Add function to distinguish shared vs static linking at runtime

This is important because extensions can only be loaded correctly when
Cantera is linked as a shared library.

include/cantera/base/global.h

@@ -87,6 +87,10 @@ void loadExtension(const std::string& extType, const std::string& name);
 //! @since New in Cantera 3.0
 void loadExtensions(const AnyMap& node);
 
+//! Returns `true` if Cantera was loaded as a shared library in the current
+//! application. Returns `false` if it was statically linked.
+bool usingSharedLibrary();
+
 //! Delete and free all memory associated with the application
 /*!
  * Delete all global data.  It should be called at the end of the

src/SConscript

@@ -113,7 +113,8 @@ if env["python_package"] == "full":
 
 
 # build the Cantera static library
-lib = build(localenv.StaticLibrary('../lib/cantera', libraryTargets,
+staticIndicator = SharedObject('extensions/canteraStatic.cpp')
+lib = build(localenv.StaticLibrary('../lib/cantera', libraryTargets + staticIndicator,
                                    SPAWN=get_spawn(localenv)))
 localenv.Depends(lib, localenv['config_h_target'])
 install('$inst_libdir', lib)
@@ -154,6 +155,8 @@ if localenv['layout'] != 'debian':
     else:
         sharedName = '../lib/cantera'
 
+    sharedIndicator = SharedObject('extensions/canteraShared.cpp')
+
     if env['CC'] == 'cl':
         # For MSVC, use the static library to create a .def file listing all
         # symbols to export in the shared library.
@@ -164,12 +167,12 @@ if localenv['layout'] != 'debian':
                                    '/IGNORE:4102'])
 
     if localenv['versioned_shared_library']:
-        lib = build(localenv.SharedLibrary(sharedName, libraryTargets,
+        lib = build(localenv.SharedLibrary(sharedName, libraryTargets + sharedIndicator,
                                            SPAWN=get_spawn(localenv),
                                            SHLIBVERSION=localenv['cantera_pure_version']))
         install(localenv.InstallVersionedLib, '$inst_libdir', lib)
     else:
-        lib = build(localenv.SharedLibrary(sharedName, libraryTargets,
+        lib = build(localenv.SharedLibrary(sharedName, libraryTargets + sharedIndicator,
                                            SPAWN=get_spawn(localenv)))
         install('$inst_libdir', lib)
 

src/base/application.cpp

@@ -402,6 +402,11 @@ std::string Application::findInputFile(const std::string& name)
 
 void Application::loadExtension(const string& extType, const string& name)
 {
+    if (!usingSharedLibrary()) {
+        throw CanteraError("Application::loadExtension",
+            "Loading extensions requires linking to the Cantera shared library\n"
+            "rather than the static library");
+    }
     if (m_loaded_extensions.count({extType, name})) {
         return;
     }

src/extensions/canteraShared.cpp

@@ -0,0 +1,16 @@
+//! @file canteraShared.cpp
+
+// This file is part of Cantera. See License.txt in the top-level directory or
+// at https://cantera.org/license.txt for license and copyright information.
+
+namespace Cantera
+{
+
+bool usingSharedLibrary()
+{
+    // This implementation of usingSharedLibrary is compiled and embedded
+    // only in the Cantera shared library
+    return true;
+}
+
+}

src/extensions/canteraStatic.cpp

@@ -0,0 +1,16 @@
+//! @file canteraStatic.cpp
+
+// This file is part of Cantera. See License.txt in the top-level directory or
+// at https://cantera.org/license.txt for license and copyright information.
+
+namespace Cantera
+{
+
+bool usingSharedLibrary()
+{
+    // This implementation of usingSharedLibrary is compiled and embedded
+    // only in the Cantera static library
+    return false;
+}
+
+}