Trim the JSON logged for Kubernetes objects by weave-npc

Remove things like "args" and "env" which bulk the logs, aren't
relevant to the function of weave-npc, and may include sensitive
information.

Use a black-list rather than a white-list because we can't be sure
what information will be necessary in future.

npc/json.go

@@ -4,7 +4,56 @@ import (
 	"encoding/json"
 )
 
+// Return JSON suitable for logging an API object.
 func js(v interface{}) string {
+	// Get the raw JSON
 	a, _ := json.Marshal(v)
+	// Convert this back into a tree of key-value maps
+	var m map[string]interface{}
+	if err := json.Unmarshal(a, &m); err != nil {
+		// If that didn't work, just return the raw version
+		return string(a)
+	}
+	// Trim some bulk, and potentially sensitive areas
+	withMap(m["metadata"], func(status map[string]interface{}) {
+		delete(status, "ownerReferences")
+	})
+	withMap(m["spec"], func(spec map[string]interface{}) {
+		delete(spec, "tolerations")
+		delete(spec, "volumes")
+		rangeSlice(spec["containers"], func(container map[string]interface{}) {
+			delete(container, "args")
+			delete(container, "command")
+			delete(container, "env")
+			delete(container, "livenessProbe")
+			delete(container, "resources")
+			delete(container, "securityContext")
+			delete(container, "volumeMounts")
+		})
+	})
+	withMap(m["status"], func(status map[string]interface{}) {
+		delete(status, "containerStatuses")
+	})
+	// Now marshall what's left to JSON
+	a, _ = json.Marshal(m)
 	return string(a)
 }
+
+// Helper function: operate on a map node from a tree of key-value maps
+func withMap(m interface{}, f func(map[string]interface{})) {
+	if v, ok := m.(map[string]interface{}); ok {
+		f(v)
+	}
+}
+
+// Helper function: operate on all nodes under i which is a slice in a
+// tree of key-value maps
+func rangeSlice(i interface{}, f func(map[string]interface{})) {
+	if s, ok := i.([]interface{}); ok {
+		for _, v := range s {
+			if m, ok := v.(map[string]interface{}); ok {
+				f(m)
+			}
+		}
+	}
+}