[core] Export CppInterOp's dispatch entry point from libCore

core/base/src/TROOT.cxx

@@ -2320,6 +2320,25 @@ void TROOT::InitInterpreter()
    fInterpreter->Initialize();
 }
 
+////////////////////////////////////////////////////////////////////////////////
+/// Return the CppInterOp API function `funcName` of ROOT's interpreter, starting
+/// the interpreter if needed. libCling implements this lookup but is loaded
+/// RTLD_LOCAL; exporting it from libCore lets a CppInterOp client such as cppjit
+/// pass libCore to Cpp::LoadDispatchAPI() and share ROOT's interpreter instead of
+/// loading its own LLVM.
+
+extern "C" void (*CppGetProcAddress(const char *funcName))()
+{
+   using GetProcAddress_t = void (*(const char *))();
+   static GetProcAddress_t *getProcAddress = [] {
+      TInterpreter::Instance(); // Sets gInterpreterLib.
+      auto f = reinterpret_cast<GetProcAddress_t *>(dlsym(gInterpreterLib, "CppGetProcAddress"));
+      // In rootcling gInterpreterLib is RTLD_DEFAULT, which may find this function.
+      return f == &CppGetProcAddress ? nullptr : f;
+   }();
+   return getProcAddress ? getProcAddress(funcName) : nullptr;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Helper function used by TClass::GetClass().
 /// This function attempts to load the dictionary for 'classname'

core/metacling/test/CMakeLists.txt

@@ -32,6 +32,12 @@ ROOT_ADD_GTEST(
 
 ROOT_ADD_GTEST(TClingLoadUnloadFile TClingLoadUnloadFileTests.cxx LIBRARIES Core RIO)
 
+if(NOT MSVC)
+  ROOT_ADD_GTEST(TClingDispatch TClingDispatchTests.cxx LIBRARIES Core ${CMAKE_DL_LIBS})
+  target_compile_definitions(TClingDispatch PRIVATE ROOT_LIBCORE="$<TARGET_FILE:Core>"
+                                                    ROOT_LIBCLING="$<TARGET_FILE:Cling>")
+endif()
+
 if(pyroot)
   ROOT_ADD_PYUNITTEST(MetaClingTests MetaClingTests.py)
 endif()

core/metacling/test/TClingDispatchTests.cxx

@@ -0,0 +1,30 @@
+#include "TInterpreter.h"
+
+#include "gtest/gtest.h"
+
+#include <dlfcn.h>
+
+using GetProcAddress_t = void (*(const char *))();
+
+// A CppInterOp client such as cppjit loads the library that exports
+// CppGetProcAddress and looks the API up by name. Through libCore it must get
+// the functions of ROOT's interpreter.
+TEST(TClingDispatch, CppGetProcAddressFromLibCore)
+{
+   void *libCore = dlopen(ROOT_LIBCORE, RTLD_LAZY | RTLD_LOCAL);
+   ASSERT_NE(libCore, nullptr) << dlerror();
+   auto getProcAddress = reinterpret_cast<GetProcAddress_t *>(dlsym(libCore, "CppGetProcAddress"));
+   ASSERT_NE(getProcAddress, nullptr) << dlerror();
+
+   auto process = reinterpret_cast<int (*)(const char *)>(getProcAddress("Process"));
+   ASSERT_NE(process, nullptr);
+   EXPECT_EQ(process("int TClingDispatchValue = 42;"), 0);
+   EXPECT_EQ(gInterpreter->Calc("TClingDispatchValue"), 42);
+   EXPECT_EQ(getProcAddress("NoSuchFunction"), nullptr);
+
+   void *libCling = dlopen(ROOT_LIBCLING, RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
+   ASSERT_NE(libCling, nullptr) << "libCling should be loaded once, by ROOT";
+   auto fromLibCling = reinterpret_cast<GetProcAddress_t *>(dlsym(libCling, "CppGetProcAddress"));
+   ASSERT_NE(fromLibCling, nullptr) << dlerror();
+   EXPECT_EQ(getProcAddress("Process"), fromLibCling("Process"));
+}