Is post_action skipped when an upstream app server returns a X-Accel-Redirect header is used?

[clintmiller]

The post_action in the config for my Nginx 1.12.1 instance is not firing (or
does not appear to be). I'm wondering if it is because my app server is
returning a X-Accel-Redirect header. The ultimate goal is track when
downloads hosted on S3 have completed.

****
Step 1: The request hits nginx at http://host/download/...
****

location ~ /download/ {
  proxy_pass http://rails-app-upstream;
  proxy_set_header Host $host;
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  post_action @finished;
}

****
Step 2: After authenticating the request for the download file and marking
the start of the download in the database, the app server returns the
request with the following headers:
****

{ 
   'X-Accel-Redirect' =>
'/s3-zip/my-bucket.s3.amazonaws.com/downloads/0001.jpg`
   'Content-Disposition' => "attachment; filename=download.jpg",
   'X-Download-Log-Id' => log.id.to_s
}

****
Step 3: Nginx sees the x-accel-redirect header and performance an internal
request to this location that is defined:
****

location ~
"^/s3-zip/(?<s3_bucket>.[a-z0-9][a-z0-9-.]*.s3.amazonaws.com)/(?<path>.*)$"
{
  # examples:
  #   s3_bucket = my-bucket.s3.amazonaws.com
  #   path = downloads/0001.mpg
  #   args = X-Amz-Credentials=...&X-Amz-Date=...
  internal;

  access_log /var/log/nginx/s3_assets-access.log main;
  error_log /var/log/nginx/s3_assets-error.log warn;

  resolver 8.8.8.8 valid=30s; # Google DNS
  resolver_timeout 10s;

  proxy_http_version 1.1;
  proxy_set_header Host $s3_bucket;
  proxy_set_header Authorization '';

  # remove amazon headers
  proxy_hide_header x-amz-id-2;
  proxy_hide_header x-amz-request-id;
  proxy_hide_header Set-Cookie;
  proxy_ignore_headers "Set-Cookie";

  # no file buffering
  proxy_buffering off;

  # bubble errors up
  proxy_intercept_errors on;

  proxy_pass https://$s3_bucket/$path?$args;
}

****
Missed Step: The following location called by the post_action in Step 1 is
never fired. Is this because of the x-accel-redirect header, or because Step
3 uses a proxy_pass, or something else? This last location calls the app
server's endpoint once nginx has completed the request to mark the download
as completed.
****

location @finished {
  internal;

  rewrite ^
/download/finish/$sent_http_x_download_log_id?bytes=$body_bytes_sent;
}

For what it's worth, this basic technique (admittedly, it has a few steps!)
works-properlyl executes the post_action- for zip-on-the-fly downloads,
using the mod_zip plugin and fetching zip file component contents from S3.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,278529,278529#msg-278529