PERL_SET_CONTEXT(t) needs to set PL_current_context from C++ too

We want to use C11 thread local storage where possible, as its read overhead
is lower. However C++ uses incompatible syntax for TLS, so we have to hide
the variable from C++ extensions, and have the read code "fall back" to the
pthread_getspecific() approach.

However, C++ extensions also need to be able to call PERL_SET_CONTEXT(t) and
have everything still work the same as C extensions. Hence they *also* need
to set PL_current_context - a variable that has to be hidden from them.
Hence change the version of the PERL_SET_CONTEXT macro used by C++
extensions from inline code to a call to Perl_set_context(), and have that
set PL_current_context (when used).

Technically this commit also fixes a bug in that Perl_set_context() is API
and should have been setting PL_current_context since that variable was
first introduced. In practice, likely nothing uses this API, as there is no
code on CPAN that calls it, and the macro PERL_SET_CONTEXT() is much more
prominent.

thread.h

@@ -387,7 +387,9 @@ extern PERL_THREAD_LOCAL void *PL_current_context;
 
 #  define PERL_GET_CONTEXT        PL_current_context
 
-/* Set our thread-specific value anyway, in case code is reading it directly. */
+/* We must also call pthread_setspecific() always, as C++ code has to read it
+ * with pthreads (the #else side just below) */
+
 #  define PERL_SET_CONTEXT(t)                                           \
     STMT_START {                                                        \
         int _eC_;                                                       \
@@ -403,14 +405,14 @@ extern PERL_THREAD_LOCAL void *PL_current_context;
 #    define PERL_GET_CONTEXT	PTHREAD_GETSPECIFIC(PL_thr_key)
 #  endif
 
+/* For C++ extensions built on a system where the C compiler provides thread
+ * local storage that call PERL_SET_CONTEXT() also need to set
+ * PL_current_context, so need to call into C code to do this.
+ * To avoid exploding code complexity, do this also on C platforms that don't
+ * support thread local storage. PERL_SET_CONTEXT is not called that often. */
+
 #  ifndef PERL_SET_CONTEXT
-#    define PERL_SET_CONTEXT(t) \
-    STMT_START {						\
-        int _eC_;						\
-        if ((_eC_ = pthread_setspecific(PL_thr_key, (void *)(t))))	\
-            Perl_croak_nocontext("panic: pthread_setspecific (%d) [%s:%d]",	\
-                                 _eC_, __FILE__, __LINE__);	\
-    } STMT_END
+#    define PERL_SET_CONTEXT(t) Perl_set_context((void*)t)
 #  endif /* PERL_SET_CONTEXT */
 #endif /* PERL_THREAD_LOCAL */
 

util.c

@@ -3762,13 +3762,18 @@ Perl_get_context(void)
 void
 Perl_set_context(void *t)
 {
-#if defined(USE_ITHREADS)
-#endif
     PERL_ARGS_ASSERT_SET_CONTEXT;
 #if defined(USE_ITHREADS)
+#  ifdef PERL_USE_THREAD_LOCAL
+    PL_current_context = t;
+#  endif
 #  ifdef I_MACH_CTHREADS
     cthread_set_data(cthread_self(), t);
 #  else
+    /* We set thread-specific value always, as C++ code has to read it with
+     * pthreads, beacuse the declaration syntax for thread local storage for C11
+     * is incompatible with C++, meaning that we can't expose the thread local
+     * variable to C++ code. */
     {
         const int error = pthread_setspecific(PL_thr_key, t);
         if (error)