Stream: don't flush empty buffers created for read errors. When we generate the last_buf buffer for an UDP upstream recv error, it does not contain any data from the wire. ngx_stream_write_filter attempts to forward it anyways, which is incorrect (e.g., UDP upstream ECONNREFUSED will be translated to an empty packet). This happens because we mark the buffer as both 'flush' and 'last_buf', and ngx_stream_write_filter has special handling for flush with certain types of connections (see d127837c714f, 32b0ba4855a6). The flags are meant to be mutually exclusive, so the fix is to ensure that flush and last_buf are not set at the same time. Reproduction: stream { upstream unreachable { server 127.0.0.1:8880; } server { listen 127.0.0.1:8998 udp; proxy_pass unreachable; } } 1 0.000000000 127.0.0.1 → 127.0.0.1 UDP 47 45588 → 8998 Len=5 2 0.000166300 127.0.0.1 → 127.0.0.1 UDP 47 51149 → 8880 Len=5 3 0.000172600 127.0.0.1 → 127.0.0.1 ICMP 75 Destination unreachable (Port unreachable) 4 0.000202400 127.0.0.1 → 127.0.0.1 UDP 42 8998 → 45588 Len=0 Fixes d127837c714f.

/*
 * Copyright (C) Nginx, Inc.
 */


#ifndef _NGX_SYSLOG_H_INCLUDED_
#define _NGX_SYSLOG_H_INCLUDED_


typedef struct {
    ngx_uint_t        facility;
    ngx_uint_t        severity;
    ngx_str_t         tag;

    ngx_addr_t        server;
    ngx_connection_t  conn;
    unsigned          busy:1;
    unsigned          nohostname:1;
} ngx_syslog_peer_t;


char *ngx_syslog_process_conf(ngx_conf_t *cf, ngx_syslog_peer_t *peer);
u_char *ngx_syslog_add_header(ngx_syslog_peer_t *peer, u_char *buf);
void ngx_syslog_writer(ngx_log_t *log, ngx_uint_t level, u_char *buf,
    size_t len);
ssize_t ngx_syslog_send(ngx_syslog_peer_t *peer, u_char *buf, size_t len);


#endif /* _NGX_SYSLOG_H_INCLUDED_ */