Improve exception messages when Windows fails to load an extension module

Src/IronPython.Modules/_ctypes/_ctypes.cs

@@ -164,6 +164,8 @@ public static int CopyComPointer(object src, object dest) {
             throw new NotImplementedException("CopyComPointer");
         }
 
+#nullable enable
+
         public static string FormatError() {
             return FormatError(get_last_error());
         }
@@ -172,6 +174,15 @@ public static string FormatError(int errorCode) {
             return new Win32Exception(errorCode).Message;
         }
 
+        private static string FormatError(int errorCode, string? fileName) {
+            string msg = FormatError(errorCode);
+            // error codes: https://docs.microsoft.com/en-us/windows/win32/debug/system-error-codes
+            if (errorCode is not (< 0 or >= 8200 or 34 or 106 or 317 or 718)) {
+                msg = msg.Replace("%1", $"'{fileName}'");
+            }
+            return msg;
+        }
+
         [SupportedOSPlatform("windows"), PythonHidden(PlatformsAttribute.PlatformFamily.Unix)]
         public static void FreeLibrary(int handle) {
             FreeLibrary(new IntPtr(handle));
@@ -187,13 +198,17 @@ public static void FreeLibrary(IntPtr handle) {
             NativeFunctions.FreeLibrary(handle);
         }
 
-#nullable enable
-
         private static object LoadDLL(string? library, int mode) {
             if (library is not null && library.IndexOf((char)0) != -1) throw PythonOps.ValueError("embedded null byte");
             IntPtr res = NativeFunctions.LoadDLL(library, mode);
             if (res == IntPtr.Zero) {
-                throw PythonOps.OSError($"cannot load library {library}");
+                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows)) {
+                    int code = NativeFunctions.GetLastError();
+                    string msg = FormatError(code, library);
+                    throw PythonExceptions.CreateThrowable(PythonExceptions.OSError, 0, msg, null, code);
+                }
+
+                throw PythonOps.OSError("cannot load library '{0}'", library);
             }
 
             return res.ToPython();

Tests/modules/type_related/test_ctypes.py

@@ -10,8 +10,9 @@
 from array import array
 import sys
 import gc
+import unittest
 
-from iptest import IronPythonTestCase, is_cli, big, myint, run_test
+from iptest import IronPythonTestCase, is_posix, is_cli, big, myint, run_test
 
 class CTypesTest(IronPythonTestCase):
     export_error_msg = "Existing exports of data: object cannot be re-sized" if is_cli else "cannot resize an array that is exporting buffers"
@@ -219,4 +220,15 @@ class TestU(Structure):
         self.assertEqual(TestU(-(1 << 64)).x, 0)
         self.assertEqual(TestU(-(1 << 64) - 1).x, 0x7fffffffffffffff)
 
+    @unittest.skipIf(is_posix, 'Windows specific test')
+    def test_loadlibrary_error(self):
+        with self.assertRaises(OSError) as cm:
+            windll.LoadLibrary(__file__)
+
+        self.assertEqual(cm.exception.errno, 8)
+        self.assertEqual(cm.exception.winerror, 193)
+        self.assertIn(" is not a valid Win32 application", cm.exception.strerror)
+        self.assertIsNone(cm.exception.filename)
+        self.assertIsNone(cm.exception.filename2)
+
 run_test(__name__)

WhatsNewInPython30.md

@@ -102,7 +102,7 @@ Changes To Exceptions
 - [x] [PEP 3110][]: Catching exceptions. You must now use `except SomeException as variable` instead of `except SomeException, variable`. Moreover, the variable is explicitly deleted when the `except` block is left.
 - [x] [PEP 3134][]: Exception chaining. There are two cases: implicit chaining and explicit chaining. Implicit chaining happens when an exception is raised in an `except` or `finally` handler block. This usually happens due to a bug in the handler block; we call this a secondary exception. In this case, the original exception (that was being handled) is saved as the `__context__` attribute of the secondary exception. Explicit chaining is invoked with this syntax: `raise SecondaryException() from primary_exception` (where `primary_exception` is any expression that produces an exception object, probably an exception that was previously caught). In this case, the primary exception is stored on the `__cause__` attribute of the secondary exception. The traceback printed when an unhandled exception occurs walks the chain of `__cause__` and `__context__` attributes and prints a separate traceback for each component of the chain, with the primary exception at the top. (Java users may recognize this behavior.)
 - [x] [PEP 3134][]: Exception objects now store their traceback as the `__traceback__` attribute. This means that an exception object now contains all the information pertaining to an exception, and there are fewer reasons to use `sys.exc_info()` (though the latter is not removed).
-- [ ] A few exception messages are improved when Windows fails to load an extension module. For example, error code 193 is now `%1 is not a valid Win32 application`. Strings now deal with non-English locales.
+- [x] A few exception messages are improved when Windows fails to load an extension module. For example, error code 193 is now `%1 is not a valid Win32 application`. Strings now deal with non-English locales.
 
 Operators And Special Methods
 ===============