Limit list_log_file response size (#265)

* Stop sending list of active log files if max Monog doc size is reached

* Use JSON structure for size calculation

Fixes Graylog2/graylog2-server#4899

api/graylog.go

@@ -16,7 +16,9 @@
 package api
 
 import (
+	"bytes"
 	"crypto/tls"
+	"encoding/json"
 	"io"
 	"net/http"
 	"strconv"
@@ -151,7 +153,20 @@ func UpdateRegistration(httpClient *http.Client, ctx *context.Ctx, status *grayl
 		registration.NodeDetails.Status = status
 		registration.NodeDetails.Metrics = metrics
 		if len(ctx.UserConfig.ListLogFiles) > 0 {
-			registration.NodeDetails.LogFileList = common.ListFiles(ctx.UserConfig.ListLogFiles)
+			fileList := common.ListFiles(ctx.UserConfig.ListLogFiles)
+			buf := new(bytes.Buffer)
+			if fileList != nil {
+				json.NewEncoder(buf).Encode(fileList)
+			}
+			fileListSize := buf.Len()
+			// Maximum MongoDB document size is 16793600 bytes so we leave some extra space for the rest of the request
+			// before we skip to send the file list.
+			if fileListSize < 10000000 {
+				registration.NodeDetails.LogFileList = fileList
+			} else {
+				log.Warn("[UpdateRegistration] Maximum file list size exceeded, skip sending list of active log files!" +
+					" Adjust list_log_file setting.")
+			}
 		}
 	}