Search for MIT krb5 installation on Windows

src/krb5/__init__.py

@@ -1,6 +1,7 @@
 # Copyright: (c) 2021 Jordan Borean (@jborean93) <jborean93@gmail.com>
 # MIT License (see LICENSE or https://opensource.org/licenses/MIT)
 
+import krb5._win_config
 from krb5._ccache import (
     CCache,
     CredentialsRetrieveFlags,

src/krb5/_win_config.py

@@ -0,0 +1,90 @@
+"""
+Using libkrb5 on Windows requires having an installation of Kerberos for Windows
+(KfW) available in the user's PATH. This module should be imported before
+anything else to check for that installation, add it to the PATH if necessary,
+and throw any errors before they manifest as cryptic missing DLL errors later
+down the import tree.
+"""
+
+import ctypes
+import os
+import shutil
+import sys
+
+#: Path to normal KfW installed bin folder
+KFW_BIN = os.path.join(
+    os.environ.get("ProgramFiles", r"C:\Program Files"),
+    "MIT",
+    "Kerberos",
+    "bin",
+)
+#: Download location for KfW
+KFW_DL = "https://web.mit.edu/KERBEROS/dist"
+
+# Mypy needs to run on both Win and non-Win so the missing attribute will fire
+# on non-Win and Win will fire with unused ignore. Instead just cache the attr
+# by name and use it as needed.
+ADD_DLL_DIR = getattr(os, "add_dll_directory", None)
+CTYPES_WIN_DLL = getattr(ctypes, "WinDLL", ctypes.CDLL)
+
+
+def _add_dll_directory(path: str) -> None:
+    if ADD_DLL_DIR:
+        ADD_DLL_DIR(path)
+
+
+def kfw_available() -> bool:
+    """Return if the main libkrb5 DLL for KfW can be loaded"""
+    try:  # to load the main libkrb5 DLL
+        if sys.maxsize > 2**32:
+            CTYPES_WIN_DLL("krb5_64.dll")
+        else:
+            CTYPES_WIN_DLL("krb5_32.dll")
+    except OSError:  # DLL is not in PATH
+        return False
+    else:  # DLL is in PATH, everything should work
+        return True
+
+
+def error_not_found() -> None:
+    """Raise an OSError detailing that KfW is missing and how to get it"""
+    raise OSError(
+        "Could not find KfW installation. Please download and install "
+        "the 64bit Kerberos for Windows MSI from %s and ensure the "
+        "'bin' folder (%s) is in your PATH." % (KFW_DL, KFW_BIN)
+    )
+
+
+def configure_windows() -> None:
+    """
+    Validate that KfW appears to be installed correctly and add it to the
+    DLL directories/PATH if necessary. In the case that it can't be located,
+    raise an error.
+    """
+    if kfw_available():
+        return  # All set, necessary DLLs should be available
+
+    if os.path.exists(KFW_BIN):  # In standard location
+        try:  # to use Python 3.8's DLL handling
+            _add_dll_directory(KFW_BIN)
+        except AttributeError:  # <3.8, use PATH
+            os.environ["PATH"] += os.pathsep + KFW_BIN
+        if kfw_available():
+            return
+
+    # Check if kinit is in the PATH which should lead us to the bin folder
+    kinit_path = shutil.which("kinit")  # KfW provided binary
+    if kinit_path:  # Non-standard install location
+        try:  # Most likely >=3.8, otherwise it would have been found already
+            _add_dll_directory(os.path.dirname(kinit_path))
+        except AttributeError:  # <3.8, corrupted installation?
+            pass
+        else:
+            if kfw_available():
+                return
+
+    error_not_found()
+
+
+if os.name == "nt":  # Make sure we have the required DLLs
+    configure_windows()