Hi, jacks.

I use post_action for something similar to this for keeping track of users who download files. I've got a location for the /download entry point like this:

location ~ /download/ {
proxy_pass http://app_pool;
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;
}

# for mod_zip and x-accel-redirect requests, proxying the
# request to S3 to fulfill the zip manifest or the x-accel-redirect URI
location ~ "^/s3-proxy/(?<s3_bucket>.[a-z0-9][a-z0-9-.]*.s3.amazonaws.com)/(?<path>.*)$" {
internal;

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";

# bubble errors up
proxy_intercept_errors on;

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


location @finished {
internal;

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

location ^~ /download/finish {
proxy_pass http://$download_postback_hostname; # variable map declared elsewhere
}

This does work for sending the post_action response after the /download request is served- with one notable caveat! It does not work for X-Accel-Redirect responses from my app server. As far as I can tell, the post_action is either (1) never called in that case, or (2) has some other issue I have been able to figure out. I've dug around in the C source for Nginx, but it gets to a spot pretty quick where I'm in over my head.

Although I've been living with this since 2017, here's my mailing list post regarding the issue from 2018: https://forum.nginx.org/read.php?2,278529

I've considered trying to engage Nginx for commercial support on this one issue, but I'm not sure what kind of appetite they may have for these types of issues.