
/*
 * Copyright (C) Igor Sysoev
 */

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


static char *ngx_http_empty_gif(ngx_conf_t *cf, ngx_command_t *cmd,
    void *conf);

static ngx_command_t  ngx_http_empty_gif_commands[] = {

    { ngx_string("empty_gif"),
      NGX_HTTP_LOC_CONF|NGX_CONF_NOARGS,
      ngx_http_empty_gif,
      0,
      0,
      NULL },

      ngx_null_command
};


/* the minimal single pixel transparent GIF, 43 bytes */

static u_char  ngx_empty_gif[] = {

    'G', 'I', 'F', '8', '9', 'a',  /* header */

                                   /* logical screen descriptor */
    0x01, 0x00,                    /* logical screen width */
    0x01, 0x00,                    /* logical screen height */
    0x80,                          /* global 1-bit color table */
    0x01,                          /* background color #1 */
    0x00,                          /* no aspect ratio */

                                   /* global color table */
    0x00, 0x00, 0x00,              /* #0: black */
    0xff, 0xff, 0xff,              /* #1: white */

                                   /* graphic control extension */
    0x21,                          /* extension introducer */
    0xf9,                          /* graphic control label */
    0x04,                          /* block size */
    0x01,                          /* transparent color is given, */
                                   /*     no disposal specified, */
                                   /*     user input is not expected */
    0x00, 0x00,                    /* delay time */
    0x01,                          /* transparent color #1 */
    0x00,                          /* block terminator */

                                   /* image descriptor */
    0x2c,                          /* image separator */
    0x00, 0x00,                    /* image left position */
    0x00, 0x00,                    /* image top position */
    0x01, 0x00,                    /* image width */
    0x01, 0x00,                    /* image height */
    0x00,                          /* no local color table, no interlaced */

                                   /* table based image data */
    0x02,                          /* LZW minimum code size, */
                                   /*     must be at least 2-bit */
    0x02,                          /* block size */
    0x4c, 0x01,                    /* compressed bytes 01_001_100, 0000000_1 */
                                   /* 100: clear code */
                                   /* 001: 1 */
                                   /* 101: end of information code */
    0x00,                          /* block terminator */

    0x3B                           /* trailer */
};


ngx_http_module_t  ngx_http_empty_gif_module_ctx = {
    NULL,                          /* preconfiguration */
    NULL,                          /* postconfiguration */

    NULL,                          /* create main configuration */
    NULL,                          /* init main configuration */

    NULL,                          /* create server configuration */
    NULL,                          /* merge server configuration */

    NULL,                          /* create location configuration */
    NULL                           /* merge location configuration */
};


ngx_module_t  ngx_http_empty_gif_module = {
    NGX_MODULE_V1,
    &ngx_http_empty_gif_module_ctx, /* module context */
    ngx_http_empty_gif_commands,   /* module directives */
    NGX_HTTP_MODULE,               /* module type */
    NULL,                          /* init master */
    NULL,                          /* init module */
    NULL,                          /* init process */
    NULL,                          /* init thread */
    NULL,                          /* exit thread */
    NULL,                          /* exit process */
    NULL,                          /* exit master */
    NGX_MODULE_V1_PADDING
};


static ngx_int_t
ngx_http_empty_gif_handler(ngx_http_request_t *r)
{
    ngx_int_t     rc;
    ngx_buf_t    *b;
    ngx_chain_t   out;

    if (r->method != NGX_HTTP_GET && r->method != NGX_HTTP_HEAD) {
        return NGX_HTTP_NOT_ALLOWED;
    }

    rc = ngx_http_discard_body(r);

    if (rc != NGX_OK && rc != NGX_AGAIN) {
        return rc;
    }

    r->headers_out.content_type.len = sizeof("image/gif") - 1;
    r->headers_out.content_type.data = (u_char *) "image/gif";

    if (r->method == NGX_HTTP_HEAD) {
        r->headers_out.status = NGX_HTTP_OK;

        rc = ngx_http_send_header(r);

        if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
            return rc;
        }
    }

    b = ngx_create_temp_buf(r->pool, sizeof(ngx_empty_gif));
    if (b == NULL) {
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }

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

    b->pos = ngx_empty_gif;
    b->last = ngx_empty_gif + sizeof(ngx_empty_gif);
    b->last_buf = 1;

    r->headers_out.status = NGX_HTTP_OK;
    r->headers_out.content_length_n = b->last - b->pos;
    r->headers_out.last_modified_time = 23349600;

    rc = ngx_http_send_header(r);

    if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
        return rc;
    }

    return ngx_http_output_filter(r, &out);;
}


static char *
ngx_http_empty_gif(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_http_core_loc_conf_t  *clcf;

    clcf = ngx_http_conf_get_module_loc_conf(cf, ngx_http_core_module);
    clcf->handler = ngx_http_empty_gif_handler;

    return NGX_CONF_OK;
}