Nginx-HTTP之ngx_http_top_header_filter

1. ngx_http_top_header_filter

该链表主要是用于构造响应消息的消息报头。

ngx_http_top_header_filter 单链表有如下模块插入了操作：

1. ngx_http_not_modified_filter_module: ngx_http_not_modified_header_filter
2. ngx_http_headers_filter_module：ngx_http_headers_filter
3. ngx_http_userid_filter_module: ngx_http_userid_filter
4. ngx_http_charset_filter_module：ngx_http_charset_header_filter
5. ngx_http_ssi_filter_module: ngx_http_ssi_header_filter
6. ngx_http_gzip_filter_module: ngx_http_gzip_header_filter
7. ngx_http_range_filter_module: ngx_http_range_header_filter
8. ngx_http_chunked_filter_module: ngx_http_chunked_header_filter
9. ngx_http_header_filter_module: ngx_http_header_filter

1.1 ngx_http_not_modified_header_filter

static ngx_int_t
ngx_http_not_modified_header_filter(ngx_http_request_t *r)
{
    if (r->headers_out.status != NGX_HTTP_OK
        || r != r->main
        || r->disable_not_modified)
    {
        return ngx_http_next_header_filter(r);
    }

    /* If-Modified-Since: 当客户端请求服务器判断自己缓存的内容是否变化时，可以设置
     * If-Modified-Since 请求头，如果服务器上的内容修改时间比这个头的时间值还新，
     * 那么将返回新的内容，否则不返回。这个与 If-Unmodified-Since 类似，是用来判断
     * 资源变化的另一种方式 */
    if (r->headers_in.if_unmodified_since
        && !ngx_http_test_if_unmodified(r))
    {
        return ngx_http_filter_finalize_request(r, NULL,
                                                NGX_HTTP_PRECONDITION_FAILED);
    }

    /* If-Match: 与 ETag 一起用来判断已缓存的内容是否被修改过。比如，客户端在获取
     * 内容时会获取一个与内容相同的实体标签 ETag （内容变化会使 ETag 变化），下次
     * 再请求同样的内容时，会在请求头的 If-Match 中包含这个 ETag，然后发给可能存有
     * 更新内容的服务器。服务器将收到的 ETag 与本地目前的 ETag 进行比较，如果匹配
     * 返回所请求内容。这种方法在断点续传的时候使用较多 */
    if (r->headers_in.if_match
        && !ngx_http_test_if_match(r, r->headers_in.if_match, 0))
    {
        return ngx_http_filter_finalize_request(r, NULL,
                                                NGX_HTTP_PRECONDITION_FAILED);
    }
 
    /* If-Modified-Since: 当客户端请求服务器判断自己缓存的内容是否变化时，可以设置
     * If-Modified-Since 头，如果服务器上的内容修改时间比这个头的时间值还要新，那么将
     * 返回新的内容，否则不返回。这个与 If-Unmodified-Since 类似，是用来判断资源变化的
     * 另一种方式。
     *
     * If-None-Match: 与 If-Match 类似，结果相反。如果 If-None-Match 中包含了 ETag 的值，
     * 服务器在进行比较后发现如果不匹配，则返回所请求的内容，否则不返回相关内容。这种
     * 方法在网页刷新的时候使用较多。
     */
    if (r->headers_in.if_modified_since || r->headers_in.if_none_match) {

        if (r->headers_in.if_modified_since
            && ngx_http_test_if_modified(r))
        {
            return ngx_http_next_header_filter(r);
        }

        if (r->headers_in.if_none_match
            && !ngx_http_test_if_match(r, r->headers_in.if_none_match, 1))
        {
            return ngx_http_next_header_filter(r);
        }

        /* not modified */

        r->headers_out.status = NGX_HTTP_NOT_MODIFIED;
        r->headers_out.status_line.len = 0;
        r->headers_out.content_type.len = 0;
        ngx_http_clear_content_length(r);
        ngx_http_clear_accept_ranges(r);

        if (r->headers_out.content_encoding) {
            r->headers_out.content_encoding->hash = 0;
            r->headers_out.content_encoding = NULL;
        }

        return ngx_http_next_header_filter(r);
    }

    return ngx_http_next_header_filter(r);
}

此时 ngx_http_next_header_filter 回调的函数为 ngx_http_headers_filter。

1.2 ngx_http_headers_filter

static ngx_int_t
ngx_http_headers_filter(ngx_http_request_t *r)
{
    ngx_str_t                 value;
    ngx_uint_t                i, safe_status;
    ngx_http_header_val_t    *h;
    ngx_http_headers_conf_t  *conf;

    if (r != r->main) {
        return ngx_http_next_header_filter(r);
    }

    /* 获取 ngx_http_headers_filter_module 模块与 location 相关的配置 */
    conf = ngx_http_get_module_loc_conf(r, ngx_http_headers_filter_module);

    /* 若对 expires、headers、trailers 都没有相应的配置，则执行
     * ngx_http_top_header_filter 链表的下一个函数 */
    if (conf->expires == NGX_HTTP_EXPIRES_OFF
        && conf->headers == NULL
        && conf->trailers == NULL)
    {
        return ngx_http_next_header_filter(r);
    }

    switch (r->headers_out.status) {

    case NGX_HTTP_OK:
    case NGX_HTTP_CREATED:
    case NGX_HTTP_NO_CONTENT:
    case NGX_HTTP_PARTIAL_CONTENT:
    case NGX_HTTP_MOVED_PERMANENTLY:
    case NGX_HTTP_MOVED_TEMPORARILY:
    case NGX_HTTP_SEE_OTHER:
    case NGX_HTTP_NOT_MODIFIED:
    case NGX_HTTP_TEMPORARY_REDIRECT:
    case NGX_HTTP_PERMANENT_REDIRECT:
        safe_status = 1;
        break;

