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

src/dll_load_error.cpp

@@ -0,0 +1,58 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+#if defined(_WIN32)
+
+#include <Windows.h>
+#include <dbghelp.h>
+#include <memory>
+#include <string>
+#pragma comment(lib, "dbghelp.lib")
+
+struct HMODULE_Deleter {
+  typedef HMODULE pointer;
+  void operator()(HMODULE h) { FreeLibrary(h); }
+};
+
+using ModulePtr = std::unique_ptr<HMODULE, HMODULE_Deleter>;
+
+std::string DetermineLoadLibraryError(const char* filename) {
+  std::string error("Error loading");
+
+  ModulePtr hModule;  // Here so that filename is valid until the next iteration
+  while (filename) {
+    error += std::string(" \"") + filename + "\"";
+
+    // 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.
+    hModule = ModulePtr{LoadLibraryEx(filename, NULL, DONT_RESOLVE_DLL_REFERENCES)};
+    if (!hModule) {
+      error += " which is missing. (Error " + std::to_string(GetLastError()) + ')';
+      return error;
+    }
+
+    // Get the address of the Import Directory
+    ULONG size{};
+    IMAGE_IMPORT_DESCRIPTOR* import_desc = reinterpret_cast<IMAGE_IMPORT_DESCRIPTOR*>(ImageDirectoryEntryToData(hModule.get(), FALSE, IMAGE_DIRECTORY_ENTRY_IMPORT, &size));
+    if (!import_desc) {
+      error += " 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 = nullptr;
+    for (; import_desc->Characteristics; import_desc++) {
+      char* dll_name = (char*)((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 = dll_name;
+        error += " which depends on";
+        break;
+      }
+    }
+  }
+  error += " But no dependency issue could be determined.";
+  return error;
+}
+
+#endif

src/dll_load_error.h

@@ -0,0 +1,3 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+std::string DetermineLoadLibraryError(const char* filename);

src/generators.cpp

@@ -30,6 +30,8 @@ std::string CurrentModulePath() {
 
   return out_path;
 }
+
+#include "dll_load_error.h"
 #endif
 
 void ThrowErrorIfSessionTerminated(bool is_session_terminated) {
@@ -132,7 +134,7 @@ struct LibraryHandle {
     auto path = CurrentModulePath() + filename;
     handle_ = LoadLibrary(path.c_str());
     if (!handle_)
-      throw std::runtime_error(std::string("Failed to load library: ") + path + " Error: " + std::to_string(GetLastError()));
+      throw std::runtime_error(std::string("Failed to load library: ") + DetermineLoadLibraryError(filename));
   };
 
   ~LibraryHandle() { FreeLibrary(handle_); }

src/models/model.h

@@ -26,7 +26,7 @@ struct State {
 
   void SetTerminate();
   void UnsetTerminate();
-  mutable bool session_terminated_{};
+  bool session_terminated_{};
   OrtValue* GetInput(const char* name);
 
   virtual void RewindTo(size_t index) { (void)index; };
@@ -135,9 +135,9 @@ struct Model : std::enable_shared_from_this<Model>, LeakChecked<Model>, External
   std::unique_ptr<Config> config_;
   std::unique_ptr<OrtSessionOptions> session_options_;
 
-  mutable DeviceInterface* p_device_{};          // The device we're running on (matches device_type_) used for things that work the same on all devices
-  mutable DeviceInterface* p_device_inputs_{};   // For some model inputs, the device might be the CPU device (all but KV cache currently for WebGPU and DML)
-  mutable DeviceInterface* p_device_kvcache_{};  // The kvcache is always allocated in device memory  (TODO: Remove in favor of just p_device_?)
+  DeviceInterface* p_device_{};          // The device we're running on (matches device_type_) used for things that work the same on all devices
+  DeviceInterface* p_device_inputs_{};   // For some model inputs, the device might be the CPU device (all but KV cache currently for WebGPU and DML)
+  DeviceInterface* p_device_kvcache_{};  // The kvcache is always allocated in device memory  (TODO: Remove in favor of just p_device_?)
 
   Ort::Allocator& allocator_cpu_{GetDeviceInterface(DeviceType::CPU)->GetAllocator()};