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After a Drupal 6 to 7 update we are unable to use private image download, symptom is we cannot access private files via system/files, while public files can be loaded.

Trying to download private file results in a 404 -- Not found error, sent from the web-server directly, not drupal. After some changes to the configuration this results instead in a Drupal Error page. The server log says

File does not exist: /home/licebase/drupal/system/files/genefinding_diagram_1_2.png, referer: https://myhost/node/761470/edit?render=overlay

The system file exists and has the correct permissions.

Centos 6 on virtualization host, PHP 5.3.3, Postgres 8.4 on dedicated DB server, Apache 2.2.15 (centos) + a lot of active modules + fields+cck

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    There never has been a good upgrade path till D8. Can you give a better description of your setup? – Brady Feb 14 '16 at 18:12
  • This is an update from a running d6 instance including tripal (see: tripal.info that should have no relevance here). Running on a cloned vm under centos, using postgres 8.4 db host, php5.3.3, apache, drush 6.7, using CCK, views and a bunch of other models. Core update was done following the standard update procedure. Please let me know, if you need any specific information. – Michael Feb 14 '16 at 18:21
  • Please be aware that bug reports are off topic here. – Mołot Feb 16 '16 at 9:47
  • Well, I am not even sure it is a bug, I am in the process of finding out, it might still be a misconfiguration. Is there any relevant input you could give me on topic? I appreciate any constructive hint. Also, who says they are off topic? Would also the disparate question of how to debug a real error be off topic here? Then this site wouldn't be very useful overall. – Michael Feb 16 '16 at 9:56
  • @Michael It's Q+A, so ask a specific, objectively answerable question, get a specific, objective answer. That's the only thing this site is useful for (intentionally so). General hints on where to start, how to debug a site, and other broadly natured requests don't fit the format very well so don't be too surprised if people don't engage here. There are forums out there if you need a back-and-forth/support style request. BTW it's the help center that lists bug reports/workarounds as off-topic - discussions about such things happen in the module issue queue, not here, for a variety of reasons – Clive Feb 16 '16 at 10:33
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Edit: We have a workaround, I hope this question can serve as a good practice guide of good to debug complex software like Drupal showing unexplainable errors or WSOD. Wish we had followed our own advice to not waste time on our futile attempts time.

tl;dr It was the core module Database logging 7.42 Logs and records system events to the database. which is interfering with private files in our specific setting. We disabled most modules including core, and brought them back one-by one. Database logging is reproducibly inducing the error on activation.

  1. Don't panic and make a debugging plan, listing options of what could be checked, and then stick to it until everything has been checked. If out of options, try to shake things up

  2. Compare your system to a vanilla system and note down what is does differently

  3. Modules might interfere with each other in unexpected ways or with your specific configuration. That doesn't mean there is a bug in any of those modules. Boil down your system to the least complex configuration possible, remove any unneeded entry in your configuration file, disable any module that can be disabled including core modules, suspect everyone.
  4. Check if error still present, if not re-enable all modules one-by-one, and note down which configuration change causes the error. Otherwise, start debugging going to 1.

Cheers

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  • Hold your horses, dude. Have you really tried everything? Nope, follow your plan and everything will work out .... – Michael Feb 16 '16 at 13:31

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