Nginx-HTTP之ngx_http_top_body_filter

1. ngx_http_top_body_filter

该链表用于构造响应消息的响应正文.

大致有以下模块在该链表中插入了自己的函数：

1. ngx_http_range_filter_module: ngx_http_range_body_filter
2. ngx_http_copy_filter_module: ngx_http_copy_filter
3. ngx_http_headers_filter_module: ngx_http_trailers_filter
4. ngx_http_charset_filter_module: ngx_http_charset_body_filter
5. ngx_http_ssi_filter_module: ngx_http_ssi_body_filter
6. ngx_http_postpone_filter_module: ngx_http_postpone_filter
7. ngx_http_gzip_filter_module：ngx_http_gzip_body_filter
8. ngx_http_chunked_filter_module：ngx_http_chunked_body_filter
9. ngx_http_write_filter_module：ngx_http_write_filter

1.1 ngx_http_range_body_filter

static ngx_int_t
ngx_http_range_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    ngx_http_range_filter_ctx_t  *ctx;

    if (in == NULL) {
        return ngx_http_next_body_filter(r, in);
    }

    /* 获取 ngx_http_range_body_filter_module 模块的上下文结构体 */
    ctx = ngx_http_get_module_ctx(r, ngx_http_range_body_filter_module);

    /* 若为 NULL 则执行链表的下一个函数 */
    if (ctx == NULL) {
        return ngx_http_next_body_filter(r, in);
    }

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

    /*
     * multipart ranges are supported only if whole body is in a single buffer
     */

    if (ngx_buf_special(in->buf)) {
        return ngx_http_next_body_filter(r, in);
    }

    if (ngx_http_range_test_overlapped(r, ctx, in) != NGX_OK) {
        return NGX_ERROR;
    }

    return ngx_http_range_multipart_body(r, ctx, in);
}

1.2 ngx_http_copy_filter

static ngx_int_t
ngx_http_copy_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    ngx_int_t                     rc;
    ngx_connection_t             *c;
    ngx_output_chain_ctx_t       *ctx;
    ngx_http_core_loc_conf_t     *clcf;
    ngx_http_copy_filter_conf_t  *conf;

    c = r->connection;

    ngx_log_debug2(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http copy filter: \"%V?%V\"", &r->uri, &r->args);

    /* 获取 ngx_http_copy_filter_module 模块的上下文结构体 */
    ctx = ngx_http_get_module_ctx(r, ngx_http_copy_filter_module);

    /* 若为 NULL，则分配 */
    if (ctx == NULL) {
        ctx = ngx_pcalloc(r->pool, sizeof(ngx_output_chain_ctx_t));
        if (ctx == NULL) {
            return NGX_ERROR;
        }

        /* 将该 ngx_http_copy_filter_module 模块的上下文结构体
         * ngx_output_chain_ctx_t 存放到 ngx_http_request_t 结构体
         * 中的 ctx 数组中，下标为单前模块的 ctx_index */
        ngx_http_set_ctx(r, ctx, ngx_http_copy_filter_module);

        conf = ngx_http_get_module_loc_conf(r, ngx_http_copy_filter_module);
        clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

        /* 标志位，为 1 表示使用 sendfile 系统调用发送文件 */
        ctx->sendfile = c->sendfile;
        ctx->need_in_memory = r->main_filter_need_in_memory
                              || r->filter_need_in_memory;
        ctx->need_in_temp = r->filter_need_temporary;

        ctx->alignment = clcf->directio_alignment;

        ctx->pool = r->pool;
        ctx->bufs = conf->bufs;
        ctx->tag = (ngx_buf_tag_t) &ngx_http_copy_filter_module;

        ctx->output_filter = (ngx_output_chain_filter_pt)
                                  ngx_http_next_body_filter;
        ctx->filter_ctx = r;

#if (NGX_HAVE_FILE_AIO)
        if (ngx_file_aio && clcf->aio == NGX_HTTP_AIO_ON) {
            ctx->aio_handler = ngx_http_copy_aio_handler;
#if (NGX_HAVE_AIO_SENDFILE)
            ctx->aio_preload = ngx_http_copy_aio_sendfile_preload;
#endif
        }
#endif

#if (NGX_THREADS)
        if (clcf->aio == NGX_HTTP_AIO_THREADS) {
            ctx->thread_handler = ngx_http_copy_thread_handler;
        }
#endif

        if (in && in->buf && ngx_buf_size(in->buf)) {
            r->request_output = 1;
        }
    }

#if (NGX_HAVE_FILE_AIO || NGX_THREADS)
    ctx->aio = r->aio;
#endif

    rc = ngx_output_chain(ctx, in);

    if (ctx->in == NULL) {
        r->buffered &= ~NGX_HTTP_COPY_BUFFERED;

    } else {
        r->buffered |= NGX_HTTP_COPY_BUFFERED;
    }

    ngx_log_debug3(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http copy filter: %i \"%V?%V\"", rc, &r->uri, &r->args);

    return rc;
}

1.2.1 ngx_output_chain

ngx_int_t
ngx_output_chain(ngx_output_chain_ctx_t *ctx, ngx_chain_t *in)
{
    off_t         bsize;
    ngx_int_t     rc, last;
    ngx_chain_t  *cl, *out, **last_out;

    if (ctx->in == NULL && ctx->busy == NULL
#if (NGX_HAVE_FILE_AIO || NGX_THREADS)
        && !ctx->aio
#endif
       )
    {
        /*
         * the short path for the case when the ctx->in and ctx->busy chains
         * are empty, the incoming chain is empty too or has the single buf
         * that does not require the copy
         */

        if (in == NULL) {
            return ctx->output_filter(ctx->filter_ctx, in);
        }

        if (in->next == NULL
#if (NGX_SENDFILE_LIMIT)
            && !(in->buf->in_file && in->buf->file_last > NGX_SENDFILE_LIMIT)
#endif
            && ngx_output_chain_as_is(ctx, in->buf))
        {
            return ctx->output_filter(ctx->filter_ctx, in);
        }
    }

    /* add the incoming buf to the chain ctx->in */

    if (in) {
        if (ngx_output_chain_add_copy(ctx->pool, &ctx->in, in) == NGX_ERROR) {
            return NGX_ERROR;
        }
    }

    out = NULL;
    last_out = &out;
    last = NGX_NONE;

    for ( ;; ) {

#if (NGX_HAVE_FILE_AIO || NGX_THREADS)
        if (ctx->aio) {
            return NGX_AGAIN;
        }
#endif

        while (ctx->in) {

            /*
             * cycle while there are the ctx->in bufs
             * and there are the free output bufs to copy in
             */

            bsize = ngx_buf_size(ctx->in->buf);

            if (bsize == 0 && !ngx_buf_special(ctx->in->buf)) {

                ngx_log_error(NGX_LOG_ALERT, ctx->pool->log, 0,
                              "zero size buf in output "
                              "t:%d r:%d f:%d %p %p-%p %p %O-%O",
                              ctx->in->buf->temporary,
                              ctx->in->buf->recycled,
                              ctx->in->buf->in_file,
                              ctx->in->buf->start,
                              ctx->in->buf->pos,
                              ctx->in->buf->last,
                              ctx->in->buf->file,
                              ctx->in->buf->file_pos,
                              ctx->in->buf->file_last);

                ngx_debug_point();

                ctx->in = ctx->in->next;

                continue;
            }

            if (ngx_output_chain_as_is(ctx, ctx->in->buf)) {

                /* move the chain link to the output chain */

                cl = ctx->in;
                ctx->in = cl->next;

                *last_out = cl;
                last_out = &cl->next;
                cl->next = NULL;

                continue;
            }

            if (ctx->buf == NULL) {

                rc = ngx_output_chain_align_file_buf(ctx, bsize);

