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

Posted by 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' =>
'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 ~
# examples:
# s3_bucket = my-bucket.s3.amazonaws.com
# path = downloads/0001.mpg
# args = X-Amz-Credentials=...&X-Amz-Date=...

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

resolver 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 {

rewrite ^

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

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