loader: Abstract and mock evmc_last_error_msg()

lib/loader/loader.c

@@ -19,12 +19,14 @@
 #define DLL_OPEN(filename) LoadLibrary(filename)
 #define DLL_CLOSE(handle) FreeLibrary(handle)
 #define DLL_GET_CREATE_FN(handle, name) (evmc_create_fn)(uintptr_t) GetProcAddress(handle, name)
+#define DLL_GET_ERROR_MSG() NULL
 #else
 #include <dlfcn.h>
 #define DLL_HANDLE void*
 #define DLL_OPEN(filename) dlopen(filename, RTLD_LAZY)
 #define DLL_CLOSE(handle) dlclose(handle)
 #define DLL_GET_CREATE_FN(handle, name) (evmc_create_fn)(uintptr_t) dlsym(handle, name)
+#define DLL_GET_ERROR_MSG() dlerror()
 #endif
 
 #define PATH_MAX_LENGTH 4096
@@ -69,11 +71,8 @@ evmc_create_fn evmc_load(const char* filename, enum evmc_loader_error_code* erro
     DLL_HANDLE handle = DLL_OPEN(filename);
     if (!handle)
     {
-#if !defined(EVMC_LOADER_MOCK) && !_WIN32
-        // If available, get the error message from dlerror().
-        last_error_msg = dlerror();
-#endif
-
+        // Get error message if available.
+        last_error_msg = DLL_GET_ERROR_MSG();
         ec = EVMC_LOADER_CANNOT_OPEN;
         goto exit;
     }

test/unittests/loader_mock.h

@@ -14,10 +14,14 @@ const char* evmc_test_library_path = NULL;
 const char* evmc_test_library_symbol = NULL;
 evmc_create_fn evmc_test_create_fn = NULL;
 
+static const char* evmc_test_last_error_msg = NULL;
+
 static int evmc_test_load_library(const char* filename)
 {
+    evmc_test_last_error_msg = NULL;
     if (filename && evmc_test_library_path && strcmp(filename, evmc_test_library_path) == 0)
         return magic_handle;
+    evmc_test_last_error_msg = "cannot load library";
     return 0;
 }
 
@@ -36,7 +40,16 @@ static evmc_create_fn evmc_test_get_symbol_address(int handle, const char* symbo
     return NULL;
 }
 
+static const char* evmc_test_get_last_error_msg()
+{
+    // Return the last error message only once.
+    const char* m = evmc_test_last_error_msg;
+    evmc_test_last_error_msg = NULL;
+    return m;
+}
+
 #define DLL_HANDLE int
 #define DLL_OPEN(filename) evmc_test_load_library(filename)
 #define DLL_CLOSE(handle) evmc_test_free_library(handle)
 #define DLL_GET_CREATE_FN(handle, name) evmc_test_get_symbol_address(handle, name)
+#define DLL_GET_ERROR_MSG() evmc_test_get_last_error_msg()

test/unittests/test_loader.cpp

@@ -137,7 +137,17 @@ TEST_F(loader, load_windows_path)
 
         evmc_loader_error_code ec;
         evmc_load(path, &ec);
-        EXPECT_EQ(ec, should_open ? EVMC_LOADER_SUCCESS : EVMC_LOADER_CANNOT_OPEN);
+        if (should_open)
+        {
+            EXPECT_EQ(ec, EVMC_LOADER_SUCCESS);
+            EXPECT_EQ(evmc_last_error_msg(), nullptr);
+        }
+        else
+        {
+            EXPECT_EQ(ec, EVMC_LOADER_CANNOT_OPEN);
+            EXPECT_STREQ(evmc_last_error_msg(), "cannot load library");
+            EXPECT_EQ(evmc_last_error_msg(), nullptr);
+        }
     }
 }