Allow the comphelper threadpool to be reset after construction.

Otherwise some pre-init components can start it, and threads get
stranded in the forkit process causing grief.

Change-Id: Ib1846f8b329b4c6b84645999dafba1252c5129c7
Reviewed-on: https://gerrit.libreoffice.org/57631
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

comphelper/source/misc/threadpool.cxx

@@ -128,13 +128,12 @@ sal_Int32 ThreadPool::getPreferredConcurrency()
     return ThreadCount;
 }
 
-// FIXME: there should be no need for this as/when our baseline
-// is >VS2015 and drop WinXP; the sorry details are here:
-// https://connect.microsoft.com/VisualStudio/feedback/details/1282596
+// Used to order shutdown, and to ensure there are no lingering
+// threads after LibreOfficeKit pre-init.
 void ThreadPool::shutdown()
 {
-    if (mbTerminate)
-        return;
+//    if (mbTerminate)
+//        return;
 
     std::unique_lock< std::mutex > aGuard( maMutex );
     shutdownLocked(aGuard);

desktop/source/lib/init.cxx

@@ -36,6 +36,7 @@
 #include <comphelper/string.hxx>
 #include <comphelper/propertysequence.hxx>
 #include <comphelper/scopeguard.hxx>
+#include <comphelper/threadpool.hxx>
 
 #include <com/sun/star/beans/XPropertySet.hpp>
 #include <com/sun/star/container/XNameAccess.hpp>
@@ -3968,7 +3969,10 @@ static int lo_initialize(LibreOfficeKit* pThis, const char* pAppPath, const char
     }
 
     if (eStage == PRE_INIT)
+    {
+        comphelper::ThreadPool::getSharedOptimalPool().shutdown();
         rtl_alloc_preInit(rtlAllocPostInit);
+    }
 
     return bInitialized;
 }