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

                if (rc != NGX_OK) {

                    if (ctx->free) {

                        /* get the free buf */

                        cl = ctx->free;
                        ctx->buf = cl->buf;
                        ctx->free = cl->next;

                        ngx_free_chain(ctx->pool, cl);

                    } else if (out || ctx->allocated == ctx->bufs.num) {

                        break;

                    } else if (ngx_output_chain_get_buf(ctx, bsize) != NGX_OK) {
                        return NGX_ERROR;
                    }
                }
            }

            rc = ngx_output_chain_copy_buf(ctx);

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

            if (rc == NGX_AGAIN) {
                if (out) {
                    break;
                }

                return rc;
            }

            /* delete the completed buf from the ctx->in chain */

            if (ngx_buf_size(ctx->in->buf) == 0) {
                ctx->in = ctx->in->next;
            }

            cl = ngx_alloc_chain_link(ctx->pool);
            if (cl == NULL) {
                return NGX_ERROR;
            }

            cl->buf = ctx->buf;
            cl->next = NULL;
            *last_out = cl;
            last_out = &cl->next;
            ctx->buf = NULL;
        }

        if (out == NULL && last != NGX_NONE) {

            if (ctx->in) {
                return NGX_AGAIN;
            }

            return last;
        }

        last = ctx->output_filter(ctx->filter_ctx, out);

        if (last == NGX_ERROR || last == NGX_DONE) {
            return last;
        }

        ngx_chain_update_chains(ctx->pool, &ctx->free, &ctx->busy, &out,
                                ctx->tag);
        last_out = &out;
    }
}

1.3 ngx_http_trailers_filter

static ngx_int_t
ngx_http_trailers_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    ngx_str_t                 value;
    ngx_uint_t                i, safe_status;
    ngx_chain_t              *cl;
    ngx_table_elt_t          *t;
    ngx_http_header_val_t    *h;
    ngx_http_headers_conf_t  *conf;

    conf = ngx_http_get_module_loc_conf(r, ngx_http_headers_filter_module);

    /* 若 trailers 为 NULL 或其他，则下一个 */
    if (in == NULL
        || conf->trailers == NULL
        || !r->expect_trailers
        || r->header_only)
    {
        return ngx_http_next_body_filter(r, in);
    }

    for (cl = in; cl; cl = cl->next) {
        if (cl->buf->last_buf) {
            break;
        }
    }

    if (cl == NULL) {
        return ngx_http_next_body_filter(r, in);
    }

    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;
    }

    h = conf->trailers->elts;
    for (i = 0; i < conf->trailers->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 (value.len) {
            t = ngx_list_push(&r->headers_out.trailers);
            if (t == NULL) {
                return NGX_ERROR;
            }

            t->key = h[i].key;
            t->value = value;
            t->hash = 1;
        }
    }

    return ngx_http_next_body_filter(r, in);
}

1.4 ngx_http_charset_body_filter

暂略

1.5 ngx_http_ssi_body_filter

暂略

1.6 ngx_http_postpone_filter

暂略

1.7 ngx_http_gzip_body_filter

暂略

1.8 ngx_http_chunked_body_filter

暂略

1.9 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 r->out 链表的第一个元素 */
    ll = &r->out;

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

    /* 若此时缓存的是 body，则此时 r->out 链表中已经缓存响应消息的消息报头 */
    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) {
        /* 生成一个 ngx_chain_t  */
        cl = ngx_alloc_chain_link(r->pool);
        if (cl == NULL) {
            return NGX_ERROR;
        }

        cl->buf = ln->buf;
        /* 将该新生成的 ngx_chain_t 添加到 r->out 链表尾部 */
        *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

        size += ngx_buf_size(cl->buf);

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

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

        /* 若 last_buf 为 1，表明这个最后一个 buf 了 */
        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);

    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;
    }

    /* 当前已经发送的字节数 */
    sent = c->sent;

