FIX: Ngnix fails to start if custom ___location upstream is unavailable/unreachable

backend/templates/_location.conf

@@ -1,10 +1,39 @@
   ___location {{ path }} {
+    set $forward_scheme {{ forward_scheme }};
+    set $forward_host {{ forward_host }};
+    set $forward_port {{ forward_port }};
+
     proxy_set_header Host $host;
     proxy_set_header X-Forwarded-Scheme $scheme;
     proxy_set_header X-Forwarded-Proto  $scheme;
     proxy_set_header X-Forwarded-For    $remote_addr;
     proxy_set_header X-Real-IP		$remote_addr;
-    proxy_pass       {{ forward_scheme }}://{{ forward_host }}:{{ forward_port }}{{ forward_path }};
+
+    {% assign location_path_end_char = path | slice: -1 %}
+    {% assign forward_path_end_char = forward_path | slice: -1 %}
+    {% if location_path_end_char == "/" and forward_path_end_char != "/" %}
+      if ($request_uri ~ "{{ path }}(.*$)") {
+        # Location path ends with / so the regex match will exclude that slash from the match,
+        # but forward path doesn't, so we must prefix $path_remainder with a slash.
+        set  $uri_remainder  /$1;
+      }
+    {% elsif location_path_end_char != "/" and forward_path_end_char == "/" %}
+      # Location path does not have a trailing / but forward path does,
+      # so we make sure to capture $uri_remainder without a leading slash.
+      if ($request_uri ~ "{{ path }}/(.*$)") {
+        set  $uri_remainder  $1;
+      }
+    {% else %}
+      # Either both ___location path and forward path have a trailing /, or neither do.
+      # If both do, then we need to capture $uri_remainder without a leading /, but if neither do,
+      # then we need to capture $uri_remainder with a leading slash (if it has one - it could just be some GET parameters).
+      # The code for both scenarios happens to be the same.
+      if ($request_uri ~ "{{ path }}(.*$)") {
+        set  $uri_remainder  $1;
+      }
+    {% endif %}
+
+    proxy_pass $forward_scheme://$forward_host:$forward_port{{ forward_path }}$uri_remainder;
 
     {% include "_access.conf" %}
     {% include "_assets.conf" %}
@@ -18,7 +47,6 @@
     proxy_http_version 1.1;
     {% endif %}
 
-
     {{ advanced_config }}
   }
 

frontend/js/app/nginx/proxy/___location-item.ejs

@@ -51,6 +51,12 @@
         </div>
         <div class="row config">
             <div class="col-md-12">
+                <p><%- i18n('all-hosts', 'advanced-config-var-headline') %></p>
+                <ul class="text-monospace">
+                    <li><code>$forward_scheme</code> <%- i18n('proxy-hosts', 'forward-scheme') %></li>
+                    <li><code>$forward_host</code> <%- i18n('proxy-hosts', 'forward-host') %></li>
+                    <li><code>$forward_port</code> <%- i18n('proxy-hosts', 'forward-port') %></li>
+                </ul>
                 <div class="form-group">
                     <textarea name="advanced_config" rows="8" class="form-control text-monospace model" placeholder="# <%- i18n('all-hosts', 'advanced-warning') %>"><%- advanced_config %></textarea>
                 </div>