Configure Windows at import time

[rharwood@redhat.com: Squash commits]
Merges: #194

gssapi/__init__.py

@@ -26,6 +26,8 @@
     low-level API functions.
 """
 
+import gssapi._win_config  # noqa
+
 from gssapi.raw.types import NameType, RequirementFlag, AddressType  # noqa
 from gssapi.raw.types import MechType, IntEnumFlagSet  # noqa
 from gssapi.raw.oids import OID  # noqa

gssapi/_win_config.py

@@ -0,0 +1,56 @@
+"""
+Using GSSAPI 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 os
+import ctypes
+
+#: 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"
+
+
+def k4w_in_path():
+    """Return if the main GSSAPI DLL for KfW is available in the PATH"""
+    try:  # to load the main GSSAPI DLL
+        ctypes.WinDLL('gssapi64.dll')
+    except OSError:  # DLL is not in PATH
+        return False
+    else:  # DLL is in PATH, everything should work
+        return True
+
+
+def error_not_found():
+    """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():
+    """
+    Validate that KfW appears to be installed correctly and add it to the
+    PATH if necessary. In the case that it can't be located, raise an error.
+    """
+    if k4w_in_path():
+        return  # All set, necessary DLLs should be available
+    if os.path.exists(KFW_BIN):  # In standard location
+        os.environ['PATH'] += os.pathsep + KFW_BIN
+        if k4w_in_path():
+            return
+    error_not_found()
+
+
+if os.name == 'nt':  # Make sure we have the required DLLs
+    configure_windows()