loader: Expose DLL load errors with evmc_last_error_msg()

Currently only works for errors by dlopen() on Linux and macos, otherwise returns NULL.

bindings/go/evmc/evmc.go

@@ -158,7 +158,13 @@ func Load(filename string) (instance *Instance, err error) {
 	case C.EVMC_LOADER_SUCCESS:
 		instance = &Instance{handle}
 	case C.EVMC_LOADER_CANNOT_OPEN:
-		err = fmt.Errorf("evmc loader: cannot open %s", filename)
+		optionalErrMsg := C.evmc_last_error_msg()
+		if optionalErrMsg != nil {
+			msg := C.GoString(optionalErrMsg)
+			err = fmt.Errorf("evmc loader: %s", msg)
+		} else {
+			err = fmt.Errorf("evmc loader: cannot open %s", filename)
+		}
 	case C.EVMC_LOADER_SYMBOL_NOT_FOUND:
 		err = fmt.Errorf("evmc loader: the EVMC create function not found in %s", filename)
 	case C.EVMC_LOADER_INVALID_ARGUMENT:

include/evmc/loader.h

@@ -1,6 +1,6 @@
 /* EVMC: Ethereum Client-VM Connector API.
- * Copyright 2018 The EVMC Authors.
- * Licensed under the Apache License, Version 2.0. See the LICENSE file.
+ * Copyright 2019 The EVMC Authors.
+ * Licensed under the Apache License, Version 2.0.
  */
 
 /**
@@ -103,6 +103,19 @@ evmc_create_fn evmc_load(const char* filename, enum evmc_loader_error_code* erro
 struct evmc_instance* evmc_load_and_create(const char* filename,
                                            enum evmc_loader_error_code* error_code);
 
+/**
+ * Returns the human-readable message describing the most recent error
+ * that occurred in EVMC loading.
+ *
+ * In case any loading function returned ::EVMC_LOADER_SUCCESS this function always returns NULL.
+ * In case of error code other than success returned, this function MAY return the error message.
+ * This function is not thread-safe.
+ *
+ * @return Error message or NULL if no additional information is available.
+ *         The returned pointer MUST NOT be freed by the caller.
+ */
+const char* evmc_last_error_msg();
+
 #if __cplusplus
 }
 #endif

lib/loader/loader.c

@@ -44,9 +44,12 @@ static void strcpy_s(char* dest, size_t destsz, const char* src)
 }
 #endif
 
+static const char* last_error_msg = NULL;
+
 
 evmc_create_fn evmc_load(const char* filename, enum evmc_loader_error_code* error_code)
 {
+    last_error_msg = NULL;  // Reset last error.
     enum evmc_loader_error_code ec = EVMC_LOADER_SUCCESS;
     evmc_create_fn create_fn = NULL;
 
@@ -66,6 +69,11 @@ 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
+
         ec = EVMC_LOADER_CANNOT_OPEN;
         goto exit;
     }
@@ -130,9 +138,15 @@ evmc_create_fn evmc_load(const char* filename, enum evmc_loader_error_code* erro
     return create_fn;
 }
 
+const char* evmc_last_error_msg()
+{
+    return last_error_msg;
+}
+
 struct evmc_instance* evmc_load_and_create(const char* filename,
                                            enum evmc_loader_error_code* error_code)
 {
+    // First load the DLL. This also resets the last_error_msg;
     evmc_create_fn create_fn = evmc_load(filename, error_code);
 
     if (!create_fn)

test/vmtester/CMakeLists.txt

@@ -18,7 +18,7 @@ add_test(NAME vmtester/help COMMAND evmc-vmtester --version --help)
 set_tests_properties(vmtester/help PROPERTIES PASS_REGULAR_EXPRESSION "Usage:")
 
 add_test(NAME vmtester/nonexistingvm COMMAND evmc-vmtester nonexistingvm)
-set_tests_properties(vmtester/nonexistingvm PROPERTIES PASS_REGULAR_EXPRESSION "Cannot open")
+set_tests_properties(vmtester/nonexistingvm PROPERTIES PASS_REGULAR_EXPRESSION "[Cc]annot open")
 
 add_test(NAME vmtester/noarg COMMAND evmc-vmtester)
 set_tests_properties(vmtester/noarg PROPERTIES PASS_REGULAR_EXPRESSION "is required")

test/vmtester/vmtester.cpp

@@ -140,8 +140,14 @@ int main(int argc, char* argv[])
         case EVMC_LOADER_SUCCESS:
             break;
         case EVMC_LOADER_CANNOT_OPEN:
-            std::cerr << "Cannot open " << evmc_module << "\n";
+        {
+            const auto error = evmc_last_error_msg();
+            if (error)
+                std::cerr << error << "\n";
+            else
+                std::cerr << "Cannot open " << evmc_module << "\n";
             return static_cast<int>(ec);
+        }
         case EVMC_LOADER_SYMBOL_NOT_FOUND:
             std::cerr << "EVMC create function not found in " << evmc_module << "\n";
             return static_cast<int>(ec);