Decouple Xhprof profiling from profiling data processing

Decouple Xhprof profiling from profiling data processing

The motivation for this patch came from trying to use xhprof to profile the
unit tests. I was able to profile specific test suites, but if I tried to
profile a complete PHPUnit run, I ended up with empty profiling data. My
initial suspicion was that this was due to some Xhprof buffer getting
exhausted. The actual reason ended up being much simpler: the XhprofTest suite
indirectly called xhprof_enable() / xhprof_disable(), which stopped xhprof and
cleared out the data, so that when I was calling xhprof_disable() at the end of
the run, there was no profiling data to return, because xhprof was not running.

For the most part the XhprofTest was already doing the right thing by trying to
avoid having side-effects or relying on xhprof. Wherever possible, test fixture
profiling data was used in lieu of actually running xhprof. But this was not
totally successful because the Xhprof class coupled the collection of data to
the processing of data. Xhprof::__construct() called xhprof_enable(), so there
was no real way around that.

I think that the right way to fix that is to decouple profiling from profiling
data analysis. Thus I renamed 'Xhprof' to 'XhprofData', and modified the class
so that it expects to be fed profiling data rather than going out and
collecting it on its own. As a result, it is now possible to profile a full
phpunit run with xhprof, and the work that went into writing fixtures for the
Xhprof unit tests pays off: the class and the tests no longer have a hard
dependency on the xhprof extension, and the tests do not have to be skipped
when it is not installed. And the tests are really testing the system under
test, rather than the xhprof extension.

Finally, I added a new Xhprof class, which really is just an extremely thin
wrapper around xhprof_enable() / xhprof_disable(). The only extra functionality
it provides is the ability to check whether xhprof is running, via
Xhprof::isEnabled(). Calling Xhprof::enable() when it is already enabled will
cause an exception to be thrown. This should help us avoid running into
situations where two components contend for control of the profiler without
realizing it. A unit test tests this behavior.

The only part of this change that is not covered by tests is the change to
ProfilerXhprof. I tested it manually and it works.

Change-Id: Ica96beded68f04083abaf48ef1ae8c726eb60fa6