    default:
        safe_status = 0;
        break;
    }

    if (conf->expires != NGX_HTTP_EXPIRES_OFF && safe_status) {
        if (ngx_http_set_expires(r, conf) != NGX_OK) {
            return NGX_ERROR;
        }
    }

    if (conf->headers) {
        h = conf->headers->elts;
        for (i = 0; i < conf->headers->nelts; i++) {

            if (!safe_status && !h[i].always) {
                continue;
            }

            if (ngx_http_complex_value(r, &h[i].value, &value) != NGX_OK) {
                return NGX_ERROR;
            }

            if (h[i].handler(r, &h[i], &value) != NGX_OK) {
                return NGX_ERROR;
            }
        }
    }

    if (conf->trailers) {
        h = conf->trailers->elts;
        for (i = 0; i < conf->trailers->nelts; i++) {

            if (!safe_status && !h[i].always) {
                continue;
            }

            r->expect_trailers = 1;
            break;
        }
    }

    return ngx_http_next_header_filter(r);
}

1.3 ngx_http_userid_filter

static ngx_int_t
ngx_http_userid_filter(ngx_http_request_t *r)
{
    ngx_http_userid_ctx_t   *ctx;
    ngx_http_userid_conf_t  *conf;

    /* 若为子请求，直接执行 ngx_http_top_header_filter 链表的下一个函数  */
    if (r != r->main) {
        return ngx_http_next_header_filter(r);
    }

    /* 获取 ngx_http_userid_filter_module 模块 loc 级别的配置 */
    conf = ngx_http_get_module_loc_conf(r, ngx_http_userid_filter_module);

    /* 若没有使能该模块，则执行下一个函数 */
    if (conf->enable < NGX_HTTP_USERID_V1) {
        return ngx_http_next_header_filter(r);
    }

    ctx = ngx_http_userid_get_uid(r, conf);

    if (ctx == NULL) {
        return NGX_ERROR;
    }

    if (ngx_http_userid_set_uid(r, ctx, conf) == NGX_OK) {
        return ngx_http_next_header_filter(r);
    }

    return NGX_ERROR;
}

1.4 ngx_http_charset_header_filter

static ngx_int_t
ngx_http_charset_header_filter(ngx_http_request_t *r)
{
    ngx_int_t                      charset, source_charset;
    ngx_str_t                      dst, src;
    ngx_http_charset_t            *charsets;
    ngx_http_charset_main_conf_t  *mcf;

    /* 若当前请求为父请求 */
    if (r == r->main) {
        charset = ngx_http_destination_charset(r, &dst);

    } else {
        charset = ngx_http_main_request_charset(r, &dst);
    }

    if (charset == NGX_ERROR) {
        return NGX_ERROR;
    }

    /* 执行下一个 */
    if (charset == NGX_DECLINED) {
        return ngx_http_next_header_filter(r);
    }

    /* charset: charset index or NGX_HTTP_NO_CHARSET */

    source_charset = ngx_http_source_charset(r, &src);

    if (source_charset == NGX_ERROR) {
        return NGX_ERROR;
    }

    /*
     * source_charset: charset index, NGX_HTTP_NO_CHARSET,
     *                 or NGX_HTTP_CHARSET_OFF
     */

    ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                   "charset: \"%V\" > \"%V\"", &src, &dst);

    if (source_charset == NGX_HTTP_CHARSET_OFF) {
        ngx_http_set_charset(r, &dst);

        return ngx_http_next_header_filter(r);
    }

    if (charset == NGX_HTTP_NO_CHARSET
        || source_charset == NGX_HTTP_NO_CHARSET)
    {
        if (source_charset != charset
            || ngx_strncasecmp(dst.data, src.data, dst.len) != 0)
        {
            goto no_charset_map;
        }

        ngx_http_set_charset(r, &dst);

        return ngx_http_next_header_filter(r);
    }

    if (source_charset == charset) {
        r->headers_out.content_type.len = r->headers_out.content_type_len;

        ngx_http_set_charset(r, &dst);

        return ngx_http_next_header_filter(r);
    }

    /* source_charset != charset */

    if (r->headers_out.content_encoding
        && r->headers_out.content_encoding->value.len)
    {
        return ngx_http_next_header_filter(r);
    }

    mcf = ngx_http_get_module_main_conf(r, ngx_http_charset_filter_module);
    charsets = mcf->charsets.elts;

    if (charsets[source_charset].tables == NULL
        || charsets[source_charset].tables[charset] == NULL)
    {
        goto no_charset_map;
    }

    r->headers_out.content_type.len = r->headers_out.content_type_len;

    ngx_http_set_charset(r, &dst);

    return ngx_http_charset_ctx(r, charsets, charset, source_charset);

no_charset_map:

    ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                  "no \"charset_map\" between the charsets \"%V\" and \"%V\"",
                  &src, &dst);

    return ngx_http_next_header_filter(r);
}

1.4.1 ngx_http_destination_charset

static ngx_int_t
ngx_http_destination_charset(ngx_http_request_t *r, ngx_str_t *name)
{
    ngx_int_t                      charset;
    ngx_http_charset_t            *charsets;
    ngx_http_variable_value_t     *vv;
    ngx_http_charset_loc_conf_t   *mlcf;
    ngx_http_charset_main_conf_t  *mcf;

