Fix failure to find libopenslide-0.dll on Windows

[simo1427]

In issue #51 I suggested doing this; it was mentioned in issue #23 as well. On every machine, on which I tried installing the library, I had to edit this line.

[simo1427]

Closed by mistake

[bgilbert]

find_library() should generally not be used at runtime. Have you tried adding the OpenSlide bin directory to the beginning of your search path?

[simo1427]

Yes but to no avail, I even tried doing so in a standalone script with ctypes.cdll.LoadLibrary in it - it still doesn’t work, but find_library continues to find the DLL.

[crisz]

After hours (if not days) of research, this is the only fix working

[jesper-molin]

I also have this issue and looked how to fix it, Python changed the default behavior in 3.8 to no longer look in PATH when resolving DLL:s on Windows. In the what's new notes, they recommend using add_dll_directory() to resolve this issue or use an absolute path, but then you need to know where to look.

The only reasonable way to search the PATH is to use find_library, if you want to protect against DLL hijacking, the project needs to deliver the win-DLL to a known location and not depending on it being in the PATH. See e.g. for another project's similar problem: beetbox/pyacoustid#54

So I think this is a good patch.

[bgilbert]

Okay, yeah, the behavior change in 3.8 seems to explain it. Does the sample code in #115 (comment) work for you?

In the what's new notes, they recommend using add_dll_directory() to resolve this issue or use an absolute path, but then you need to know where to look.

add_dll_directory() seems reasonable. OpenSlide Python assumes that OpenSlide is provided by some external mechanism, which on Windows is almost certainly under the control of the application.

[simo1427]

I pointed the path to the bin directory of the openslide installation and I can confirm that the quoted code worked.

[alessiamarcolini]

Hello maintainers, I was wondering when is this PR going to be merged :) thank you

[bgilbert]

@alessiamarcolini On Python 3.8 and later, applications should use the new os.add_dll_directory() Python function to add the OpenSlide DLL directory to the search path before importing openslide. We'll update the documentation to include example code, but for now, see #115 (comment).

Since this PR bypasses the new Python API, I'll close. Thanks for sending!