ob_end_flush(): failed to delete and flush buffer. No buffer to delete or flush

[adrianbj]

I get this error when PHP's output_buffering is set to 0, which seems to be quite common on shared hosts.

I think it is likely related to this: http://php.net/manual/en/function.ob-get-level.php#109001 and the fact that ob_get_level returns one at the beginning of the script.

The error comes from this line: https://github.com/nette/tracy/blob/master/src/Tracy/Debugger.php#L436

Let me know if you have any trouble reproducing - it may only be occurring because I am manually calling Debugger::shutdownHandler() from another script.

[dg]

There is @, so it should not produce rror.

[adrianbj]

Sorry for the delay in replying - I was on vacation last week.

It doesn't produce the error with the @, but the problem is that it still results in a blank page being returned. I can fix it by replacing:

while (ob_get_level() > self::$obLevel) {

with

while (ob_get_level() > 1) {

here: https://github.com/nette/tracy/blob/master/src/Tracy/Debugger.php#L430

I am not sure if that is the correct/appropriate solution, but it is the only way I can seem to prevent a blank screen from being returned in certain situations.

Is there any other info I can provide?

[dg]

Can you describe in which situations is returned blank screen? Because setting output_buffering = 0 is fine.

[adrianbj]

Ok, I have tracked it down (I think), although it may be a little tricky for you to replicate. The problem only occurs when I manually call Debugger::shutdownHandler() which I have do for a specific situation - can explain more if you want. Now the other thing required to cause the blank screen is to do a header location redirect and call exit on the line after the header call. If I remove that exit, then it works fine.

Does that give you enough insight to work from? If not, let me know and I'll set up a site so you can test.

Thanks!

[adrianbj]

@dg - would you consider adding:

if(!ob_get_level()) ob_start();

just before this line:

tracy/src/Tracy/Debugger.php

Line 149 in 577c63b

self::$obLevel = ob_get_level();

This is in line with what is suggested here: http://php.net/manual/en/function.ob-get-level.php#109001

It seems to fix everything at my end. Can you see any possible side-effects?

[dg]

ob_start() is not good idea, because it blocks server output until connection is closed.

Simply, I don't understand where is problem, so I am unable to solve it.

[adrianbj]

ob_start() is not good idea, because it blocks server output until connection is closed.

Good point!

OK, I'll try to explain in more detail. It all relates back to this issue: #185

In my last comment you'll see that I am now calling Debugger::shutdownHandler() before my script shuts down so I can get the AJAX bar to work. Without that, the AJAX bar doesn't show. The side-effect of calling Debugger::shutdownHandler() however is the problem described in the issue where I get a blank page because of the Debugger::removeOutputBuffers method.

Maybe manually calling Debugger::shutdownHandler() messes with the integrity of self::$obLevel?

I do know that if I add 1 to the value of self::$obLevel then everything works just fine.

[dg]

shutdownHandler() should flush all buffers created after Tracy is enabled. So ob_get_level() should return the same value before Tracy\Debugger::enable(), after enable(), before shutdownHandler() (*) and after shutdownHandler(). Can you check it?

*) before shutdownHandler() it can be greater, but after that and when you call shutdownHandler() two times, value should be the same as before enable().

[adrianbj]

I am getting:

before enable: 0
after enable: 0
before shutdown: 1
after shutdown: 0

[dg]

It seems ok. Can you dump ob_get_status(TRUE) before manual shutdown?

[adrianbj]

    [0] => Array
        (
            [name] => default output handler
            [type] => 0
            [flags] => 112
            [level] => 0
            [chunk_size] => 0
            [buffer_size] => 16384
            [buffer_used] => 0
        )

[dg]

So after enable() you code starts an output buffer, shutdownHandler() flushes and closes it (via ob_end_flush()).

This should not produce ob_end_flush(): failed to delete and flush buffer, because buffer existed and was closed.

At the same time, it should not result in a blank page, because it is flushed.

So I'm stuck :-)

For what purpose do you use the buffer? Page cache?

You can try to render Bar this way instead of calling shutdownHandler:

Debugger::getBar()->render();
Debugger::$showBar = FALSE;

[adrianbj]

For what purpose do you use the buffer? Page cache?

I believe the issue is starting here: https://github.com/processwire/processwire/blob/50f2834a0ae2a9f9f2ac53439184ebdc04db2f54/wire/core/admin.php#L89

Removing that ob_start() does fix things, but it also seems like your solution of:

Debugger::getBar()->render();
Debugger::$showBar = FALSE;

instead of calling the shutdown handler also works, so thank you very much for that!

Just an FYI - as I think I mentioned in that other issue, all of this is only problem when using that SessionHandlerDB module: https://github.com/processwire/processwire/blob/50f2834a0ae2a9f9f2ac53439184ebdc04db2f54/wire/modules/Session/SessionHandlerDB/SessionHandlerDB.module

[adrianbj]

Sorry to bring this up again @dg - maybe I should have started a new issue, but since it only seems to affect me, I thought it better to add here. I just noticed that even though:

Debugger::getBar()->render();
Debugger::$showBar = FALSE;

gets the AJAX bar working, it doesn't get the Redirect working. Again this is only an issue with that SessionHandlerDB module.

It might be good to note that I am having to add:

if(ob_get_level() == 0) ob_start();

before those two lines, otherwise php header redirects don't work at all unless php.ini has output_buffering on by default.

Any thoughts on what might be preventing the Redirect bar from working?

Here is what a dump of $_SESSION['_tracy'] looks like:

with SessionHandlerDB off:

and with it on:

These are on exactly the same page after the same redirect. I don't know why the redirect index is NULL and the bar index has three items.

I don't want to take up anymore of your time on this, but if you have any quick ideas of why it might be happening, that would be greatly appreciated.

Thanks!