Handle error conditions better in NetworksWithin

Previously, NetworksWithin would allow you to look up an IPv6 network in
an IPv4 database. In this case it would return invalid data.

As the method doesn't have an error return value, it will now return a
Networks with Err() set. When the Networks has an error, iteration will
stop and the Network call will fail.

Author: oschwald

traverse.go

@@ -1,6 +1,7 @@
 package maxminddb
 
 import (
+	"fmt"
 	"net"
 )
 
@@ -51,6 +52,15 @@ func (r *Reader) Networks() *Networks {
 // If the provided network is contained within a network in the database, the
 // iterator will iterate over exactly one network, the containing network.
 func (r *Reader) NetworksWithin(network *net.IPNet) *Networks {
+	if r.Metadata.IPVersion == 4 && network.IP.To4() == nil {
+		return &Networks{
+			err: fmt.Errorf(
+				"error getting networks with '%s': you attempted to use an IPv6 network in an IPv4-only database",
+				network.String(),
+			),
+		}
+	}
+
 	ip := network.IP
 	prefixLength, _ := network.Mask.Size()
 
@@ -76,6 +86,9 @@ func (r *Reader) NetworksWithin(network *net.IPNet) *Networks {
 // returns true if there is another network to be processed and false if there
 // are no more networks or if there is an error.
 func (n *Networks) Next() bool {
+	if n.err != nil {
+		return false
+	}
 	for len(n.nodes) > 0 {
 		node := n.nodes[len(n.nodes)-1]
 		n.nodes = n.nodes[:len(n.nodes)-1]
@@ -115,6 +128,9 @@ func (n *Networks) Next() bool {
 // decoding the data for the network. It takes a pointer to a result value to
 // decode the network's data into.
 func (n *Networks) Network(result interface{}) (*net.IPNet, error) {
+	if n.err != nil {
+		return nil, n.err
+	}
 	if err := n.reader.retrieveData(n.lastNode.pointer, result); err != nil {
 		return nil, err
 	}

traverse_test.go

@@ -152,22 +152,17 @@ var tests = []networkTest{
 		},
 	},
 	{
-		Network:  "::/0",
-		Database: "ipv4",
+		Network:  "1.1.1.16/28",
+		Database: "mixed",
 		Expected: []string{
-			"101:101::/32",
-			"101:102::/31",
-			"101:104::/30",
-			"101:108::/29",
-			"101:110::/28",
-			"101:120::/32",
+			"1.1.1.16/28",
 		},
 	},
 	{
-		Network:  "101:104::/30",
+		Network:  "1.1.1.4/30",
 		Database: "ipv4",
 		Expected: []string{
-			"101:104::/30",
+			"1.1.1.4/30",
 		},
 	},
 }