DNSTransaction *tx, DNSState *state)
     if (tx->de_state != NULL)
         DetectEngineStateFree(tx->de_state);
 
+    if (state->iter == tx)
+        state->iter = NULL;
+
     DNSDecrMemcap(sizeof(DNSTransaction), state);
     SCFree(tx);
     SCReturn;

src/app-layer-dns-common.h

@@ -156,6 +156,7 @@ typedef struct DNSTransaction_ {
 typedef struct DNSState_ {
     TAILQ_HEAD(, DNSTransaction_) tx_list;  /**< transaction list */
     DNSTransaction *curr;                   /**< ptr to current tx */
+    DNSTransaction *iter;
     uint64_t transaction_max;
     uint32_t unreplied_cnt;                 /**< number of unreplied requests in a row */
     uint32_t memuse;                        /**< state memuse, for comparing with