    /* 若还没有设置当前响应的 Content-Type 头  */
    if (r->headers_out.content_type.len == 0) {
        return NGX_DECLINED;
    }

    if (r->headers_out.override_charset
        && r->headers_out.override_charset->len)
    {
        *name = *r->headers_out.override_charset;

        charset = ngx_http_get_charset(r, name);

        if (charset != NGX_HTTP_NO_CHARSET) {
            return charset;
        }

        ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                      "unknown charset \"%V\" to override", name);

        return NGX_DECLINED;
    }

    mlcf = ngx_http_get_module_loc_conf(r, ngx_http_charset_filter_module);
    charset = mlcf->charset;

    /* 若没有开启 charset */
    if (charset == NGX_HTTP_CHARSET_OFF) {
        return NGX_DECLINED;
    }

    if (r->headers_out.charset.len) {
        if (mlcf->override_charset == 0) {
            return NGX_DECLINED;
        }

    } else {
        if (ngx_http_test_content_type(r, &mlcf->types) == NULL) {
            return NGX_DECLINED;
        }
    }

    if (charset < NGX_HTTP_CHARSET_VAR) {
        mcf = ngx_http_get_module_main_conf(r, ngx_http_charset_filter_module);
        charsets = mcf->charsets.elts;
        *name = charsets[charset].name;
        return charset;
    }

    vv = ngx_http_get_indexed_variable(r, charset - NGX_HTTP_CHARSET_VAR);

    if (vv == NULL || vv->not_found) {
        return NGX_ERROR;
    }

    name->len = vv->len;
    name->data = vv->data;

    return ngx_http_get_charset(r, name);
}

1.5 ngx_http_ssi_header_filter

static ngx_int_t
ngx_http_ssi_header_filter(ngx_http_request_t *r)
{
    ngx_http_ssi_ctx_t       *ctx;
    ngx_http_ssi_loc_conf_t  *slcf;

    /* 获取 ngx_http_ssi_filter_module 模块 loc 级别的配置 */
    slcf = ngx_http_get_module_loc_conf(r, ngx_http_ssi_filter_module);

    /* 若没有启用该模块 */
    if (!slcf->enable
        || r->headers_out.content_length_n == 0
        || ngx_http_test_content_type(r, &slcf->types) == NULL)
    {
        return ngx_http_next_header_filter(r);
    }

    ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_ssi_ctx_t));
    if (ctx == NULL) {
        return NGX_ERROR;
    }

    ngx_http_set_ctx(r, ctx, ngx_http_ssi_filter_module);


    ctx->value_len = slcf->value_len;
    ctx->last_out = &ctx->out;

    ctx->encoding = NGX_HTTP_SSI_ENTITY_ENCODING;
    ctx->output = 1;

    ctx->params.elts = ctx->params_array;
    ctx->params.size = sizeof(ngx_table_elt_t);
    ctx->params.nalloc = NGX_HTTP_SSI_PARAMS_N;
    ctx->params.pool = r->pool;

    ctx->timefmt = ngx_http_ssi_timefmt;
    ngx_str_set(&ctx->errmsg,
                "[an error occurred while processing the directive]");

    r->filter_need_in_memory = 1;

    if (r == r->main) {
        ngx_http_clear_content_length(r);
        ngx_http_clear_accept_ranges(r);

        if (!slcf->last_modified) {
            ngx_http_clear_last_modified(r);
            ngx_http_clear_etag(r);

        } else {
            ngx_http_weak_etag(r);
        }
    }

    return ngx_http_next_header_filter(r);
}

1.6 ngx_http_gzip_header_filter

static ngx_int_t
ngx_http_gzip_header_filter(ngx_http_request_t *r)
{
    ngx_table_elt_t       *h;
    ngx_http_gzip_ctx_t   *ctx;
    ngx_http_gzip_conf_t  *conf;

    conf = ngx_http_get_module_loc_conf(r, ngx_http_gzip_filter_module);

    /* 若没有使能该模块 */
    if (!conf->enable
        || (r->headers_out.status != NGX_HTTP_OK
            && r->headers_out.status != NGX_HTTP_FORBIDDEN
            && r->headers_out.status != NGX_HTTP_NOT_FOUND)
        || (r->headers_out.content_encoding
            && r->headers_out.content_encoding->value.len)
        || (r->headers_out.content_length_n != -1
            && r->headers_out.content_length_n < conf->min_length)
        || ngx_http_test_content_type(r, &conf->types) == NULL
        || r->header_only)
    {
        return ngx_http_next_header_filter(r);
    }

    r->gzip_vary = 1;

#if (NGX_HTTP_DEGRADATION)
    {
    ngx_http_core_loc_conf_t  *clcf;

    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

    if (clcf->gzip_disable_degradation && ngx_http_degraded(r)) {
        return ngx_http_next_header_filter(r);
    }
    }
#endif

    if (!r->gzip_tested) {
        if (ngx_http_gzip_ok(r) != NGX_OK) {
            return ngx_http_next_header_filter(r);
        }

    } else if (!r->gzip_ok) {
        return ngx_http_next_header_filter(r);
    }

    ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_gzip_ctx_t));
    if (ctx == NULL) {
        return NGX_ERROR;
    }

    ngx_http_set_ctx(r, ctx, ngx_http_gzip_filter_module);

    ctx->request = r;
    ctx->buffering = (conf->postpone_gzipping != 0);

    ngx_http_gzip_filter_memory(r, ctx);

    h = ngx_list_push(&r->headers_out.headers);
    if (h == NULL) {
        return NGX_ERROR;
    }

    h->hash = 1;
    ngx_str_set(&h->key, "Content-Encoding");
    ngx_str_set(&h->value, "gzip");
    r->headers_out.content_encoding = h;

    r->main_filter_need_in_memory = 1;

    ngx_http_clear_content_length(r);
    ngx_http_clear_accept_ranges(r);
    ngx_http_weak_etag(r);

    return ngx_http_next_header_filter(r);
}

1.7 ngx_http_range_header_filter

static ngx_int_t
ngx_http_range_header_filter(ngx_http_request_t *r)
{
    time_t                        if_range_time;
    ngx_str_t                    *if_range, *etag;
    ngx_uint_t                    ranges;
    ngx_http_core_loc_conf_t     *clcf;
    ngx_http_range_filter_ctx_t  *ctx;

    if (r->http_version < NGX_HTTP_VERSION_10
        || r->headers_out.status != NGX_HTTP_OK
        || (r != r->main && !r->subrequest_ranges)
        || r->headers_out.content_length_n == -1
        || !r->allow_ranges)
    {
        return ngx_http_next_header_filter(r);
    }

