Fix invalid "Network is unreachable" errors

[antlarr]

Sometimes, in systems with both ipv4 and ipv6 addresses but where the
network doesn't support ipv6, Curl_is_connected returns an error
(intermittently) even if the ipv4 socket connects successfully.

This happens because there's a for-loop that iterates on the sockets
but the error variable is not resetted when the ipv4 is checked and
is ok.

This patch fixes this problem by setting error to 0 when checking the
second socket and not having a result yet.

[jay]

Ref: https://curl.haxx.se/mail/lib-2016-05/0019.html

We use that variable later on though and I think in that case we don't want to reset it

diff --git a/lib/connect.c b/lib/connect.c
index 8dfe9e2..39fd064 100644
--- a/lib/connect.c
+++ b/lib/connect.c
@@ -722,7 +722,7 @@ CURLcode Curl_is_connected(struct connectdata *conn,
   struct SessionHandle *data = conn->data;
   CURLcode result = CURLE_OK;
   long allow;
-  int error = 0;
+  int error_save = 0;
   struct timeval now;
   int rc;
   int i;
@@ -749,6 +749,7 @@ CURLcode Curl_is_connected(struct connectdata *conn,
   }

   for(i=0; i<2; i++) {
+    int error = 0;
     const int other = i ^ 1;
     if(conn->tempsock[i] == CURL_SOCKET_BAD)
       continue;
@@ -817,6 +818,7 @@ CURLcode Curl_is_connected(struct connectdata *conn,
      * address" for the given host. But first remember the latest error.
      */
     if(error) {
+      error_save = error;
       data->state.os_errno = error;
       SET_SOCKERRNO(error);
       if(conn->tempaddr[i]) {
@@ -859,7 +861,7 @@ CURLcode Curl_is_connected(struct connectdata *conn,
       hostname = conn->host.name;

     failf(data, "Failed to connect to %s port %ld: %s",
-        hostname, conn->port, Curl_strerror(conn, error));
+        hostname, conn->port, Curl_strerror(conn, error_save));
   }

   return result;

[jay]

I took another look at this just now and it seems that with your patch there's no path for what I thought could happen to actually happen.

[antlarr]

I thought so, but just in case I commited your patch too. Should I revert it?

[bagder]

No need, I'll just merge the first patch and leave out the second one. Thanks!