Fix calculate reader headroom

Author: nekohasekai

common/network/thread.go

@@ -98,6 +98,29 @@ func CalculateFrontHeadroom(writer any) int {
 	return headroom
 }
 
+func calculateReaderFrontHeadroom(reader any) int {
+	var headroom int
+	for {
+		if reader == nil {
+			break
+		}
+		if lazyRoom, isLazy := reader.(LazyHeadroom); isLazy && lazyRoom.LazyHeadroom() {
+			return DefaultHeadroom
+		}
+		if headroomWriter, needHeadroom := reader.(FrontHeadroom); needHeadroom {
+			headroom += headroomWriter.FrontHeadroom()
+		}
+		if upstreamWriter, hasUpstreamWriter := reader.(WithUpstreamReader); hasUpstreamWriter {
+			reader = upstreamWriter.UpstreamReader()
+		} else if upstream, hasUpstream := reader.(common.WithUpstream); hasUpstream {
+			reader = upstream.Upstream()
+		} else {
+			break
+		}
+	}
+	return headroom
+}
+
 func CalculateRearHeadroom(writer any) int {
 	var headroom int
 	for {
@@ -131,14 +154,15 @@ type WriterWithMTU interface {
 
 func CalculateMTU(reader any, writer any) int {
 	readerMTU := calculateReaderMTU(reader)
+	readerHeadroom := calculateReaderFrontHeadroom(reader)
 	writerMTU := calculateWriterMTU(writer)
 	if readerMTU > writerMTU {
-		return readerMTU
+		return readerMTU + readerHeadroom
 	}
 	if writerMTU > buf.BufferSize {
 		return 0
 	}
-	return writerMTU
+	return writerMTU + readerHeadroom
 }
 
 func calculateReaderMTU(reader any) int {