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

    /* 指向 ngx_linux_sendfile_chain 函数, 将响应消息发送给客户端 */
    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);
    }

    /* 将 r->out 链表中的元素（即 ngx_chain_t）插入到 r->pool->chain 链表中 */
    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;
}

1.9.1 c->send_chain: ngx_linux_sendfile_chain

ngx_chain_t *
ngx_linux_sendfile_chain(ngx_connection_t *c, ngx_chain_t *in, off_t limit)
{
    int            tcp_nodelay;
    off_t          send, prev_send;
    size_t         file_size, sent;
    ssize_t        n;
    ngx_err_t      err;
    ngx_buf_t     *file;
    ngx_event_t   *wev;
    ngx_chain_t   *cl;
    ngx_iovec_t    header;
    struct iovec   headers[NGX_IOVS_PREALLOCATE];

    /* 获取该连接的写事件 */
    wev = c->write;

    if (!wev->ready) {
        return in;
    }

    /* the maximum limit size is 2G-1 - the page size */

    if (limit == 0 || limit > (off_t) (NGX_SENDFILE_MAXSIZE - ngx_pagesize)) {
        limit = NGX_SENDFILE_MAXSIZE - ngx_pagesize;
    }

    send = 0;

    header.iovs = headers;
    header.nalloc = NGX_IOVS_PREALLOCATE;

    for ( ;; ) {
        prev_send = send;

        /* create the iovec and coalesce the neighbouring bufs */

        /* 将 in 中的数据（除在文件中的数据外）构造成 iovec */
        cl = ngx_output_chain_to_iovec(&header, in, limit - send, c->log);

        if (cl == NGX_CHAIN_ERROR) {
            return NGX_CHAIN_ERROR;
        }

        send += header.size;

        /* set TCP_CORK if there is a header before a file */

        if (c->tcp_nopush == NGX_TCP_NOPUSH_UNSET
            && header.count != 0
            && cl
            && cl->buf->in_file)
        {
            /* the TCP_CORK and TCP_NODELAY are mutually exclusive */

            if (c->tcp_nodelay == NGX_TCP_NODELAY_SET) {

                tcp_nodelay = 0;

                if (setsockopt(c->fd, IPPROTO_TCP, TCP_NODELAY,
                               (const void *) &tcp_nodelay, sizeof(int)) == -1)
                {
                    err = ngx_socket_errno;

                    /*
                     * there is a tiny chance to be interrupted, however,
                     * we continue a processing with the TCP_NODELAY
                     * and without the TCP_CORK
                     */

                    if (err != NGX_EINTR) {
                        wev->error = 1;
                        ngx_connection_error(c, err,
                                             "setsockopt(TCP_NODELAY) failed");
                        return NGX_CHAIN_ERROR;
                    }

                } else {
                    c->tcp_nodelay = NGX_TCP_NODELAY_UNSET;

                    ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, 0,
                                   "no tcp_nodelay");
                }
            }

            if (c->tcp_nodelay == NGX_TCP_NODELAY_UNSET) {

                if (ngx_tcp_nopush(c->fd) == NGX_ERROR) {
                    err = ngx_socket_errno;

                    /*
                     * there is a tiny chance to be interrupted, however,
                     * we continue a processing without the TCP_CORK
                     */

                    if (err != NGX_EINTR) {
                        wev->error = 1;
                        ngx_connection_error(c, err,
                                             ngx_tcp_nopush_n " failed");
                        return NGX_CHAIN_ERROR;
                    }

                } else {
                    c->tcp_nopush = NGX_TCP_NOPUSH_SET;

                    ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, 0,
                                   "tcp_nopush");
                }
            }
        }

        /* get the file buf */

        /* 这里开始获取文件中的数据 */
        if (header.count == 0 && cl && cl->buf->in_file && send < limit) {
            file = cl->buf;

