tdf#114815 pyuno: avoid 2 threads initing python in parallel

According to the crash reports, it's possible for the grammar checking
thread to call GetGrammarChecker, instantiating lightproof, at the same
time as the main thread instantiates LngSvcMgr, which also instantiates
(some?) (all?) grammar checkers.

Ensure that pyuno_loader::CreateInstance() initialises Python only
once with a C++11 thread safe static.

Change-Id: I5b1faba9107355c508831a078366e4a29fdbfadf
(cherry picked from commit 5357ca82)
Reviewed-on: https://gerrit.libreoffice.org/49115Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

pyuno/source/loader/pyuno_loader.cxx

@@ -176,11 +176,10 @@ static void prependPythonPath( const OUString & pythonPathBootstrap )
     osl_setEnvironment(envVar.pData, envValue.pData);
 }
 
-Reference< XInterface > CreateInstance( const Reference< XComponentContext > & ctx )
+struct PythonInit
 {
-    Reference< XInterface > ret;
-
-    if( ! Py_IsInitialized() )
+PythonInit() {
+    if (! Py_IsInitialized()) // may be inited by getComponentContext() already
     {
         OUString pythonPath;
         OUString pythonHome;
@@ -236,9 +235,22 @@ Reference< XInterface > CreateInstance( const Reference< XComponentContext > & c
         // PyThreadAttach below.
         PyThreadState_Delete(tstate);
     }
+}
+};
+
+Reference<XInterface> CreateInstance(const Reference<XComponentContext> & ctx)
+{
+    // tdf#114815 thread-safe static to init python only once
+    static PythonInit s_Init;
+
+    Reference< XInterface > ret;
 
     PyThreadAttach attach( PyInterpreterState_Head() );
     {
+        // note: this can't race against getComponentContext() because
+        // either (in soffice.bin) CreateInstance() must be called before
+        // getComponentContext() can be called, or (in python.bin) the other
+        // way around
         if( ! Runtime::isInitialized() )
         {
             Runtime::initialize( ctx );