From 4feee8aa70060588f63b1576410bebc7e03ac9cb Mon Sep 17 00:00:00 2001
From: Alex Ioannidis <a.ioannidis@cern.ch>
Date: Tue, 24 Mar 2026 11:06:24 +0100
Subject: [PATCH] fix(previewer): bypass IIPImage server for simple image
 preview

* When we don't have tiles (e.g. for restricted records), we currently
  rely on nginx to interpret the 404/500 from IIPImage to fall back to
  the Flask application. This didn't work well in cases where IIPImage
  fails with a segfault or non-standard response.
* We now explicitly add to the IIIF URL for the simple image preview
  (used for restricted records), a `__skip_image_server=1` querystring
  parameter, so that nginx can interpret it directly and immediately
  skip/fallback to the Flask IIIF serving.
* We also enable buffering on nginx, so we can properly handle upstream
  errors and fallback properly to Flask. This is fine since tile payloads
  from IIPImage are relatively samll.
---
 docker/nginx/conf.d/default.conf             | 13 +++++++++++--
 site/zenodo_rdm/previewer/image_previewer.py | 10 +++++++++-
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/docker/nginx/conf.d/default.conf b/docker/nginx/conf.d/default.conf
index 3e397e5c9..eb2fc215a 100644
--- a/docker/nginx/conf.d/default.conf
+++ b/docker/nginx/conf.d/default.conf
@@ -181,6 +181,15 @@ server {
 
       # Keep the original URI to pass to the fallback location in case of error
       set $original_uri $request_uri;
+
+      # Skip IIPImage, route directly to Flask (strip the param before fallback).
+      # NOTE: $uri strips the entire querystring. This is fine since IIIF Image
+      # API uses path segments, not query params. If future query params are
+      # needed, this will need a more targeted stripping approach.
+      if ($arg___skip_image_server = "1") {
+        set $original_uri $uri;
+        return 418;
+      }
       # Construct the final URL. This converts:
       #
       #   - /api/iiif/record:1:image.png/... -> /api/iiif/1_/__/_/image.png.ptif/...
@@ -191,7 +200,7 @@ server {
       fastcgi_pass image_server;
       include fastcgi_params;
 
-      fastcgi_buffering off;
+      fastcgi_buffering on;
 
       fastcgi_param PATH_INFO $fastcgi_script_name;
       fastcgi_param REQUEST_METHOD $request_method;
@@ -204,7 +213,7 @@ server {
 
       # In case of an error, fallback to the Flask/Invenio application
       fastcgi_intercept_errors on;
-      error_page 404 500 502 503 504 = @iiif_fallback;
+      error_page 404 418 500 502 503 504 = @iiif_fallback;
     }
   }
 
diff --git a/site/zenodo_rdm/previewer/image_previewer.py b/site/zenodo_rdm/previewer/image_previewer.py
index 964317d7b..5b8a6f975 100644
--- a/site/zenodo_rdm/previewer/image_previewer.py
+++ b/site/zenodo_rdm/previewer/image_previewer.py
@@ -5,6 +5,7 @@
 from copy import deepcopy
 from datetime import datetime, timedelta, timezone
 from os.path import splitext
+from urllib.parse import urlencode, urlparse, urlunparse
 
 from flask import current_app, render_template
 
@@ -149,9 +150,16 @@ def preview(file):
             else "jpg"
         )
         size = current_app.config["IIIF_SIMPLE_PREVIEWER_SIZE"]
-        tpl_ctx["iiif_simple_url"] = (
+        iiif_simple_url = (
             f"{file.data['links']['iiif_base']}/full/{size}/0/default.{format}"
         )
+        unprocessed = tile_status is None
+        if unprocessed:
+            parsed = urlparse(iiif_simple_url)
+            qs = urlencode({"__skip_image_server": "1"})
+            existing_qs = f"{parsed.query}&{qs}" if parsed.query else qs
+            iiif_simple_url = urlunparse(parsed._replace(query=existing_qs))
+        tpl_ctx["iiif_simple_url"] = iiif_simple_url
 
         # Add banner message if needed
         if record.is_draft: