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.