nginx sometimes close connection to grpc upstream when keepalive is set

[peanutgyz]

when grpc server response
 SETTINGS[0], WINDOW_UPDATE[0], PING[0], HEADERS[1], DATA[1], HEADERS[1]  in
one tcp packet, 

nginx want to send settings ack and call function
ngx_http_grpc_send_settings_ack, write some data in ctx->out

when finalize upstream grpc request, because out != NULL, nginx can't set
connection to be keepalive and closed connection.

expect connection to be persistent .

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,280159,280159#msg-280159