lib/logstorage: add Fields.ClearUpToCapacity() function for clearing all the fields up to Fields.Fields capacity

Use this function in cases when references to the underlying byte slice at Fields.Fields entries must be cleared,
so the underlying byte slice could be released by Go GC.

This is a follow-up for 63a68ed

See also #891

Author: valyala

app/vlinsert/loki/loki_json.go

@@ -82,7 +82,12 @@ func parseJSONRequest(data []byte, lmp insertutil.LogMessageProcessor, msgFields
 	}
 
 	fieldsTmp := logstorage.GetFields()
-	defer logstorage.PutFields(fieldsTmp)
+	defer func() {
+		// Explicitly clear fieldsTmp up to its' capacity in order to free up
+		// all the references to the original byte slice, so it could be freed by Go GC.
+		fieldsTmp.ClearUpToCapacity()
+		logstorage.PutFields(fieldsTmp)
+	}()
 
 	var msgParser *logstorage.JSONParser
 	if parseMessage {

app/vlinsert/loki/pb.go

@@ -50,7 +50,12 @@ func decodeStream(src []byte, pushLogs pushLogsHandler) error {
 	// }
 
 	fs := logstorage.GetFields()
-	defer logstorage.PutFields(fs)
+	defer func() {
+		// Explicitly clear fs up to its' capacity in order to free up
+		// all the references to the original byte slice, so it could be freed by Go GC.
+		fs.ClearUpToCapacity()
+		logstorage.PutFields(fs)
+	}()
 
 	labels, ok, err := easyproto.GetString(src, 1)
 	if err != nil {

app/vlinsert/opentelemetry/pb.go

@@ -55,11 +55,9 @@ func decodeResourceLogs(src []byte, pushLogs pushLogsHandler) (err error) {
 
 	fs := logstorage.GetFields()
 	defer func() {
-		// Manually clear all the fields up to capacity in order to remove references to src and help GC freeing up the used memory.
-		// The logstorage.PutFields() clears only the references up to len(fs.Fields).
-		clear(fs.Fields[:cap(fs.Fields)])
-		fs.Fields = fs.Fields[:0]
-
+		// Explicitly clear fs up to its' capacity in order to free up
+		// all the references to the original byte slice, so it could be freed by Go GC.
+		fs.ClearUpToCapacity()
 		logstorage.PutFields(fs)
 	}()
 

lib/logstorage/rows.go

@@ -368,6 +368,15 @@ func (f *Fields) Reset() {
 	f.Fields = f.Fields[:0]
 }
 
+// ClearUpToCapacity clears f.Fields up to its' capacity.
+//
+// This function is useful in order to make sure f.Fields do not reference underlying byte slices,
+// so they could be freed by Go GC.
+func (f *Fields) ClearUpToCapacity() {
+	clear(f.Fields[:cap(f.Fields)])
+	f.Fields = f.Fields[:0]
+}
+
 // Add adds (name, value) field to f.
 func (f *Fields) Add(name, value string) {
 	f.Fields = append(f.Fields, Field{