Mixed inbound: Handle immediately closing connection gracefully (#4297)

Co-authored-by: RPRX <[email protected]>

proxy/socks/server.go

@@ -2,6 +2,7 @@ package socks
 
 import (
 	"context"
+	goerrors "errors"
 	"io"
 	"time"
 
@@ -78,7 +79,13 @@ func (s *Server) Process(ctx context.Context, network net.Network, conn stat.Con
 	switch network {
 	case net.Network_TCP:
 		firstbyte := make([]byte, 1)
-		conn.Read(firstbyte)
+		if n, err := conn.Read(firstbyte); n == 0 {
+			if goerrors.Is(err, io.EOF) {
+				errors.LogInfo(ctx, "Connection closed immediately, likely health check connection")
+				return nil
+			}
+			return errors.New("failed to read from connection").Base(err)
+		}
 		if firstbyte[0] != 5 && firstbyte[0] != 4 { // Check if it is Socks5/4/4a
 			errors.LogDebug(ctx, "Not Socks request, try to parse as HTTP request")
 			return s.httpServer.ProcessWithFirstbyte(ctx, network, conn, dispatcher, firstbyte...)