    /* 获取 ngx_http_core_module 模块 loc 级别的配置 */
    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

    if (clcf->max_ranges == 0) {
        return ngx_http_next_header_filter(r);
    }

    if (r->headers_in.range == NULL
        || r->headers_in.range->value.len < 7
        || ngx_strncasecmp(r->headers_in.range->value.data,
                           (u_char *) "bytes=", 6)
           != 0)
    {
        goto next_filter;
    }

    if (r->headers_in.if_range) {

        if_range = &r->headers_in.if_range->value;

        if (if_range->len >= 2 && if_range->data[if_range->len - 1] == '"') {

            if (r->headers_out.etag == NULL) {
                goto next_filter;
            }

            etag = &r->headers_out.etag->value;

            ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                           "http ir:%V etag:%V", if_range, etag);

            if (if_range->len != etag->len
                || ngx_strncmp(if_range->data, etag->data, etag->len) != 0)
            {
                goto next_filter;
            }

            goto parse;
        }

        if (r->headers_out.last_modified_time == (time_t) -1) {
            goto next_filter;
        }

        if_range_time = ngx_parse_http_time(if_range->data, if_range->len);

        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                       "http ir:%T lm:%T",
                       if_range_time, r->headers_out.last_modified_time);

        if (if_range_time != r->headers_out.last_modified_time) {
            goto next_filter;
        }
    }

parse:

    ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_range_filter_ctx_t));
    if (ctx == NULL) {
        return NGX_ERROR;
    }

    ctx->offset = r->headers_out.content_offset;

    ranges = r->single_range ? 1 : clcf->max_ranges;

    switch (ngx_http_range_parse(r, ctx, ranges)) {

    case NGX_OK:
        ngx_http_set_ctx(r, ctx, ngx_http_range_body_filter_module);

        r->headers_out.status = NGX_HTTP_PARTIAL_CONTENT;
        r->headers_out.status_line.len = 0;

        if (ctx->ranges.nelts == 1) {
            return ngx_http_range_singlepart_header(r, ctx);
        }

        return ngx_http_range_multipart_header(r, ctx);

    case NGX_HTTP_RANGE_NOT_SATISFIABLE:
        return ngx_http_range_not_satisfiable(r);

    case NGX_ERROR:
        return NGX_ERROR;

    default: /* NGX_DECLINED */
        break;
    }

next_filter:

    /* Accept-Ranges: 可以请求网页实体的一个或多个子范围字段，
     * 值为 bytes 表示支持断点续传，为 none 表示不支持 */
    r->headers_out.accept_ranges = ngx_list_push(&r->headers_out.headers);
    if (r->headers_out.accept_ranges == NULL) {
        return NGX_ERROR;
    }

    r->headers_out.accept_ranges->hash = 1;
    ngx_str_set(&r->headers_out.accept_ranges->key, "Accept-Ranges");
    ngx_str_set(&r->headers_out.accept_ranges->value, "bytes");

    return ngx_http_next_header_filter(r);
}

1.8 ngx_http_chunked_header_filter

static ngx_int_t
ngx_http_chunked_header_filter(ngx_http_request_t *r)
{
    ngx_http_core_loc_conf_t       *clcf;
    ngx_http_chunked_filter_ctx_t  *ctx;

    if (r->headers_out.status == NGX_HTTP_NOT_MODIFIED
        || r->headers_out.status == NGX_HTTP_NO_CONTENT
        || r->headers_out.status < NGX_HTTP_OK
        || r != r->main
        || r->method == NGX_HTTP_HEAD)
    {
        return ngx_http_next_header_filter(r);
    }

    if (r->headers_out.content_length_n == -1
        || r->expect_trailers)
    {
        clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

        if (r->http_version >= NGX_HTTP_VERSION_11
            && clcf->chunked_transfer_encoding)
        {
            if (r->expect_trailers) {
                ngx_http_clear_content_length(r);
            }

            r->chunked = 1;

            ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_chunked_filter_ctx_t));
            if (ctx == NULL) {
                return NGX_ERROR;
            }

            ngx_http_set_ctx(r, ctx, ngx_http_chunked_filter_module);

        } else if (r->headers_out.content_length_n == -1) {
            r->keepalive = 0;
        }
    }

    return ngx_http_next_header_filter(r);
}

1.9 ngx_http_header_filter

static ngx_int_t
ngx_http_header_filter(ngx_http_request_t *r)
{
    u_char                    *p;
    size_t                     len;
    ngx_str_t                  host, *status_line;
    ngx_buf_t                 *b;
    ngx_uint_t                 status, i, port;
    ngx_chain_t                out;
    ngx_list_part_t           *part;
    ngx_table_elt_t           *header;
    ngx_connection_t          *c;
    ngx_http_core_loc_conf_t  *clcf;
    ngx_http_core_srv_conf_t  *cscf;
    u_char                     addr[NGX_SOCKADDR_STRLEN];

    /* 标志位，为 1 表示该 header 已经发送了 */
    if (r->header_sent) {
        return NGX_OK;
    }

