Delay salt initialization for audit backends

audit/format_json.go

@@ -4,12 +4,15 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+
+	"github.com/hashicorp/vault/helper/salt"
 )
 
 // JSONFormatWriter is an AuditFormatWriter implementation that structures data into
 // a JSON format.
 type JSONFormatWriter struct {
-	Prefix string
+	Prefix   string
+	SaltFunc func() (*salt.Salt, error)
 }
 
 func (f *JSONFormatWriter) WriteRequest(w io.Writer, req *AuditRequestEntry) error {
@@ -43,3 +46,7 @@ func (f *JSONFormatWriter) WriteResponse(w io.Writer, resp *AuditResponseEntry)
 	enc := json.NewEncoder(w)
 	return enc.Encode(resp)
 }
+
+func (f *JSONFormatWriter) Salt() (*salt.Salt, error) {
+	return f.SaltFunc()
+}