Windows: Show more useful DLL load errors to say exactly what DLL is missing

onnxruntime/core/platform/windows/dll_load_error.cc

@@ -0,0 +1,61 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#include <windows.h>
+#include <dbghelp.h>
+#pragma comment(lib, "dbghelp.lib")
+#include <iostream>
+#include <memory>
+#include <string>
+#include "dll_load_error.h"
+
+struct HMODULE_Deleter {
+  typedef HMODULE pointer;
+  void operator()(HMODULE h) { FreeLibrary(h); }
+};
+
+using ModulePtr = std::unique_ptr<HMODULE, HMODULE_Deleter>;
+
+// If a DLL fails to load, this will try loading the DLL and then its dependencies recursively
+// until it finds a missing file, then will report which file is missing and what the dependency
+// chain is.
+std::wstring DetermineLoadLibraryError(const wchar_t* filename_in, DWORD flags) {
+  std::wstring error(L"Error loading");
+
+  std::wstring filename{filename_in};
+  while (filename.size()) {
+    error += std::wstring(L" \"") + filename + L"\"";
+
+    // We use DONT_RESOLVE_DLL_REFERENCES instead of LOAD_LIBRARY_AS_DATAFILE because the latter will not process the import table
+    // and will result in the IMAGE_IMPORT_DESCRIPTOR table names being uninitialized.
+    ModulePtr hModule = ModulePtr{LoadLibraryExW(filename.c_str(), NULL, flags | DONT_RESOLVE_DLL_REFERENCES)};
+    if (!hModule) {
+      error += L" which is missing.";
+      return error;
+    }
+
+    // Get the address of the Import Directory
+    ULONG size{};
+    IMAGE_IMPORT_DESCRIPTOR* import_desc = reinterpret_cast<IMAGE_IMPORT_DESCRIPTOR*>(ImageDirectoryEntryToData(hModule.get(), TRUE, IMAGE_DIRECTORY_ENTRY_IMPORT, &size));
+    if (!import_desc) {
+      error += L" No import directory found.";  // This is unexpected, and I'm not sure how it could happen but we handle it just in case.
+      return error;
+    }
+
+    // Iterate through the import descriptors to see which dependent DLL can't load
+    filename.clear();
+    flags = 0;  // Dependent libraries are relative, and flags like LOAD_WITH_ALTERED_SEARCH_PATH is undefined for those.
+    for (; import_desc->Characteristics; import_desc++) {
+      const char* dll_name = reinterpret_cast<const char*>(reinterpret_cast<const BYTE*>(hModule.get()) + import_desc->Name);
+      // Try to load the dependent DLL, and if it fails, we loop again with this as the DLL and we'll be one step closer to the missing file.
+      ModulePtr hDepModule{LoadLibrary(dll_name)};
+      if (!hDepModule) {
+        filename = std::wstring(dll_name, dll_name + strlen(dll_name));
+        error += L" which depends on";
+        break;
+      }
+    }
+  }
+  error += L" But no dependency issue could be determined.";
+  return error;
+}

onnxruntime/core/platform/windows/dll_load_error.h

@@ -0,0 +1 @@
+std::wstring DetermineLoadLibraryError(const wchar_t* filename, DWORD flags);

onnxruntime/core/platform/windows/env.cc

@@ -39,6 +39,7 @@ limitations under the License.
 #include <wil/Resource.h>
 
 #include "core/platform/path_lib.h"  // for LoopDir()
+#include "core/platform/windows/dll_load_error.h"
 
 EXTERN_C IMAGE_DOS_HEADER __ImageBase;
 
@@ -704,17 +705,18 @@ Status WindowsEnv::LoadDynamicLibrary(const PathString& wlibrary_filename, bool
     static constexpr DWORD bufferLength = 64 * 1024;
     std::wstring s(bufferLength, '\0');
     FormatMessageW(
-        FORMAT_MESSAGE_FROM_SYSTEM |
-            FORMAT_MESSAGE_IGNORE_INSERTS,
+        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
         NULL,
         error_code,
         MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
         (LPWSTR)s.data(),
-        0, NULL);
+        bufferLength, NULL);
+    s.erase(std::remove(s.begin(), s.end(), L'\r'), s.end());
+    s.erase(std::remove(s.begin(), s.end(), L'\n'), s.end());
     std::wostringstream oss;
-    oss << L"LoadLibrary failed with error " << error_code << L" \"" << s.c_str() << L"\" when trying to load \"" << wlibrary_filename << L"\"";
+    oss << DetermineLoadLibraryError(wlibrary_filename.c_str(), LOAD_WITH_ALTERED_SEARCH_PATH)
+        << L" (Error " << error_code << ": \"" << s.c_str() << "\")";
     std::wstring errmsg = oss.str();
-    // TODO: trim the ending '\r' and/or '\n'
     common::Status status(common::ONNXRUNTIME, common::FAIL, ToUTF8String(errmsg));
     return status;
   }