    /* 设置该标志位为 1 */
    r->header_sent = 1;

    /* 若为子请求，则直接返回 */
    if (r != r->main) {
        return NGX_OK;
    }

    if (r->http_version < NGX_HTTP_VERSION_10) {
        return NGX_OK;
    }

    /* 若当前方法为 HEAD 则仅发送 header */
    if (r->method == NGX_HTTP_HEAD) {
        r->header_only = 1;
    }

    if (r->headers_out.last_modified_time != -1) {
        if (r->headers_out.status != NGX_HTTP_OK
            && r->headers_out.status != NGX_HTTP_PARTIAL_CONTENT
            && r->headers_out.status != NGX_HTTP_NOT_MODIFIED)
        {
            r->headers_out.last_modified_time = -1;
            r->headers_out.last_modified = NULL;
        }
    }

    /* 状态行的长度 */
    len = sizeof("HTTP/1.x ") - 1 + sizeof(CRLF) - 1
          /* the end of the header */
          + sizeof(CRLF) - 1;

    /* status line */

    if (r->headers_out.status_line.len) {
        len += r->headers_out.status_line.len;
        status_line = &r->headers_out.status_line;
#if (NGX_SUPPRESS_WARN)
        status = 0;
#endif

    } else {

        status = r->headers_out.status;

        if (status >= NGX_HTTP_OK
            && status < NGX_HTTP_LAST_2XX)
        {
            /* 2XX */

            /* 204：该响应没有响应正文 */
            if (status == NGX_HTTP_NO_CONTENT) {
                r->header_only = 1;
                ngx_str_null(&r->headers_out.content_type);
                r->headers_out.last_modified_time = -1;
                r->headers_out.last_modified = NULL;
                r->headers_out.content_length = NULL;
                r->headers_out.content_length_n = -1;
            }

            /* 计算该状态在 ngx_http_status_lines 数组中的索引 */
            status -= NGX_HTTP_OK;
            /* 从该数组中 status 索引处获取字符串形式的状态行 */
            status_line = &ngx_http_status_lines[status];
            len += ngx_http_status_lines[status].len;

        } else if (status >= NGX_HTTP_MOVED_PERMANENTLY
                   && status < NGX_HTTP_LAST_3XX)
        {
            /* 3XX */

            if (status == NGX_HTTP_NOT_MODIFIED) {
                r->header_only = 1;
            }

            status = status - NGX_HTTP_MOVED_PERMANENTLY + NGX_HTTP_OFF_3XX;
            status_line = &ngx_http_status_lines[status];
            len += ngx_http_status_lines[status].len;

        } else if (status >= NGX_HTTP_BAD_REQUEST
                   && status < NGX_HTTP_LAST_4XX)
        {
            /* 4XX */
            status = status - NGX_HTTP_BAD_REQUEST
                            + NGX_HTTP_OFF_4XX;

            status_line = &ngx_http_status_lines[status];
            len += ngx_http_status_lines[status].len;

        } else if (status >= NGX_HTTP_INTERNAL_SERVER_ERROR
                   && status < NGX_HTTP_LAST_5XX)
        {
            /* 5XX */
            status = status - NGX_HTTP_INTERNAL_SERVER_ERROR
                            + NGX_HTTP_OFF_5XX;

            status_line = &ngx_http_status_lines[status];
            len += ngx_http_status_lines[status].len;

        } else {
            len += NGX_INT_T_LEN + 1 /* SP */;
            status_line = NULL;
        }

        if (status_line && status_line->len == 0) {
            status = r->headers_out.status;
            len += NGX_INT_T_LEN + 1 /* SP */;
            status_line = NULL;
        }
    }

    /* 获取 ngx_http_core_module 模块在 loc 级别的配置 */
    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

    if (r->headers_out.server == NULL) {
        if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_ON) {
            /* Server: web 服务器表明自己是什么版本及版本信息 */
            len += sizeof(ngx_http_server_full_string) - 1;

        } else if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_BUILD) {
            len += sizeof(ngx_http_server_build_string) - 1;

        } else {
            len += sizeof(ngx_http_server_string) - 1;
        }
    }

    if (r->headers_out.date == NULL) {
        /* Date: 发送该 HTTP 消息的时间 */
        len += sizeof("Date: Mon, 28 Sep 1970 06:00:00 GMT" CRLF) - 1;
    }

    if (r->headers_out.content_type.len) {
        len += sizeof("Content-Type: ") - 1
               + r->headers_out.content_type.len + 2;

        if (r->headers_out.content_type_len == r->headers_out.content_type.len
            && r->headers_out.charset.len)
        {
            len += sizeof("; charset=") - 1 + r->headers_out.charset.len;
        }
    }

    if (r->headers_out.content_length == NULL
        && r->headers_out.content_length_n >= 0)
    {
        /* Content-Length: 实体内容的大小 */
        len += sizeof("Content-Length: ") - 1 + NGX_OFF_T_LEN + 2;
    }

    if (r->headers_out.last_modified == NULL
        && r->headers_out.last_modified_time != -1)
    {
        /* Last-Modified: 指定所访问内容的最后更新时间 */
        len += sizeof("Last-Modified: Mon, 28 Sep 1970 06:00:00 GMT" CRLF) - 1;
    }

    c = r->connection;

    if (r->headers_out.location
        && r->headers_out.location->value.len
        && r->headers_out.location->value.data[0] == '/'
        && clcf->absolute_redirect)
    {
        r->headers_out.location->hash = 0;

        if (clcf->server_name_in_redirect) {
            cscf = ngx_http_get_module_srv_conf(r, ngx_http_core_module);
            host = cscf->server_name;

        } else if (r->headers_in.server.len) {
            host = r->headers_in.server;

