MB-45378: Fix static init fiasco with ExecutorPool & phosphor

When converting platform to be statically linked, a crash is seen
during shutdown of ep-engine_ep_unit_tests.DcpConnMapTest tests on
macOS:

(lldb) bt
* thread #22, name = 'NonIoPool2', stop reason = signal SIGABRT
  * frame #0: 0x00007fff693f733a libsystem_kernel.dylib` __pthread_kill  + 10
    frame #1: 0x00007fff694b3e60 libsystem_pthread.dylib` pthread_kill  + 430
    frame #2: 0x00007fff6937e808 libsystem_c.dylib` abort  + 120
    frame #3: 0x00007fff665dd458 libc++abi.dylib` abort_message  + 231
    frame #4: 0x00007fff665ce8a7 libc++abi.dylib` demangling_terminate_handler()  + 238
    frame #5: 0x00007fff681095b1 libobjc.A.dylib` _objc_terminate()  + 104
    frame #6: 0x00007fff665dc887 libc++abi.dylib` std::__terminate(void (*)())  + 8
    frame #7: 0x00007fff665df1a2 libc++abi.dylib` __cxxabiv1::failed_throw(__cxxabiv1::__cxa_exception*)  + 27
    frame #8: 0x00007fff665df169 libc++abi.dylib` __cxa_throw  + 113
    frame #9: 0x00007fff665b955b libc++.1.dylib` std::__1::__throw_system_error(int, char const*)  + 77
    frame #10: 0x00007fff665b054d libc++.1.dylib` std::__1::mutex::lock()  + 29
    frame #11: 0x000000010a78af00 libphosphor.dylib` phosphor::TraceLog::lock(this=0x000000010a797b30)  + 16 at trace_log.h:250
    frame #12: 0x000000010a78aedf libphosphor.dylib` std::__1::lock_guard<phosphor::TraceLog>::lock_guard(this=0x000070000b00be98, __m=0x000000010a797b30)  + 15 at __mutex_base:91
    frame #13: 0x000000010a787c49 libphosphor.dylib` std::__1::lock_guard<phosphor::TraceLog>::lock_guard(this=0x000070000b00be98, __m=0x000000010a797b30)  + 9 at __mutex_base:91
    frame #14: 0x000000010a788a2c libphosphor.dylib` phosphor::TraceLog::deregisterThread(this=0x000000010a797b30)  + 28 at trace_log.cc:222
    frame #15: 0x000000010023ec6d ep-engine_ep_unit_tests` CBRegisteredThreadFactory::newThread(this=0x000000010ddc3c00)>&&)::'lambda'()::operator()()  + 93 at folly_executorpool.cc:49

The ExecutorPool is shutting down all its background threads, during
which each thread calls phosphor::TraceLog::deregisterThread() to
remove this thread from the set folly is tracking. However TraceLog is
a singleton and it has already been destructed, so accessing it's
mutex member variable results in an exception being thrown.

This is due to a change in the static initialisation (and
deinitialization) order between ExecutorPool and
phosphor::TraceLog. Both are Mayer singletons, but phosphor::TraceLog
is first accessed (and hence initialised) _after_ ExecutorPool - when
the ExecutorPool threads first register their threads. As such,
TraceLog will be destroyed before ExecutorPool (destruction is in
reverse construction order).

Solve by explicilty accessing (and hence initializing) TraceLog before
ExecutorPool is created in ep_unit_tests_main.cc.

(Note this problem doesn't occur in memcached as we explicilty
initialise tracing before any buckets are created.)

Change-Id: I1953129cce0d05a42f0790724c470e38b2dd0701
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/152326
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Paolo Cocchi <paolo.cocchi@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@couchbase.com>

Author: daverigby

engines/ep/tests/module_tests/ep_unit_tests_main.cc

@@ -26,6 +26,7 @@
 #include <memcached/config_parser.h>
 #include <memcached/server_core_iface.h>
 #include <memcached/server_log_iface.h>
+#include <phosphor/phosphor.h>
 #include <platform/cb_arena_malloc.h>
 #include <platform/cbassert.h>
 
@@ -163,6 +164,14 @@ int main(int argc, char **argv) {
         env.engineFileDescriptors = env.reservedFileDescriptors * 2;
     }
 
+    // Ensure phosphor TraceLog singleton is initialised before we run any
+    // tests - specifically before we create the ExecutorPool singleton and
+    // its background threads. If TraceLog is *not* initialised before
+    // ExecutorPool, then it will also be destroyed before ExecutorPool; which
+    // then results in ExecutorPool crashing when it attempts to unregister
+    // worker threads from phosphor.
+    phosphor::TraceLog::getInstance();
+
     auto ret = RUN_ALL_TESTS();
 
     globalBucketLogger.reset();