error ngx_http_send_header

[Dk Jack]

Hi,
I am trying to send a custom response in my module when I encounter a
request for a specific location. For example, I have setup my location as
follows:

  server {
    listen 9999;
      location /__my_module {
      set_mymodule_location;
      log_not_found off;
    }
  }

In my location handler, I am trying to respond to a request for this
location. I get the response I expect at the client. However, I am seeing
the following error message logged in error.log:

2018/05/16 00:38:07 [alert] 225#225: *158 header already sent, client:
127.0.0.1, server: , request: "GET /__my_module HTTP/1.1", host:
"localhost:9999"

Can someone let me know how I can prevent this error from showing up. Also,
the return value from ngx_http_send_header is always an NGX_ERROR. Not sure
why...

FWIW, my response buffer is always less than 512 bytes. My code is included
at the end of this email. Any help is appreciated. Thanks

Dk.

int
SendResponse(ngx_http_request_t *r, ngx_uint_t http_status,
             const char *data, unsigned int dlen)
{
  ngx_buf_t *buf = ngx_create_temp_buf(r->pool, 512);

  if (NULL == buf) {
    ngx_log_error(NGX_LOG_ERR, log, 0,
                  "allocation failure");
    return NGX_HTTP_INTERNAL_SERVER_ERROR;
  }

  buf->last = ngx_copy(buf->start, (unsigned char*) data, dlen);
  ngx_log_t *log = r->connection->log;

  ngx_chain_t *out_chain = ngx_alloc_chain_link(r->pool);
  if (NULL == out_chain) {
    ngx_log_error(NGX_LOG_ERR, log, 0,
                  "failed to alloc buffer chain");
    return NGX_HTTP_INTERNAL_SERVER_ERROR;
  }

  out_chain->buf = buf;
  out_chain->next = NULL;
  buf->last_buf = 1;
  buf->last_in_chain = 1;

  ngx_int_t rc;
  r->headers_out.status = http_status;
  r->headers_out.content_length_n = buf->last - buf->start;
  r->headers_out.content_type.len = sizeof("text/plain") - 1;
  r->headers_out.content_type.data = (u_char *) "text/plain";
  rc = ngx_http_send_header(r);

#if 0 // send_header always returns NGX_ERROR.
  if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
    ngx_log_error(NGX_LOG_ERR, log, 0,
                  "send header failed. rc=%i", rc);
    return NGX_HTTP_INTERNAL_SERVER_ERROR;
  }
#endif

  rc = ngx_http_output_filter(r, out_chain);
  if (rc != NGX_OK) {
    ngx_log_error(NGX_LOG_ERR, log, 0,
                  "send response buffer failed. rc=%i", rc);
    return NGX_HTTP_INTERNAL_SERVER_ERROR;
  }

  return 0;
}
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