        } else {
            host.len = NGX_SOCKADDR_STRLEN;
            host.data = addr;

            if (ngx_connection_local_sockaddr(c, &host, 0) != NGX_OK) {
                return NGX_ERROR;
            }
        }

        port = ngx_inet_get_port(c->local_sockaddr);

        len += sizeof("Location: https://") - 1
               + host.len
               + r->headers_out.location->value.len + 2;

        if (clcf->port_in_redirect) {

#if (NGX_HTTP_SSL)
            if (c->ssl)
                port = (port == 443) ? 0 : port;
            else
#endif
                port = (port == 80) ? 0 : port;

        } else {
            port = 0;
        }

        if (port) {
            len += sizeof(":65535") - 1;
        }

    } else {
        ngx_str_null(&host);
        port = 0;
    }

    if (r->chunked) {
        len += sizeof("Transfer-Encoding: chunked" CRLF) - 1;
    }

    if (r->headers_out.status == NGX_HTTP_SWITCHING_PROTOCOLS) {
        len += sizeof("Connection: upgrade" CRLF) - 1;

    } else if (r->keepalive) {
        len += sizeof("Connection: keep-alive" CRLF) - 1;

        /*
         * MSIE and Opera ignore the "Keep-Alive: timeout=<N>" header.
         * MSIE keeps the connection alive for about 60-65 seconds.
         * Opera keeps the connection alive very long.
         * Mozilla keeps the connection alive for N plus about 1-10 seconds.
         * Konqueror keeps the connection alive for about N seconds.
         */

        if (clcf->keepalive_header) {
            len += sizeof("Keep-Alive: timeout=") - 1 + NGX_TIME_T_LEN + 2;
        }

    } else {
        len += sizeof("Connection: close" CRLF) - 1;
    }

#if (NGX_HTTP_GZIP)
    if (r->gzip_vary) {
        if (clcf->gzip_vary) {
            len += sizeof("Vary: Accept-Encoding" CRLF) - 1;

        } else {
            r->gzip_vary = 0;
        }
    }
#endif

    /* 从 headers_out.headers 链表中取出第一个 数组元素 */
    part = &r->headers_out.headers.part;
    /* header 执行该数组 part 的第一个元素 */
    header = part->elts;

    /* 统计响应消息中响应头部的总大小 */
    for (i = 0; /* void */; i++) {

        if (i >= part->nelts) {
            if (part->next == NULL) {
                break;
            }

            part = part->next;
            header = part->elts;
            i = 0;
        }

        if (header[i].hash == 0) {
            continue;
        }

        len += header[i].key.len + sizeof(": ") - 1 + header[i].value.len
               + sizeof(CRLF) - 1;
    }

    /* 为该响应消息分配临时缓存 */
    b = ngx_create_temp_buf(r->pool, len);
    if (b == NULL) {
        return NGX_ERROR;
    }

    /* 响应消息分四部分：状态行、消息报头、空行和响应正文 */
    /* "HTTP/1.x " */
    b->last = ngx_cpymem(b->last, "HTTP/1.1 ", sizeof("HTTP/1.x ") - 1);

    /* status line */
    if (status_line) {
        b->last = ngx_copy(b->last, status_line->data, status_line->len);

    } else {
        b->last = ngx_sprintf(b->last, "%03ui ", status);
    }
    *b->last++ = CR; *b->last++ = LF;

    /* 下面依次向该缓存中写入消息报头 */
    if (r->headers_out.server == NULL) {
        if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_ON) {
            p = ngx_http_server_full_string;
            len = sizeof(ngx_http_server_full_string) - 1;

        } else if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_BUILD) {
            p = ngx_http_server_build_string;
            len = sizeof(ngx_http_server_build_string) - 1;

        } else {
            p = ngx_http_server_string;
            len = sizeof(ngx_http_server_string) - 1;
        }

        b->last = ngx_cpymem(b->last, p, len);
    }

    if (r->headers_out.date == NULL) {
        b->last = ngx_cpymem(b->last, "Date: ", sizeof("Date: ") - 1);
        b->last = ngx_cpymem(b->last, ngx_cached_http_time.data,
                             ngx_cached_http_time.len);

        *b->last++ = CR; *b->last++ = LF;
    }

    if (r->headers_out.content_type.len) {
        b->last = ngx_cpymem(b->last, "Content-Type: ",
                             sizeof("Content-Type: ") - 1);
        p = b->last;
        b->last = ngx_copy(b->last, r->headers_out.content_type.data,
                           r->headers_out.content_type.len);

        if (r->headers_out.content_type_len == r->headers_out.content_type.len
            && r->headers_out.charset.len)
        {
            b->last = ngx_cpymem(b->last, "; charset=",
                                 sizeof("; charset=") - 1);
            b->last = ngx_copy(b->last, r->headers_out.charset.data,
                               r->headers_out.charset.len);

            /* update r->headers_out.content_type for possible logging */

            r->headers_out.content_type.len = b->last - p;
            r->headers_out.content_type.data = p;
        }

        *b->last++ = CR; *b->last++ = LF;
    }

    if (r->headers_out.content_length == NULL
        && r->headers_out.content_length_n >= 0)
    {
        b->last = ngx_sprintf(b->last, "Content-Length: %O" CRLF,
                              r->headers_out.content_length_n);
    }

    if (r->headers_out.last_modified == NULL
        && r->headers_out.last_modified_time != -1)
    {
        b->last = ngx_cpymem(b->last, "Last-Modified: ",
                             sizeof("Last-Modified: ") - 1);
        b->last = ngx_http_time(b->last, r->headers_out.last_modified_time);

