Multiple Xdebug dumps for single request

Question

I expect my server to make one Xdebug dump per request using the php.ini

xdebug.auto_trace=1
xdebug.collect_params=4
xdebug.trace_format=1
xdebug.collect_return=1
xdebug.collect_assignments=1
xdebug.trace_options=0
xdebug.trace_output_dir=/tmp/
xdebug.trace_output_name=xdebug.%R.%U
xdebug.var_display_max_data=16000

I also dump the unique id generated by Apache into a log file which shows:

"GET / HTTP/1.1" "XIZB4KSTHJac6j8l3z6SOwAAAAA"

So far so good - single log entry should mean a single log file. However, my Xdebug dump folder now has more than a single xdebug dump:

xdebug._.XIZB4KSTHJac6j8l3z6SOwAAAAA.0943bc.xt  xdebug._.XIZB4KSTHJac6j8l3z6SOwAAAAA.0e4362.xt
xdebug._.XIZB4KSTHJac6j8l3z6SOwAAAAA.0c6269.xt  xdebug._.XIZB4KSTHJac6j8l3z6SOwAAAAA.xt
xdebug._.XIZB4KSTHJac6j8l3z6SOwAAAAA.0363ec.xt  xdebug._.XIZB4KSTHJac6j8l3z6SOwAAAAA.0cb868.xt

Of which the one matching xdebug._.XIZB4KSTHJac6j8l3z6SOwAAAAA.xt seems to be the legit one based on the content. The other xdebugs are way smaller and do not seem to contain the core calls needed to process the request. However, they do seem related as they share the same id.

What are those extra xdebugs and how can I merge them into the proper xdebug (in case they are actually related to the same request)?

Posting the whole trace files would be a bit much and I generate trace files on code I did not write myself and can consequently not tell you any implementation specifics. However, I dumped below one of the short 'weird' trace files.

Version: 2.6.0
File format: 4
TRACE START [2019-01-08 19:53:33]
1       1119    0       0.056006        797816  MyBB->__destruct        1               /opt/bitnami/apps/mybb/htdocs/inc/class_core.php        0       0
2       1120    0       0.056014        797816  function_exists 0               /opt/bitnami/apps/mybb/htdocs/inc/class_core.php        590     1       'run_shutdown'
2       1120    1       0.056021        797816
2       1120    R                       TRUE
2       1121    0       0.056026        797816  run_shutdown    1               /opt/bitnami/apps/mybb/htdocs/inc/class_core.php        592     0
2       1121    1       0.056032        797816
1       1119    1       0.056035        797816
1       1119    R                       NULL
                        0.056350        38208
TRACE END   [2019-01-08 19:53:33]

Answer 1

From your sample trace and some researches around the topic I MIGHT found an answer to the behaviour you described:

As in https://stackoverflow.com/a/17995870/2833639 stated, shutdown functions run out of the "normal" request stack and debug_backtrace() from registered shutdown function in PHP tells us basically, that you cannot trace within a shutdown function.

Your sample trace tells, that the destructor method of an Object of the class "MyBB" was called.

My ASSUMTION here is that object destructors, like shutdown functions, are handled out of the "normal" stack so Xdebug creates different trace files for each seperate trace of the request (normal trace + x shutdown/destructor calls). Of course I found nothing in the docs about it, but due to the nature of object destruction this seems pretty plausible. Most likely these seperate traces happen after the response of the HTTP-request has taken place, but this depends on the specific implementation.

(I know that answers shouldn't be based on guesses or thougts but I felt that it could be usefull here, so feel free to prove me right or wrong on this point)