Nginx http filter异常排查

问题：

• 访问异常

root@cloud:/usr/local/nginx# curl -i http://localhost/test.html
curl: (52) Empty reply from server

• 错误日志

2016/09/11 13:17:03 [alert] 63560#0: worker process 63663 exited on signal 11 (core dumped)

• dmesg信息

[265950.220943] nginx[63663]: segfault at 128 ip 000000000048259d sp 00007ffde898eab0 error 4 in nginx[400000+a5000]

• core dump设置
  
  默认Worker进程用户nobody:nogroup，无法写coredump. 需在nginx.conf配置：

worker_rlimit_core 1024m; #1G
working_directory /tmp/core #保证nobody:nogroup有W权限

执行"sbin/nginx -s reload"重新加载配置。

• core dump分析

• 使用readelf工具
  
  core dump是ELF格式文件，你可以用readelf -a 查看core dump文件，但哪只是一堆数据。
• 使用gdb工具

gdb <command> <core>

一般会直接显示出错的代码位置

• 位置1

  conf = ngx_http_conf_get_module_loc_conf(r,ngx_http_myfilter_module); #应该是ngx_http_get_module_loc_conf(...)
  

• 位置2

  myfilter_ctx_t *ctx;
  ctx = ngx_http_get_module_ctx(r, ngx_http_myfilter_module);
  if (ctx->add_prefix != 1) { 应该先判断ctx == NULL
  	return next_body_filter(r, in);
  }
  

Nginx http filter示例源码

ngx_addon_name=ngx_http_myfilter_module
HTTP_FILTER_MODULES="$HTTP_FILTER_MODULES ngx_http_myfilter_module"
NGX_ADDON_SRCS="$NGX_ADDON_SRCS $ngx_addon_dir/ngx_http_myfilter_module.c"

#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_http.h>

static ngx_str_t filter_prefix = ngx_string("[my filter prefix]");
static ngx_http_output_header_filter_pt next_header_filter;
static ngx_http_output_body_filter_pt next_body_filter;

typedef struct {
	ngx_flag_t enable;
} myfilter_conf_t;

typedef struct {
	ngx_int_t add_prefix;
} myfilter_ctx_t;

static ngx_int_t myfilter_header_filter(ngx_http_request_t *r);
static ngx_int_t myfilter_body_filter(ngx_http_request_t *r, ngx_chain_t *in);
static ngx_int_t myfilter_init(ngx_conf_t *cf);

static void *myfilter_create_loc_conf(ngx_conf_t *cf);
static char *myfilter_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child);

static ngx_http_module_t myfilter_conf = {
	NULL,
	myfilter_init,
	NULL,
	NULL,
	NULL,
	NULL,
	myfilter_create_loc_conf,
	myfilter_merge_loc_conf
};

static ngx_command_t myfilter_commands[] = {
	{
		ngx_string("add_prefix"),
		NGX_HTTP_MAIN_CONF | NGX_HTTP_SRV_CONF | NGX_HTTP_LOC_CONF | NGX_CONF_FLAG,
		ngx_conf_set_flag_slot,
		NGX_HTTP_LOC_CONF_OFFSET,
		offsetof(myfilter_conf_t, enable),
		NULL
	},
	ngx_null_command
};

ngx_module_t ngx_http_myfilter_module = {
	NGX_MODULE_V1,
	&myfilter_conf,
	myfilter_commands,
	NGX_HTTP_MODULE,
	NULL,
	NULL,
	NULL,
	NULL,
	NULL,
	NULL,
	NULL,
	NGX_MODULE_V1_PADDING
};

static ngx_int_t myfilter_init(ngx_conf_t *cf){
	next_header_filter = ngx_http_top_header_filter;
	next_body_filter = ngx_http_top_body_filter;

	ngx_http_top_header_filter = myfilter_header_filter;
	ngx_http_top_body_filter = myfilter_body_filter;

	return NGX_OK;
}

static ngx_int_t myfilter_header_filter(ngx_http_request_t *r){
	myfilter_conf_t *conf;
	myfilter_ctx_t *ctx;

	if(r->headers_out.status != NGX_HTTP_OK){
		return next_header_filter(r);
	}

	ctx = ngx_http_get_module_ctx(r, ngx_http_myfilter_module);
	if(ctx){
		return next_header_filter(r);
	}

	conf = ngx_http_get_module_loc_conf(r,ngx_http_myfilter_module);
	if(conf == NULL || conf->enable == 0){
		return next_header_filter(r);
	}

	ctx = ngx_pcalloc(r->pool, sizeof(myfilter_ctx_t));
	if(ctx == NULL){
		return NGX_ERROR;
	}
	ctx->add_prefix = 0;
	ngx_http_set_ctx(r, ctx, ngx_http_myfilter_module);

	r->headers_out.status = NGX_HTTP_MOVED_TEMPORARILY;
	ngx_str_t keys = ngx_string("Location");
	ngx_str_t vals = ngx_string("http://www.163.com");
	ngx_table_elt_t *headers = ngx_list_push(&r->headers_out.headers);
	if(headers){
		headers->key.data = keys.data;
		headers->key.len = keys.len;
		headers->value.data = vals.data;
		headers->value.len = vals.len;
		headers->hash = 1;
	}

//	if (r->headers_out.content_type.len >= sizeof("text/plain") - 1
//			&& ngx_strncasecmp(r->headers_out.content_type.data,
//					(u_char *) "text/plain", sizeof("text/plain") - 1) == 0) {
		ctx->add_prefix = 1;
		if(r->headers_out.content_length_n > 0){
			r->headers_out.content_length_n += filter_prefix.len;
		}
//	}

	return next_header_filter(r);
}

static ngx_int_t myfilter_body_filter(ngx_http_request_t *r, ngx_chain_t *in) {
	myfilter_ctx_t *ctx;
	ctx = ngx_http_get_module_ctx(r, ngx_http_myfilter_module);
	if (ctx == NULL || ctx->add_prefix != 1) {
		return next_body_filter(r, in);
	}

	ctx->add_prefix = 2;
	ngx_buf_t *b = ngx_create_temp_buf(r->pool, filter_prefix.len);
	b->start = b->pos = filter_prefix.data;
	b->last = b->pos + filter_prefix.len;

	ngx_chain_t *cl = ngx_alloc_chain_link(r->pool);
	cl->buf = b;
	cl->next = in;
	return next_body_filter(r, cl);
}

static void *myfilter_create_loc_conf(ngx_conf_t *cf) {
	myfilter_conf_t *mycf;
	mycf = (myfilter_conf_t *)ngx_pcalloc(cf->pool, sizeof(myfilter_conf_t));
	if(mycf == NULL) return NULL;
	mycf->enable = NGX_CONF_UNSET;
	return mycf;
}

static char *myfilter_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child){
	myfilter_conf_t *prev = (myfilter_conf_t *)parent;
	myfilter_conf_t *conf = (myfilter_conf_t *)child;

	ngx_conf_merge_value(conf->enable, prev->enable, 0);
	return NGX_CONF_OK;
}