Use a standard search path for DLL loading in windows (#235)

Since Python 3.8, ctypes uses an altered search path when loading DLLs
in Windows which only includes the application directory, the system32
directory, and directories manually specified with add_dll_directory.
None of those suit us.  The standard search path can be specified with
`winmode=0`.

This commit adds a new function to loads libraries with `winmode=0`
and makes all modules that load shared libraries use it.

NEWS.rst

@@ -67,6 +67,11 @@ Version 0.7.0 (upcoming)
 
 * Microscope is now dependent on Python 3.7 or later.
 
+* Python 3.8 changed the default DLL search path in Windows which
+  caused all cameras, deformable mirrors, and Linkam stage to fail to
+  load.  This version restores the use of Windows standard search
+  path.
+
 
 Version 0.6.0 (2021/01/14)
 --------------------------

microscope/_utils.py

@@ -17,6 +17,9 @@
 ## You should have received a copy of the GNU General Public License
 ## along with Microscope.  If not, see <http://www.gnu.org/licenses/>.
 
+import ctypes
+import os
+import sys
 import threading
 import typing
 
@@ -43,6 +46,36 @@
         )
 
 
+def library_loader(
+    libname: str, dlltype: typing.Type[ctypes.CDLL] = ctypes.CDLL, **kwargs
+) -> ctypes.CDLL:
+    """Load shared library.
+
+    This exists mainly to search for DLL in Windows using a standard
+    search path, i.e, search for dlls in ``PATH``.
+
+    Args:
+        libname: file name or path of the library to be loaded as
+            required by `dlltype`
+        dlltype: the class of shared library to load.  Typically,
+            ``CDLL`` but sometimes ``WinDLL`` in windows.
+        kwargs: other arguments passed on to ``dlltype``.
+    """
+    # Python 3.8 in Windows uses an altered search path.  Their
+    # reasons is security and I guess it would make sense if we
+    # installed the DLLs we need ourselves but we don't.  `winmode=0`
+    # restores the use of the standard search path.  See issue #235.
+    if (
+        os.name == "nt"
+        and sys.version_info >= (3.8)
+        and "winmode" not in kwargs
+    ):
+        winmode_kwargs = {"winmode": 0}
+    else:
+        winmode_kwargs = {}
+    return dlltype(libname, **winmode_kwargs, **kwargs)
+
+
 class OnlyTriggersOnceOnSoftwareMixin(microscope.abc.TriggerTargetMixin):
     """Utility mixin for devices that only trigger "once" with software.
 

microscope/_wrappers/BMC.py

@@ -24,12 +24,14 @@
 import os
 from ctypes import c_char, c_char_p, c_double, c_int, c_uint, c_uint32
 
+import microscope._utils
+
 
 if os.name == "nt":  # is windows
     # Not actually tested yet
-    SDK = ctypes.WinDLL("BMC2")
+    SDK = microscope._utils.library_loader("BMC2", ctypes.WinDLL)
 else:
-    SDK = ctypes.CDLL("libBMC.so.3")
+    SDK = microscope._utils.library_loader("libBMC.so.3")
 
 
 # Definitions from BMCDefs.h

microscope/_wrappers/asdk.py

@@ -24,13 +24,14 @@
 import os
 from ctypes import c_char_p, c_double, c_int, c_size_t, c_uint32
 
+import microscope._utils
+
 
 if os.name == "nt":  # is windows
     _libname = "ASDK"
 else:
     _libname = "libasdk.so"  # Not actually tested yet
-
-SDK = ctypes.CDLL(_libname)
+SDK = microscope._utils.library_loader(_libname)
 
 
 class DM(ctypes.Structure):

microscope/_wrappers/dcamapi4.py

@@ -34,11 +34,13 @@
 import enum
 import os
 
+import microscope._utils
+
 
 if os.name == "nt":
-    _LIB = ctypes.WinDLL("dcamapi")
+    _LIB = microscope._utils.library_loader("dcamapi", ctypes.WinDLL)
 else:
-    _LIB = ctypes.CDLL("libdcamapi.so")
+    _LIB = microscope._utils.library_loader("libdcamapi.so", ctypes.CDLL)
 
 
 if os.name == "nt":

microscope/_wrappers/mirao52e.py

@@ -22,9 +22,11 @@
 
 import ctypes
 
+import microscope._utils
+
 
 # Vendor only supports Windows
-SDK = ctypes.WinDLL("mirao52e")
+SDK = microscope._utils.library_loader("mirao52e", ctypes.WinDLL)
 
