//......

if (r->keepalive) {
    if (clcf->keepalive_timeout == 0) {
        r->keepalive = 0;

    } else if (r->connection->requests >= clcf->keepalive_requests) {
        r->keepalive = 0;

    } else if (ngx_current_msec - r->connection->start_time
               > clcf->keepalive_time)
    {
        r->keepalive = 0;

    } else if (r->headers_in.msie6
               && r->method == NGX_HTTP_POST
               && (clcf->keepalive_disable
                   & NGX_HTTP_KEEPALIVE_DISABLE_MSIE6))
    {
        /*
         * MSIE may wait for some time if an response for
         * a POST request was sent over a keepalive connection
         */
        r->keepalive = 0;

    } else if (r->headers_in.safari
               && (clcf->keepalive_disable
                   & NGX_HTTP_KEEPALIVE_DISABLE_SAFARI))
    {
        /*
         * Safari may send a POST request to a closed keepalive
         * connection and may stall for some time, see
         *     https://bugs.webkit.org/show_bug.cgi?id=5760
         */
        r->keepalive = 0;
    }
}

//......

ngx_str_t  *value;

if (clcf->keepalive_timeout != NGX_CONF_UNSET_MSEC) {
    return "is duplicate";
}

value = cf->args->elts;

clcf->keepalive_timeout = ngx_parse_time(&value[1], 0);

if (clcf->keepalive_timeout == (ngx_msec_t) NGX_ERROR) {
    return "invalid value";
}

if (cf->args->nelts == 2) {
    return NGX_CONF_OK;
}

clcf->keepalive_header = ngx_parse_time(&value[2], 1);

if (clcf->keepalive_header == (time_t) NGX_ERROR) {
    return "invalid value";
}

return NGX_CONF_OK;

if (r->header_sent) {
    return NGX_OK;
}

//......

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);
}

//......

//......

wev = c->write;
wev->handler = ngx_http_empty_handler;

if (b->pos < b->last) {

    ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0, "pipelined request");

    c->log->action = "reading client pipelined request line";

    r = ngx_http_create_request(c);
    if (r == NULL) {
        ngx_http_close_connection(c);
        return;
    }

    r->pipeline = 1;

    c->data = r;

    c->sent = 0;
    c->destroyed = 0;
    c->pipeline = 1;

    if (rev->timer_set) {
        ngx_del_timer(rev);
    }

    rev->handler = ngx_http_process_request_line;
    ngx_post_event(rev, &ngx_posted_events);
    return;
}

//......

rev->handler = ngx_http_keepalive_handler;

//......

c->idle = 1;
ngx_reusable_connection(c, 1);

ngx_add_timer(rev, clcf->keepalive_timeout);

if (rev->ready) {
    ngx_post_event(rev, &ngx_posted_events);
}

c = rev->data;

ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0, "http keepalive handler");

if (rev->timedout || c->close) {
    ngx_http_close_connection(c);
    return;
}

if (ngx_event_flags & NGX_USE_KQUEUE_EVENT) {
    if (rev->pending_eof) {
        c->log->handler = NULL;
        ngx_log_error(NGX_LOG_INFO, c->log, rev->kq_errno,
                      "kevent() reported that client %V closed "
                      "keepalive connection", &c->addr_text);

b = c->buffer;
size = b->end - b->start;

if (b->pos == NULL) {

    /*
     * The c->buffer's memory was freed by ngx_http_set_keepalive().
     * However, the c->buffer->start and c->buffer->end were not changed
     * to keep the buffer size.
     */

    b->pos = ngx_palloc(c->pool, size);
    if (b->pos == NULL) {
        ngx_http_close_connection(c);
        return;
    }

    b->start = b->pos;
    b->last = b->pos;
    b->end = b->pos + size;
}

/*
 * MSIE closes a keepalive connection with RST flag
 * so we ignore ECONNRESET here.
 */

c->log_error = NGX_ERROR_IGNORE_ECONNRESET;
ngx_set_socket_errno(0);

n = c->recv(c, b->last, size);
c->log_error = NGX_ERROR_INFO;

if (n == NGX_AGAIN) {
    if (ngx_handle_read_event(rev, 0) != NGX_OK) {
        ngx_http_close_connection(c);
        return;
    }

    /*
     * Like ngx_http_set_keepalive() we are trying to not hold
     * c->buffer's memory for a keepalive connection.
     */

    if (ngx_pfree(c->pool, b->start) == NGX_OK) {

        /*
         * the special note that c->buffer's memory was freed
         */

        b->pos = NULL;
    }

    return;
}

if (n == NGX_ERROR) {
    ngx_http_close_connection(c);
    return;
}

c->log->handler = NULL;

if (n == 0) {
    ngx_log_error(NGX_LOG_INFO, c->log, ngx_socket_errno,
                  "client %V closed keepalive connection", &c->addr_text);
    ngx_http_close_connection(c);
    return;
}

b->last += n;

c->log->handler = ngx_http_log_error;
c->log->action = "reading client request line";

c->idle = 0;
ngx_reusable_connection(c, 0);

c->data = ngx_http_create_request(c);
if (c->data == NULL) {
    ngx_http_close_connection(c);
    return;
}

c->sent = 0;
c->destroyed = 0;

ngx_del_timer(rev);

rev->handler = ngx_http_process_request_line;
ngx_http_process_request_line(rev);