WP Super Cache sometimes ignites a blank Home Page! Need to restart Apache

Upgraded WP yesterday to 2.9.1 and WP Super Cache to 0.9.9 – was getting blank pages on everything. Turned the plugin OFF without deactivating and everything seemed to start working. Found this thread and decided to keep the plugin off for a bit until the issue is resolved. But when I came to the office this morning I was greatly disappointed to find my site loading blank pages. Only home page was loading fine – the rest were blank. Restarted Apache – no go. I de-activated the plugin – no go. Re-activated it again without turning it on – the pages re-appeared. Nothing in the httpd error log.

Any ideas on what’s going on? Should I clear cache tables?

max – I don’t know what’s going on there unfortunately. Can you try debug it with the functions in the admin page? Turn on caching again then.

You should see if log the cache serving process. Maybe there’s something in there..

Thanks for the quick reply, Donncha.

I set the debug to 4 and here is what I have (in ascending order):
Output buffer callback
404 file not found not cached
Created output buffer
Created output buffer
Output buffer callback
Output buffer callback
Output buffer callback
Created output buffer
404 file not found not cached
Buffer is blank. Output buffer may have been corrupted by another plugin or this is a redirected URL. Look for text ‘ob_start’ in the files of your plugins directory.
Created output buffer
Output buffer callback
Output buffer callback
Created output buffer
No closing html tag. Not caching.
Buffer is blank. Output buffer may have been corrupted by another plugin or this is a redirected URL. Look for text ‘ob_start’ in the files of your plugins directory.
…

I noticed it mentioned that URL might be redirected. Does it matter if the root directory of the site is actually a symlink? Or this is irrelevant?

Thank you!

The symlink shouldn’t matter, the redirect is a web redirect. A 301 or 302 usually.

Have you looked for the ob_start function in your plugins directory?

Are there files being created in wp-content/supercache/HOSTNAME/ … ? Are they 0 byte/empty files? That might explain why you’re seeing blank pages. It shouldn’t happen because of existing checks but perhaps I need to check for that.

I had the same experience
So I left debugging logs.

12:43:06 / Cookie detected: comment_author_e118a5144ffaddbfed4914cc1c278eea
12:43:06 / Cookie detected: comment_author_email_e118a5144ffaddbfed4914cc1c278eea
12:43:06 / Cookie detected: wordpress_test_cookie
12:43:06 / Cookie detected: wordpress_logged_in_e118a5144ffaddbfed4914cc1c278eea
12:43:06 / No wp-cache file exists. Must generate a new one.
12:43:06 / Cookie detected: comment_author_e118a5144ffaddbfed4914cc1c278eea
12:43:06 / Cookie detected: comment_author_email_e118a5144ffaddbfed4914cc1c278eea
12:43:06 / Cookie detected: wordpress_test_cookie
12:43:06 / Cookie detected: wordpress_logged_in_e118a5144ffaddbfed4914cc1c278eea
12:43:06 / In WP Cache Phase 2
21:43:06 / Setting up WordPress actions
21:43:06 / Created output buffer
21:43:06 / Cookie detected: comment_author_e118a5144ffaddbfed4914cc1c278eea
21:43:06 / Cookie detected: comment_author_email_e118a5144ffaddbfed4914cc1c278eea
21:43:06 / Cookie detected: wordpress_test_cookie
21:43:06 / Cookie detected: wordpress_logged_in_e118a5144ffaddbfed4914cc1c278eea
21:43:06 / scheduled wp_cache_gc for 10 seconds time.
21:43:10 / Output buffer callback
21:43:10 / Output buffer callback
12:43:10 /favicon.ico Cookie detected: comment_author_e118a5144ffaddbfed4914cc1c278eea
12:43:10 /favicon.ico Cookie detected: comment_author_email_e118a5144ffaddbfed4914cc1c278eea
12:43:10 /favicon.ico Cookie detected: wordpress_test_cookie
12:43:10 /favicon.ico Cookie detected: wordpress_logged_in_e118a5144ffaddbfed4914cc1c278eea
12:43:10 /favicon.ico No wp-cache file exists. Must generate a new one.
12:43:10 /favicon.ico Cookie detected: comment_author_e118a5144ffaddbfed4914cc1c278eea
12:43:10 /favicon.ico Cookie detected: comment_author_email_e118a5144ffaddbfed4914cc1c278eea
12:43:10 /favicon.ico Cookie detected: wordpress_test_cookie
12:43:10 /favicon.ico Cookie detected: wordpress_logged_in_e118a5144ffaddbfed4914cc1c278eea
12:43:10 /favicon.ico In WP Cache Phase 2
21:43:10 /favicon.ico Setting up WordPress actions
21:43:10 /favicon.ico Created output buffer
21:43:10 /favicon.ico Cookie detected: comment_author_e118a5144ffaddbfed4914cc1c278eea
21:43:10 /favicon.ico Cookie detected: comment_author_email_e118a5144ffaddbfed4914cc1c278eea
21:43:10 /favicon.ico Cookie detected: wordpress_test_cookie
21:43:10 /favicon.ico Cookie detected: wordpress_logged_in_e118a5144ffaddbfed4914cc1c278eea
21:43:13 /favicon.ico Output buffer callback
21:43:13 /favicon.ico 404 file not found not cached

bask43 – the last message for / is “Output buffer callback” – do you have one of the “do not cache” checkboxes enabled on the admin page?
Do you have coarse file locking enabled?

Doncha,

I don’t have “do not cache” option enabled. Also, there are no files in the cache folder at all (the folder is writable). And I have 13 ob_start() functions in different plugins.

Unfortunately it could be one of those plugins that’s causing the problem. You could either disable them one by one and test when it works or look up the ob_start command in the php manual (use Google) and try to fix them yourself.

You could also edit wp-cache-phase2.php and find the “Output buffer callback” message and add more debugging around there and in the functions called from there to discover where the plugin is stopping.

I’m having a similar problem and it appears to have something to do with wp-cache-base.php in the supercache directory. I’ve been adding echos to various parts of the code to see how far everything executes, and nothing seems to happen after this line in wp-cache-phase1.php:
include( WPCACHEHOME . 'wp-cache-base.php');

wp-cache-base.php just declares a $known_headers array and the CacheMeta class if it doesn’t already exist. That’s it. Putting echos in this file seemed to fix the blank page problem for a while (!!!) so it’s hard to tell how this file is affecting things, if at all. Strange. Once a page was loaded properly after adding an echo, the file could be set back exactly as it was and everything worked fine. Again, for a while. I’m waiting for this to happen again.

grantp – are you using xcache, APC, Eaccelerator or another PHP compiler/optimizer? I’ve seen problems like this happen when the compiled PHP bytecode just refuses to work, sometimes. It is very hard to debug.

Same issue here. Occassionally a blank home page will be served up. Refreshing fixes but am now getting crawl errors from googleBot when a page it accesses is blank (I assume) I have deactivated the plugin for now and that has fixed the issue.

I’m having the same issue, but slightly different. I’ll occasionally get the white pages thing, but if I do nothing…the pages come back in about 4-5 minutes. It seems to be in correlation to the garbage collection and expired files timeframe. I’ll keep track, but every time the pages come back, it seems as though garbage collection and expired files were just cleaned a minute or two before. For now, I have set expiration to 86400 (a day). We’ll see how that works.

Coplan – is your site running on a slow disk like NFS? It shouldn’t matter because files are written to temporary files first, then renamed to the final filename. Should be super fast.

On the other hand, if it’s something to do with cache cleanup, are you seeing these files as a logged in user or anonymous user?