dns: extend CURLOPT_RESOLVE syntax for adding non-permanent entries

Extend the syntax of CURLOPT_RESOLVE strings: allow using a '+' prefix
(similar to the existing '-' prefix for removing entries) to add
DNS cache entries that will time out just like entries that are added
by libcurl itself.

Append " (non-permanent)" to info log message in case a non-permanent
entry is added.

Adjust relevant comments to reflect the new behavior.

Adjust documentation.

Extend unit1607 to test the new functionality.

Closes #6294

1 files changed, 24 insertions, 12 deletions

diff --git a/lib/hostip.c b/lib/hostip.c
index ab1f6df05..50fb42f21 100644
--- a/lib/hostip.c
+++ b/lib/hostip.c
@@ -444,7 +444,7 @@ Curl_cache_addr(struct Curl_easy *data,
   dns->addr = addr; /* this is the address(es) */
   time(&dns->timestamp);
   if(dns->timestamp == 0)
-    dns->timestamp = 1;   /* zero indicates CURLOPT_RESOLVE entry */
+    dns->timestamp = 1;   /* zero indicates permanent CURLOPT_RESOLVE entry */
 
   /* Store the resolved data in our DNS cache. */
   dns2 = Curl_hash_add(data->dns.hostcache, entry_id, entry_len + 1,
@@ -916,17 +916,24 @@ CURLcode Curl_loadhostpairs(struct Curl_easy *data)
       char *addr_end;
       char *port_ptr;
       char *end_ptr;
+      bool permanent = TRUE;
+      char *host_begin;
       char *host_end;
       unsigned long tmp_port;
       bool error = true;
 
-      host_end = strchr(hostp->data, ':');
+      host_begin = hostp->data;
+      if(host_begin[0] == '+') {
+        host_begin++;
+        permanent = FALSE;
+      }
+      host_end = strchr(host_begin, ':');
       if(!host_end ||
-         ((host_end - hostp->data) >= (ptrdiff_t)sizeof(hostname)))
+         ((host_end - host_begin) >= (ptrdiff_t)sizeof(hostname)))
         goto err;
 
-      memcpy(hostname, hostp->data, host_end - hostp->data);
-      hostname[host_end - hostp->data] = '\0';
+      memcpy(hostname, host_begin, host_end - host_begin);
+      hostname[host_end - host_begin] = '\0';
 
       port_ptr = host_end + 1;
       tmp_port = strtoul(port_ptr, &end_ptr, 10);
@@ -1008,18 +1015,22 @@ CURLcode Curl_loadhostpairs(struct Curl_easy *data)
       if(data->share)
         Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);
 
-      /* See if its already in our dns cache */
+      /* See if it's already in our dns cache */
       dns = Curl_hash_pick(data->dns.hostcache, entry_id, entry_len + 1);
 
       if(dns) {
         infof(data, "RESOLVE %s:%d is - old addresses discarded!\n",
                 hostname, port);
-        /* delete old entry entry, there are two reasons for this
+        /* delete old entry, there are two reasons for this
          1. old entry may have different addresses.
          2. even if entry with correct addresses is already in the cache,
             but if it is close to expire, then by the time next http
             request is made, it can get expired and pruned because old
-            entry is not necessarily marked as added by CURLOPT_RESOLVE. */
+            entry is not necessarily marked as permanent.
+         3. when adding a non-permanent entry, we want it to remove and
+            replace an existing permanent entry.
+         4. when adding a non-permanent entry, we want it to get a "fresh"
+            timeout that starts _now_. */
 
         Curl_hash_delete(data->dns.hostcache, entry_id, entry_len + 1);
       }
@@ -1027,10 +1038,11 @@ CURLcode Curl_loadhostpairs(struct Curl_easy *data)
       /* put this new host in the cache */
       dns = Curl_cache_addr(data, head, hostname, port);
       if(dns) {
-        dns->timestamp = 0; /* mark as added by CURLOPT_RESOLVE */
+        if(permanent)
+          dns->timestamp = 0; /* mark as permanent */
         /* release the returned reference; the cache itself will keep the
          * entry alive: */
-            dns->inuse--;
+        dns->inuse--;
       }
 
       if(data->share)
@@ -1040,8 +1052,8 @@ CURLcode Curl_loadhostpairs(struct Curl_easy *data)
         Curl_freeaddrinfo(head);
         return CURLE_OUT_OF_MEMORY;
       }
-      infof(data, "Added %s:%d:%s to DNS cache\n",
-            hostname, port, addresses);
+      infof(data, "Added %s:%d:%s to DNS cache%s\n",
+            hostname, port, addresses, permanent ? "" : " (non-permanent)");
 
       /* Wildcard hostname */
       if(hostname[0] == '*' && hostname[1] == '\0') {