am 028ccf5d: Merge "Avoid multiple dns lookups for the same query"

* commit '028ccf5d40dd9a945ea92aa79822c08c6f6aa1d2':
  Avoid multiple dns lookups for the same query

libc/netbsd/resolv/res_cache.c

@@ -1170,6 +1170,15 @@ entry_equals( const Entry*  e1, const Entry*  e2 )
  * inlined in the Entry structure.
  */
 
+/* Maximum time for a thread to wait for an pending request */
+#define PENDING_REQUEST_TIMEOUT 20;
+
+typedef struct pending_req_info {
+    unsigned int                hash;
+    pthread_cond_t              cond;
+    struct pending_req_info*    next;
+} PendingReqInfo;
+
 typedef struct resolv_cache {
     int              max_entries;
     int              num_entries;
@@ -1178,6 +1187,7 @@ typedef struct resolv_cache {
     unsigned         generation;
     int              last_id;
     Entry*           entries;
+    PendingReqInfo   pending_requests;
 } Cache;
 
 typedef struct resolv_cache_info {
@@ -1191,6 +1201,107 @@ typedef struct resolv_cache_info {
 
 #define  HTABLE_VALID(x)  ((x) != NULL && (x) != HTABLE_DELETED)
 
+static void
+_cache_flush_pending_requests_locked( struct resolv_cache* cache )
+{
+    struct pending_req_info *ri, *tmp;
+    if (cache) {
+        ri = cache->pending_requests.next;
+
+        while (ri) {
+            tmp = ri;
+            ri = ri->next;
+            pthread_cond_broadcast(&tmp->cond);
+
+            pthread_cond_destroy(&tmp->cond);
+            free(tmp);
+        }
+
+        cache->pending_requests.next = NULL;
+    }
+}
+
+/* return 0 if no pending request is found matching the key
+ * if a matching request is found the calling thread will wait
+ * and return 1 when released */
+static int
+_cache_check_pending_request_locked( struct resolv_cache* cache, Entry* key )
+{
+    struct pending_req_info *ri, *prev;
+    int exist = 0;
+
+    if (cache && key) {
+        ri = cache->pending_requests.next;
+        prev = &cache->pending_requests;
+        while (ri) {
+            if (ri->hash == key->hash) {
+                exist = 1;
+                break;
+            }
+            prev = ri;
+            ri = ri->next;
+        }
+
+        if (!exist) {
+            ri = calloc(1, sizeof(struct pending_req_info));
+            if (ri) {
+                ri->hash = key->hash;
+                pthread_cond_init(&ri->cond, NULL);
+                prev->next = ri;
+            }
+        } else {
+            struct timespec ts = {0,0};
+            ts.tv_sec = _time_now() + PENDING_REQUEST_TIMEOUT;
+            int rv = pthread_cond_timedwait(&ri->cond, &cache->lock, &ts);
+        }
+    }
+
+    return exist;
+}
+
+/* notify any waiting thread that waiting on a request
+ * matching the key has been added to the cache */
+static void
+_cache_notify_waiting_tid_locked( struct resolv_cache* cache, Entry* key )
+{
+    struct pending_req_info *ri, *prev;
+
+    if (cache && key) {
+        ri = cache->pending_requests.next;
+        prev = &cache->pending_requests;
+        while (ri) {
+            if (ri->hash == key->hash) {
+                pthread_cond_broadcast(&ri->cond);
+                break;
+            }
+            prev = ri;
+            ri = ri->next;
+        }
+
+        // remove item from list and destroy
+        if (ri) {
+            prev->next = ri->next;
+            pthread_cond_destroy(&ri->cond);
+            free(ri);
+        }
+    }
+}
+
+/* notify the cache that the query failed */
+void
+_resolv_cache_query_failed( struct resolv_cache* cache,
+                   const void* query,
+                   int         querylen)
+{
+    Entry    key[1];
+
+    if (cache && entry_init_key(key, query, querylen)) {
+        pthread_mutex_lock(&cache->lock);
+        _cache_notify_waiting_tid_locked(cache, key);
+        pthread_mutex_unlock(&cache->lock);
+    }
+}
+
 static void
 _cache_flush_locked( Cache*  cache )
 {
@@ -1208,6 +1319,9 @@ _cache_flush_locked( Cache*  cache )
         }
     }
 
+    // flush pending request
+    _cache_flush_pending_requests_locked(cache);
+
     cache->mru_list.mru_next = cache->mru_list.mru_prev = &cache->mru_list;
     cache->num_entries       = 0;
     cache->last_id           = 0;
@@ -1491,7 +1605,17 @@ _resolv_cache_lookup( struct resolv_cache*  cache,
 
     if (e == NULL) {
         XLOG( "NOT IN CACHE");
-        goto Exit;
+        // calling thread will wait if an outstanding request is found
+        // that matching this query
+        if (!_cache_check_pending_request_locked(cache, key)) {
+            goto Exit;
+        } else {
+            lookup = _cache_lookup_p(cache, key);
+            e = *lookup;
+            if (e == NULL) {
+                goto Exit;
+            }
+        }
     }
 
     now = _time_now();
@@ -1594,6 +1718,7 @@ _resolv_cache_add( struct resolv_cache*  cache,
     _cache_dump_mru(cache);
 #endif
 Exit:
+    _cache_notify_waiting_tid_locked(cache, key);
     pthread_mutex_unlock( &cache->lock );
 }
 

libc/netbsd/resolv/res_send.c

@@ -646,6 +646,9 @@ res_nsend(res_state statp,
 		errno = terrno;
 	return (-1);
  fail:
+#if USE_RESOLV_CACHE
+	_resolv_cache_query_failed(cache, buf, buflen);
+#endif
 	res_nclose(statp);
 	return (-1);
 }

libc/private/resolv_cache.h

@@ -95,4 +95,10 @@ _resolv_cache_add( struct resolv_cache*  cache,
                    const void*           answer,
                    int                   answerlen );
 
+/* Notify the cache a request failed */
+extern void
+_resolv_cache_query_failed( struct resolv_cache* cache,
+                   const void* query,
+                   int         querylen);
+
 #endif /* _RESOLV_CACHE_H_ */