Fixed DNS resolution in case all sendto operations fail

DNS resolver now counts how many sendto operations are successful
during one DNS server attempt cycle (attempting to contact DNS
servers from 0 to last index, including stack specific ones). If all
socket sendto operations fail, DNS resolution is aborted.

If one or more are successful, DNS resolution is continued until
total network attempts count is reached or all sendto operations
fails during a cycle.

Author: Mika Leppänen

features/netsocket/nsapi_dns.cpp

@@ -80,14 +80,15 @@ struct DNS_QUERY {
     uint8_t dns_server;
     uint8_t retries;
     uint8_t total_attempts;
+    uint8_t send_success;
     uint8_t count;
     dns_state state;
 };
 
 static void nsapi_dns_cache_add(const char *host, nsapi_addr_t *address, uint32_t ttl);
 static nsapi_size_or_error_t nsapi_dns_cache_find(const char *host, nsapi_version_t version, nsapi_addr_t *address);
 
-static nsapi_error_t nsapi_dns_get_server_addr(NetworkStack *stack, uint8_t *index, uint8_t *total_attempts, SocketAddress *dns_addr);
+static nsapi_error_t nsapi_dns_get_server_addr(NetworkStack *stack, uint8_t *index, uint8_t *total_attempts, uint8_t *send_success, SocketAddress *dns_addr);
 
 static void nsapi_dns_query_async_create(void *ptr);
 static nsapi_error_t nsapi_dns_query_async_delete(int unique_id);
@@ -384,7 +385,7 @@ static nsapi_error_t nsapi_dns_cache_find(const char *host, nsapi_version_t vers
     return ret_val;
 }
 
-static nsapi_error_t nsapi_dns_get_server_addr(NetworkStack *stack, uint8_t *index, uint8_t *total_attempts, SocketAddress *dns_addr)
+static nsapi_error_t nsapi_dns_get_server_addr(NetworkStack *stack, uint8_t *index, uint8_t *total_attempts, uint8_t *send_success, SocketAddress *dns_addr)
 {
     bool dns_addr_set = false;
 
@@ -393,8 +394,10 @@ static nsapi_error_t nsapi_dns_get_server_addr(NetworkStack *stack, uint8_t *ind
     }
 
     if (*index >= DNS_SERVERS_SIZE + DNS_STACK_SERVERS_NUM) {
-        if (*total_attempts) {
+        // If there are total attempts left and send to has been successful at least once on this round
+        if (*total_attempts && *send_success) {
             *index = 0;
+            *send_success = 0;
         } else {
             return NSAPI_ERROR_NO_ADDRESS;
         }
@@ -453,11 +456,12 @@ static nsapi_size_or_error_t nsapi_dns_query_multiple(NetworkStack *stack, const
     uint8_t retries = MBED_CONF_NSAPI_DNS_RETRIES;
     uint8_t index = 0;
     uint8_t total_attempts = MBED_CONF_NSAPI_DNS_TOTAL_ATTEMPTS;
+    uint8_t send_success = 0;
 
     // check against each dns server
     while (true) {
         SocketAddress dns_addr;
-        err = nsapi_dns_get_server_addr(stack, &index, &total_attempts, &dns_addr);
+        err = nsapi_dns_get_server_addr(stack, &index, &total_attempts, &send_success, &dns_addr);
         if (err != NSAPI_ERROR_OK) {
             break;
         }
@@ -474,6 +478,8 @@ static nsapi_size_or_error_t nsapi_dns_query_multiple(NetworkStack *stack, const
             continue;
         }
 
+        send_success++;
+
         if (total_attempts) {
             total_attempts--;
         }
@@ -650,6 +656,7 @@ nsapi_value_or_error_t nsapi_dns_query_multiple_async(NetworkStack *stack, const
     query->dns_server = 0;
     query->retries = MBED_CONF_NSAPI_DNS_RETRIES + 1;
     query->total_attempts =  MBED_CONF_NSAPI_DNS_TOTAL_ATTEMPTS;
+    query->send_success = 0;
     query->dns_message_id = 0;
     query->socket_timeout = 0;
     query->total_timeout = MBED_CONF_NSAPI_DNS_TOTAL_ATTEMPTS * MBED_CONF_NSAPI_DNS_RESPONSE_WAIT_TIME + 500;
@@ -971,7 +978,7 @@ static void nsapi_dns_query_async_send(void *ptr)
 
     while (true) {
         SocketAddress dns_addr;
-        nsapi_size_or_error_t err = nsapi_dns_get_server_addr(query->stack, &(query->dns_server), &(query->total_attempts), &dns_addr);
+        nsapi_size_or_error_t err = nsapi_dns_get_server_addr(query->stack, &(query->dns_server), &(query->total_attempts), &(query->send_success), &dns_addr);
         if (err != NSAPI_ERROR_OK) {
             nsapi_dns_query_async_resp(query, NSAPI_ERROR_TIMEOUT, NULL);
             free(packet);
@@ -987,6 +994,8 @@ static void nsapi_dns_query_async_send(void *ptr)
         }
     }
 
+    query->send_success++;
+
     if (query->total_attempts) {
         query->total_attempts--;
     }