Hierarchical CPU profiling for MongoDB

One day it will be easy to get hierarchical CPU profile results for open-source databases using open-source profiling tools. Support for CPU profiling via Google perftools can be compiled into MongoDB via the --use-cpu-profiler option. Given the use of a compiler toolchain in a nonstandard location I also used --extrapath and --extralib to help it find libunwind. However, the profiler output file was mostly empty after doing this and did not have any per-thread results.

For future reference, profiling was started and stopped via:

echo "db.runCommand({ _cpuProfilerStart: { profileFilename: '/path/to/output' } })" | bin/mongo admin
echo "db.runCommand({ _cpuProfilerStop:1})" | bin/mongo admin

Eventually I remembered that it might help to add a call to ProfilerRegisterThread() at the start of each new thread. I did something similar in the Google patch for MySQL many years ago. And now I have hierarchical CPU profiles to help understand the source of performance regressions in MongoDB 2.6. I updated JIRA 6628 with details from this and then was asked to create JIRA 13683. The change is to handleIncomingMsg():
        static void* handleIncomingMsg(void* arg) {
            TicketHolderReleaser connTicketReleaser( &Listener::globalTicketHolder );
            ::ProfilerRegisterThread(); // Add this