 
 TRUE = 1  # TRUE MroBoolean value

microscope/cameras/_SDK3.py

@@ -23,6 +23,8 @@
 import os
 from ctypes import POINTER, c_double, c_int, c_uint, c_void_p
 
+import microscope._utils
+
 
 #### typedefs
 AT_H = ctypes.c_int
@@ -34,12 +36,20 @@
 _stdcall_libraries = {}
 
 if os.name == "nt":  # is windows
-    _stdcall_libraries["ATCORE"] = ctypes.WinDLL("atcore")
-    _stdcall_libraries["ATUTIL"] = ctypes.WinDLL("atutility")
+    _stdcall_libraries["ATCORE"] = microscope._utils.library_loader(
+        "atcore", ctypes.WinDLL
+    )
+    _stdcall_libraries["ATUTIL"] = microscope._utils.library_loader(
+        "atutility", ctypes.WinDLL
+    )
     CALLBACKTYPE = ctypes.WINFUNCTYPE(c_int, AT_H, POINTER(AT_WC), c_void_p)
 else:
-    _stdcall_libraries["ATCORE"] = ctypes.CDLL("atcore.so")
-    _stdcall_libraries["ATUTIL"] = ctypes.CDLL("atutility.so")
+    _stdcall_libraries["ATCORE"] = microscope._utils.library_loader(
+        "atcore.so"
+    )
+    _stdcall_libraries["ATUTIL"] = microscope._utils.library_loader(
+        "atutility.so"
+    )
     CALLBACKTYPE = ctypes.CFUNCTYPE(c_int, AT_H, POINTER(AT_WC), c_void_p)
 
 #### Defines

microscope/cameras/atmcd.py

@@ -64,6 +64,7 @@
 from numpy.ctypeslib import ndpointer
 
 import microscope
+import microscope._utils
 import microscope.abc
 
 
@@ -88,9 +89,9 @@
 else:
     _dllName = "atmcd64d"
 if os.name == "nt":  # is windows
-    _dll = ctypes.WinDLL(_dllName)
+    _dll = microscope._utils.library_loader(_dllName, ctypes.WinDLL)
 else:
-    _dll = ctypes.CDLL(_dllName + ".so")
+    _dll = microscope._utils.library_loader(_dllName + ".so")
 
 # Andor's types
 at_32 = c_long

microscope/cameras/pvcam.py

@@ -144,6 +144,7 @@
 import Pyro4
 
 import microscope
+import microscope._utils
 import microscope.abc
 
 
@@ -676,11 +677,12 @@ class md_frame(ctypes.Structure):
 
 if os.name == "nt":  # is windows
     if platform.architecture()[0] == "32bit":
-        _lib = ctypes.WinDLL("pvcam32")
+        _lib = microscope._utils.library_loader("pvcam32", ctypes.WinDLL)
     else:
-        _lib = ctypes.WinDLL("pvcam64")
+        _lib = microscope._utils.library_loader("pvcam64", ctypes.WinDLL)
 else:
-    _lib = ctypes.CDLL("pvcam.so")
+    _lib = microscope._utils.library_loader("pvcam.so")
+
 
 ### Functions ###
 STRING = ctypes.c_char_p

microscope/stages/linkam.py

@@ -48,6 +48,7 @@
 from enum import Enum, IntEnum
 
 import microscope
+import microscope._utils
 import microscope.abc
 
 
@@ -975,9 +976,10 @@ def get_sdk_version():
     def init_sdk():
         """Initialise the SDK and set up event callbacks"""
         if os.name == "nt":  # is windows
-            __class__._lib = ctypes.CDLL("LinkamSDK.dll")
+            _libname = "LinkamSDK.dll"
         else:  # assuming Linux.  Not tested.
-            __class__._lib = ctypes.CDLL("libLinkamSDK.so")
+            _libname = "libLinkamSDK.so"
+        __class__._lib = microscope._utils.library_loader(_libname)
         _lib = __class__._lib
         """Initialise the SDK, and create and set the callbacks."""
         # Omit conditional pending a fix for ctypes issues when optimisations in use.