        *b->last++ = CR; *b->last++ = LF;
    }

    if (host.data) {

        p = b->last + sizeof("Location: ") - 1;

        b->last = ngx_cpymem(b->last, "Location: http",
                             sizeof("Location: http") - 1);

#if (NGX_HTTP_SSL)
        if (c->ssl) {
            *b->last++ ='s';
        }
#endif

        *b->last++ = ':'; *b->last++ = '/'; *b->last++ = '/';
        b->last = ngx_copy(b->last, host.data, host.len);

        if (port) {
            b->last = ngx_sprintf(b->last, ":%ui", port);
        }

        b->last = ngx_copy(b->last, r->headers_out.location->value.data,
                           r->headers_out.location->value.len);

        /* update r->headers_out.location->value for possible logging */

        r->headers_out.location->value.len = b->last - p;
        r->headers_out.location->value.data = p;
        ngx_str_set(&r->headers_out.location->key, "Location");

        *b->last++ = CR; *b->last++ = LF;
    }

    if (r->chunked) {
        b->last = ngx_cpymem(b->last, "Transfer-Encoding: chunked" CRLF,
                             sizeof("Transfer-Encoding: chunked" CRLF) - 1);
    }

    if (r->headers_out.status == NGX_HTTP_SWITCHING_PROTOCOLS) {
        b->last = ngx_cpymem(b->last, "Connection: upgrade" CRLF,
                             sizeof("Connection: upgrade" CRLF) - 1);

    } else if (r->keepalive) {
        b->last = ngx_cpymem(b->last, "Connection: keep-alive" CRLF,
                             sizeof("Connection: keep-alive" CRLF) - 1);

        if (clcf->keepalive_header) {
            b->last = ngx_sprintf(b->last, "Keep-Alive: timeout=%T" CRLF,
                                  clcf->keepalive_header);
        }

    } else {
        b->last = ngx_cpymem(b->last, "Connection: close" CRLF,
                             sizeof("Connection: close" CRLF) - 1);
    }

#if (NGX_HTTP_GZIP)
    if (r->gzip_vary) {
        b->last = ngx_cpymem(b->last, "Vary: Accept-Encoding" CRLF,
                             sizeof("Vary: Accept-Encoding" CRLF) - 1);
    }
#endif

    part = &r->headers_out.headers.part;
    header = part->elts;

    for (i = 0; /* void */; i++) {

        if (i >= part->nelts) {
            if (part->next == NULL) {
                break;
            }

            part = part->next;
            header = part->elts;
            i = 0;
        }

        if (header[i].hash == 0) {
            continue;
        }

        b->last = ngx_copy(b->last, header[i].key.data, header[i].key.len);
        *b->last++ = ':'; *b->last++ = ' ';

        b->last = ngx_copy(b->last, header[i].value.data, header[i].value.len);
        *b->last++ = CR; *b->last++ = LF;
    }

    ngx_log_debug2(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "%*s", (size_t) (b->last - b->pos), b->pos);

    /* the end of HTTP header */
    *b->last++ = CR; *b->last++ = LF;

    r->header_size = b->last - b->pos;

    if (r->header_only) {
        b->last_buf = 1;
    }

    out.buf = b;
    out.next = NULL;

    return ngx_http_write_filter(r, &out);
}

1.9.1 ngx_http_write_filter

ngx_int_t
ngx_http_write_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    off_t                      size, sent, nsent, limit;
    ngx_uint_t                 last, flush, sync;
    ngx_msec_t                 delay;
    ngx_chain_t               *cl, *ln, **ll, *chain;
    ngx_connection_t          *c;
    ngx_http_core_loc_conf_t  *clcf;

    c = r->connection;

    if (c->error) {
        return NGX_ERROR;
    }

    size = 0;
    flush = 0;
    sync = 0;
    last = 0;
    /* 指向该链表的头部 */
    ll = &r->out;

    /* find the size, the flush point and the last link of the saved chain */

    for (cl = r->out; cl; cl = cl->next) {
        ll = &cl->next;

        ngx_log_debug7(NGX_LOG_DEBUG_EVENT, c->log, 0,
                       "write old buf t:%d f:%d %p, pos %p, size: %z "
                       "file: %O, size: %O",
                       cl->buf->temporary, cl->buf->in_file,
                       cl->buf->start, cl->buf->pos,
                       cl->buf->last - cl->buf->pos,
                       cl->buf->file_pos,
                       cl->buf->file_last - cl->buf->file_pos);

#if 1
        if (ngx_buf_size(cl->buf) == 0 && !ngx_buf_special(cl->buf)) {
            ngx_log_error(NGX_LOG_ALERT, c->log, 0,
                          "zero size buf in writer "
                          "t:%d r:%d f:%d %p %p-%p %p %O-%O",
                          cl->buf->temporary,
                          cl->buf->recycled,
                          cl->buf->in_file,
                          cl->buf->start,
                          cl->buf->pos,
                          cl->buf->last,
                          cl->buf->file,
                          cl->buf->file_pos,
                          cl->buf->file_last);

            ngx_debug_point();
            return NGX_ERROR;
        }
#endif

        size += ngx_buf_size(cl->buf);

        if (cl->buf->flush || cl->buf->recycled) {
            flush = 1;
        }

        if (cl->buf->sync) {
            sync = 1;
        }

        if (cl->buf->last_buf) {
            last = 1;
        }
    }

    /* add the new chain to the existent one */

    for (ln = in; ln; ln = ln->next) {
        /* 分配一个 ngx_chain_t  */
        cl = ngx_alloc_chain_link(r->pool);
        if (cl == NULL) {
            return NGX_ERROR;
        }