            /* coalesce the neighbouring file bufs */

            /* 统计该文件的大小 */
            file_size = (size_t) ngx_chain_coalesce_file(&cl, limit - send);

            send += file_size;
#if 1
            if (file_size == 0) {
                ngx_debug_point();
                return NGX_CHAIN_ERROR;
            }
#endif

            /* 将该文件中的数据发送出去 */
            n = ngx_linux_sendfile(c, file, file_size);

            if (n == NGX_ERROR) {
                return NGX_CHAIN_ERROR;
            }

            if (n == NGX_DONE) {
                /* thread task posted */
                return in;
            }

            sent = (n == NGX_AGAIN) ? 0 : n;

        } else {
            n = ngx_writev(c, &header);

            if (n == NGX_ERROR) {
                return NGX_CHAIN_ERROR;
            }

            sent = (n == NGX_AGAIN) ? 0 : n;
        }

        /* 更新已经发送的字节数 */
        c->sent += sent;

        in = ngx_chain_update_sent(in, sent);

        if (n == NGX_AGAIN) {
            wev->ready = 0;
            return in;
        }

        if ((size_t) (send - prev_send) != sent) {

            /*
             * sendfile() on Linux 4.3+ might be interrupted at any time,
             * and provides no indication if it was interrupted or not,
             * so we have to retry till an explicit EAGAIN
             *
             * sendfile() in threads can also report less bytes written
             * than we are prepared to send now, since it was started in
             * some point in the past, so we again have to retry
             */

            send = prev_send + sent;
            continue;
        }

        if (send >= limit || in == NULL) {
            return in;
        }
    }
}

1.9.2 ngx_output_chain_to_iovec

ngx_chain_t *
ngx_output_chain_to_iovec(ngx_iovec_t *vec, ngx_chain_t *in, size_t limit,
    ngx_log_t *log)
{
    size_t         total, size;
    u_char        *prev;
    ngx_uint_t     n;
    struct iovec  *iov;

    iov = NULL;
    prev = NULL;
    total = 0;
    n = 0;

    for ( /* void */ ; in && total < limit; in = in->next) {

        if (ngx_buf_special(in->buf)) {
            continue;
        }

        /* 若该buf中的数据是在文件中，则跳出该循环 */
        if (in->buf->in_file) {
            break;
        }

        if (!ngx_buf_in_memory(in->buf)) {
            ngx_log_error(NGX_LOG_ALERT, log, 0,
                          "bad buf in output chain "
                          "t:%d r:%d f:%d %p %p-%p %p %O-%O",
                          in->buf->temporary,
                          in->buf->recycled,
                          in->buf->in_file,
                          in->buf->start,
                          in->buf->pos,
                          in->buf->last,
                          in->buf->file,
                          in->buf->file_pos,
                          in->buf->file_last);

            ngx_debug_point();

            return NGX_CHAIN_ERROR;
        }

        size = in->buf->last - in->buf->pos;

        if (size > limit - total) {
            size = limit - total;
        }

        if (prev == in->buf->pos) {
            iov->iov_len += size;

        } else {
            if (n == vec->nalloc) {
                break;
            }

            iov = &vec->iovs[n++];

            iov->iov_base = (void *) in->buf->pos;
            iov->iov_len = size;
        }

        prev = in->buf->pos + size;
        total += size;
    }

    vec->count = n;
    vec->size = total;

    return in;
}

1.9.3 ngx_writev

ssize_t
ngx_writev(ngx_connection_t *c, ngx_iovec_t *vec)
{
    ssize_t    n;
    ngx_err_t  err;

eintr:

    n = writev(c->fd, vec->iovs, vec->count);

    ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
                   "writev: %z of %uz", n, vec->size);

    if (n == -1) {
        err = ngx_errno;

        switch (err) {
        case NGX_EAGAIN:
            ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, err,
                           "writev() not ready");
            return NGX_AGAIN;

        case NGX_EINTR:
            ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, err,
                           "writev() was interrupted");
            goto eintr;

        default:
            c->write->error = 1;
            ngx_connection_error(c, err, "writev() failed");
            return NGX_ERROR;
        }
    }

    return n;
}

1.9.4 ngx_chain_update_sent

ngx_chain_t *
ngx_chain_update_sent(ngx_chain_t *in, off_t sent)
{
    off_t  size;

    for ( /* void */ ; in; in = in->next) {

        if (ngx_buf_special(in->buf)) {
            continue;
        }

        if (sent == 0) {
            break;
        }

        size = ngx_buf_size(in->buf);

        /* 若已将 buf 中的数据都发送出去 */
        if (sent >= size) {
            sent -= size;

            if (ngx_buf_in_memory(in->buf)) {
                /* 则更新 pos 指针的位置 */
                in->buf->pos = in->buf->last;
            }

            if (in->buf->in_file) {
                in->buf->file_pos = in->buf->file_last;
            }

            continue;
        }

        if (ngx_buf_in_memory(in->buf)) {
            in->buf->pos += (size_t) sent;
        }

        if (in->buf->in_file) {
            in->buf->file_pos += sent;
        }

        break;
    }

    return in;
}

1.9.5 ngx_chain_coalesce_file

off_t
ngx_chain_coalesce_file(ngx_chain_t **in, off_t limit)
{
    off_t         total, size, aligned, fprev;
    ngx_fd_t      fd;
    ngx_chain_t  *cl;

    total = 0;

    cl = *in;
    fd = cl->buf->file->fd;

    do {
        /* 计算当前需要发送的字节数 */
        size = cl->buf->file_last - cl->buf->file_pos;

        if (size > limit - total) {
            size = limit - total;

            aligned = (cl->buf->file_pos + size + ngx_pagesize - 1)
                       & ~((off_t) ngx_pagesize - 1);

            if (aligned <= cl->buf->file_last) {
                size = aligned - cl->buf->file_pos;
            }

            total += size;
            break;
        }

        total += size;
        fprev = cl->buf->file_pos + size;
        cl = cl->next;

    } while (cl
             && cl->buf->in_file
             && total < limit
             && fd == cl->buf->file->fd
             && fprev == cl->buf->file_pos);

    *in = cl;

    return total;
}

1.9.6 ngx_linux_sendfile

static ssize_t
ngx_linux_sendfile(ngx_connection_t *c, ngx_buf_t *file, size_t size)
{
#if (NGX_HAVE_SENDFILE64)
    off_t      offset;
#else
    int32_t    offset;
#endif
    ssize_t    n;
    ngx_err_t  err;

#if (NGX_THREADS)

    if (file->file->thread_handler) {
        return ngx_linux_sendfile_thread(c, file, size);
    }

#endif

#if (NGX_HAVE_SENDFILE64)
    offset = file->file_pos;
#else
    offset = (int32_t) file->file_pos;
#endif

eintr:

    ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
                   "sendfile: @%O %uz", file->file_pos, size);

    n = sendfile(c->fd, file->file->fd, &offset, size);

    if (n == -1) {
        err = ngx_errno;

        switch (err) {
        case NGX_EAGAIN:
            ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, err,
                           "sendfile() is not ready");
            return NGX_AGAIN;

        case NGX_EINTR:
            ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, err,
                           "sendfile() was interrupted");
            goto eintr;

        default:
            c->write->error = 1;
            ngx_connection_error(c, err, "sendfile() failed");
            return NGX_ERROR;
        }
    }

    if (n == 0) {
        /*
         * if sendfile returns zero, then someone has truncated the file,
         * so the offset became beyond the end of the file
         */

        ngx_log_error(NGX_LOG_ALERT, c->log, 0,
                      "sendfile() reported that \"%s\" was truncated at %O",
                      file->file->name.data, file->file_pos);

        return NGX_ERROR;
    }

    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0, "sendfile: %z of %uz @%O",
                   n, size, file->file_pos);

    return n;
}