Nginx use temp file as request body in post subrequest

[salmaanp]

I'm building an nginx module and trying to create a post subrequest from the
main request. The response from the main request buffers is saved to a nginx
temp file and a post subrequest is created. The body of the post subrequest
is from the nginx temp file instead of buffers. 

```
    ngx_http_request_t *sr;
    ngx_http_post_subrequest_t *ps;

    ps = ngx_palloc(r->pool, sizeof(ngx_http_post_subrequest_t));
    if (ps == NULL) {
        ngx_log_stderr(0, "ngx_palloc failed");
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }
    //sub-request callback
    ps->handler = subrequest_done;
    ps->data = last_buf;



    ngx_http_test_conf_t *conf = ngx_http_get_module_loc_conf(r,
ngx_http_test_filter_module);
    if (ngx_http_subrequest(r, &conf->test_location, NULL, &sr, ps,
NGX_HTTP_SUBREQUEST_IN_MEMORY) != NGX_OK)
    {
        ngx_log_stderr(0, "failed to create subrequest");
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }
    ngx_str_t post_str = ngx_string("POST");
    sr->method = NGX_HTTP_POST;
    sr->method_name = post_str;
    sr->request_body_in_file_only = 1;
    ngx_http_set_ctx(sr, ctx, ngx_http_test_filter_module);

    // Create request body and assign temp file
    sr->request_body = ngx_pcalloc(r->pool,
sizeof(ngx_http_request_body_t));
    if (sr->request_body == NULL) {
        ngx_log_stderr(0, "request body ngx_pcalloc failed");
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }

    sr->request_body->temp_file = ctx->temp_file;
    //sr->request_body->temp_file->offset = 0; (?)


    sr->header_only = 1;
    //sr->filter_need_in_memory = 1;

    sr->headers_in.content_length_n = ctx->file_size;

    ngx_log_stderr(0, "Created sub-request.");
    return NGX_OK;


```

However this does not send out the request body in the subrequest, the
request gets stuck after finishing the handshake. If I use request buffers,
it works fine.

```
    sr->request_body->bufs->buf = payload_buf;
    sr->request_body->bufs->next = NULL;
    sr->request_body->buf = payload_buf;
```

Is there something I'm missing when using the temp file? I've verified the
temp file is there and valid. Does anyone know if using a temp file is
supported in post subrequest? or how else to achieve this. I was wondering
if the post subrequest can stream the request body as new data is added to
the temp file. Something like proxy buffering on, response buffers are added
to temp file until the last one and the post subrequest keeps streaming the
data from the tempfile. Cant use request buffers in the request body since I
will run out of buffers after 64K, and the response can be larger than that.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,290940,290940#msg-290940