        /* 指向保存着将要发送的数据的缓存的首地址 */
        cl->buf = ln->buf;
        /* 将该 cl 添加到 r->out 链表中 */
        *ll = cl;
        ll = &cl->next;

        ngx_log_debug7(NGX_LOG_DEBUG_EVENT, c->log, 0,
                       "write new buf t:%d f:%d %p, pos %p, size: %z "
                       "file: %O, size: %O",
                       cl->buf->temporary, cl->buf->in_file,
                       cl->buf->start, cl->buf->pos,
                       cl->buf->last - cl->buf->pos,
                       cl->buf->file_pos,
                       cl->buf->file_last - cl->buf->file_pos);

#if 1
        if (ngx_buf_size(cl->buf) == 0 && !ngx_buf_special(cl->buf)) {
            ngx_log_error(NGX_LOG_ALERT, c->log, 0,
                          "zero size buf in writer "
                          "t:%d r:%d f:%d %p %p-%p %p %O-%O",
                          cl->buf->temporary,
                          cl->buf->recycled,
                          cl->buf->in_file,
                          cl->buf->start,
                          cl->buf->pos,
                          cl->buf->last,
                          cl->buf->file,
                          cl->buf->file_pos,
                          cl->buf->file_last);

            ngx_debug_point();
            return NGX_ERROR;
        }
#endif

        /* 计算该 cl->buf 中缓存的数据大小 */
        size += ngx_buf_size(cl->buf);

        if (cl->buf->flush || cl->buf->recycled) {
            flush = 1;
        }

        if (cl->buf->sync) {
            sync = 1;
        }

        if (cl->buf->last_buf) {
            last = 1;
        }
    }

    *ll = NULL;

    ngx_log_debug3(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http write filter: l:%ui f:%ui s:%O", last, flush, size);

    /* 获取 ngx_http_core_module 模块 loc 级别的配置  */
    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

    /*
     * avoid the output if there are no last buf, no flush point,
     * there are the incoming bufs and the size of all bufs
     * is smaller than "postpone_output" directive
     */

    if (!last && !flush && in && size < (off_t) clcf->postpone_output) {
        return NGX_OK;
    }

    if (c->write->delayed) {
        c->buffered |= NGX_HTTP_WRITE_BUFFERED;
        return NGX_AGAIN;
    }

    if (size == 0
        && !(c->buffered & NGX_LOWLEVEL_BUFFERED)
        && !(last && c->need_last_buf))
    {
        if (last || flush || sync) {
            for (cl = r->out; cl; /* void */) {
                ln = cl;
                cl = cl->next;
                ngx_free_chain(r->pool, ln);
            }

            r->out = NULL;
            c->buffered &= ~NGX_HTTP_WRITE_BUFFERED;

            return NGX_OK;
        }

        ngx_log_error(NGX_LOG_ALERT, c->log, 0,
                      "the http output chain is empty");

        ngx_debug_point();

        return NGX_ERROR;
    }

    if (r->limit_rate) {
        if (r->limit_rate_after == 0) {
            r->limit_rate_after = clcf->limit_rate_after;
        }

        limit = (off_t) r->limit_rate * (ngx_time() - r->start_sec + 1)
                - (c->sent - r->limit_rate_after);

        if (limit <= 0) {
            c->write->delayed = 1;
            delay = (ngx_msec_t) (- limit * 1000 / r->limit_rate + 1);
            ngx_add_timer(c->write, delay);

            c->buffered |= NGX_HTTP_WRITE_BUFFERED;

            return NGX_AGAIN;
        }

        if (clcf->sendfile_max_chunk
            && (off_t) clcf->sendfile_max_chunk < limit)
        {
            limit = clcf->sendfile_max_chunk;
        }

    } else {
        limit = clcf->sendfile_max_chunk;
    }

    sent = c->sent;

    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http write filter limit %O", limit);

    chain = c->send_chain(c, r->out, limit);

    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http write filter %p", chain);

    if (chain == NGX_CHAIN_ERROR) {
        c->error = 1;
        return NGX_ERROR;
    }

    if (r->limit_rate) {

        nsent = c->sent;

        if (r->limit_rate_after) {

            sent -= r->limit_rate_after;
            if (sent < 0) {
                sent = 0;
            }

            nsent -= r->limit_rate_after;
            if (nsent < 0) {
                nsent = 0;
            }
        }

        delay = (ngx_msec_t) ((nsent - sent) * 1000 / r->limit_rate);

        if (delay > 0) {
            limit = 0;
            c->write->delayed = 1;
            ngx_add_timer(c->write, delay);
        }
    }

    if (limit
        && c->write->ready
        && c->sent - sent >= limit - (off_t) (2 * ngx_pagesize))
    {
        c->write->delayed = 1;
        ngx_add_timer(c->write, 1);
    }

    for (cl = r->out; cl && cl != chain; /* void */) {
        ln = cl;
        cl = cl->next;
        ngx_free_chain(r->pool, ln);
    }

    r->out = chain;

    if (chain) {
        c->buffered |= NGX_HTTP_WRITE_BUFFERED;
        return NGX_AGAIN;
    }

    c->buffered &= ~NGX_HTTP_WRITE_BUFFERED;

    if ((c->buffered & NGX_LOWLEVEL_BUFFERED) && r->postponed == NULL) {
        return NGX_AGAIN;
    }

    return NGX_OK;
}

1.9.2 ngx_alloc_chain_link

ngx_chain_t *
ngx_alloc_chain_link(ngx_pool_t *pool)
{
    ngx_chain_t  *cl;

    cl = pool->chain;

    if (cl) {
        pool->chain = cl->next;
        return cl;
    }

    /* 重新分配一个 ngx_chain_t */
    cl = ngx_palloc(pool, sizeof(ngx_chain_t));
    if (cl == NULL) {
        return NULL;
    }

    return cl;
}