Issue #396: treat registry.consul.kvpath as prefix

This patch extends the behavior of the registry.consul.kvpath
and treats it as a prefix instead of a single key. fabio will
now list the key and all available subkeys in alphabetical order
and combine them for the routing table. Each subsection will get
header which contains key name.

Todo: the ui needs to handle that as well

docs/content/ref/registry.consul.kvpath.md

@@ -8,6 +8,26 @@ The Consul KV path is watched for changes which get appended to
 the routing table. This allows for manual overrides and weighted
 round-robin routes.
 
+As of version 1.5.7 fabio will treat the kv path as a prefix and
+combine the values of the key itself and all its subkeys in 
+alphabetical order.
+
+To see all updates you may want to set [`-log.routes.format`](/ref/log.routes.format/)
+to `all`.
+
+You can modify the content of the routes with the `consul` tool or via
+the [Consul API](https://www.consul.io/api/index.html):
+
+```
+consul put fabio/config "route add svc /maint http://5.6.7.8:5000\nroute add svc / http://1.2.3.4:5000\n"
+
+# fabio >= 1.5.7 supports prefix match
+consul put fabio/config/maint    "route add svc /maint http://5.6.7.8:5000"
+consul put fabio/config/catchall "route add svc / http://1.2.3.4:5000"
+
+consul delete fabio/config/maint
+```
+
 The default is
 
 	registry.consul.kvpath = /fabio/config

fabio.properties

@@ -609,7 +609,9 @@
 #
 # The consul KV path is watched for changes which get appended to
 # the routing table. This allows for manual overrides and weighted
-# round-robin routes.
+# round-robin routes. The key itself (e.g. fabio/config) and all
+# subkeys (e.g. fabio/config/foo and fabio/config/bar) are combined
+# in alphabetical order.
 #
 # The default is
 #

registry/consul/kv.go

@@ -15,7 +15,7 @@ func watchKV(client *api.Client, path string, config chan string) {
 	var lastValue string
 
 	for {
-		value, index, err := getKV(client, path, lastIndex)
+		value, index, err := listKV(client, path, lastIndex)
 		if err != nil {
 			log.Printf("[WARN] consul: Error fetching config from %s. %v", path, err)
 			time.Sleep(time.Second)
@@ -30,6 +30,23 @@ func watchKV(client *api.Client, path string, config chan string) {
 	}
 }
 
+func listKV(client *api.Client, path string, waitIndex uint64) (string, uint64, error) {
+	q := &api.QueryOptions{RequireConsistent: true, WaitIndex: waitIndex}
+	kvpairs, meta, err := client.KV().List(path, q)
+	if err != nil {
+		return "", 0, err
+	}
+	if len(kvpairs) == 0 {
+		return "", meta.LastIndex, nil
+	}
+	var s []string
+	for _, kvpair := range kvpairs {
+		val := "# --- " + kvpair.Key + "\n" + strings.TrimSpace(string(kvpair.Value))
+		s = append(s, val)
+	}
+	return strings.Join(s, "\n\n"), meta.LastIndex, nil
+}
+
 func getKV(client *api.Client, key string, waitIndex uint64) (string, uint64, error) {
 	q := &api.QueryOptions{RequireConsistent: true, WaitIndex: waitIndex}
 	kvpair, meta, err := client.KV().Get(key, q)