Cannot load MediaInfo.dll (windows) from Python 3.8

[anjos]

Apparently, there is a nasty bug/miscommunication starting with Python 3.8 regarding the way to search for DLLs under Windows. As a result, conda-forge builds for this package under win-64/python3.8 are broken (log of the build: https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=410706&view=logs&jobId=5be07ae1-d8ba-5406-47b6-8e3a3a12f825&j=5be07ae1-d8ba-5406-47b6-8e3a3a12f825&t=0bf03e01-0bec-5b85-5316-b1633322e895)

The problem is rather trivial: ctypes.WinDLL does not find MediaInfo.dll (details and solution here: https://stackoverflow.com/questions/59330863/cant-import-dll-module-in-python).

How to fix: When the DLL is loaded, one has to set winmode=0 to recover the old DLL finding behaviour, around line 314 of pymediainfo/__init__.py (v5.1.0).

I can add this as a patch to the build instructions, but I thought you may want to consider this to be pushed upstream, @sbraz.

As expected, things work OK with Python 3.7 and somehow with Python 3.9 as well. I could not test it yet with Python 3.10. Please let me know how you want to proceed.

[sbraz]

Hi,
According to the documentation, winmode=0 is the default. I guess the documentation is wrong?

It's strange that the AppVeyor builds never failed. I'll look into it.

[anjos]

Yes, that seems to be wrong in the documentation as explained in the StackOverflow thread above (it is actually set to None instead, and that makes a difference). I had a look at the AppVeyor builds, but in that case you copy the dll to somewhere with ps: "Copy-Item -Path MediaInfo.dll -Destination ${Env:PYTHON}"? I wonder if that does not bypass this fault somehow?

[anjos]

Patch tested on conda-forge: conda-forge/pymediainfo-feedstock@96fda70

[sbraz]

Sorry for the delay, I tried the following:

• Installed Python 3.8.10 on Windows 10
• Ran pip.exe install pymediainfo (pip installed pymediainfo-5.1.0-py3-none-win_amd64.whl)
• Ran C:\Python38\python.exe -c "import pymediainfo; print(pymediainfo.MediaInfo.parse('test.png'))" which worked as expected.

This probably works because the DLL is in the wheel. However, what failed for me was:

git clone https://github.com/sbraz/pymediainfo/
cd pymediainfo
# Downloaded MediaInfo.dll
C:\Python38\python.exe -c "import pymediainfo; print(pymediainfo.MediaInfo.parse('test.png'))"

It resulted in:

OSError: Failed to load library from MediaInfo.dll - Could not find module 'MediaInfo.dll' (or one of its dependencies). Try using the full path with constructor syntax.

I also tried with a range of Python versions: 3.6 and 3.7 work; 3.8, 3.9 and 3.10 fail.

Settingwinmode=0 fixes the problem with >= 3.8 versions but is not accepted on 3.6 and 3.7 so the patch will need to include an if sys.version < (3, 8).

[sbraz]

I've found a better way: using the absolute path fixes the problem:

diff --git a/pymediainfo/__init__.py b/pymediainfo/__init__.py
index 9c18679..a2458c0 100644
--- a/pymediainfo/__init__.py
+++ b/pymediainfo/__init__.py
@@ -310,6 +310,7 @@ class MediaInfo:
             library_paths = (library_file,)
         exceptions = []
         for library_path in library_paths:
+            library_path = os.path.abspath(library_path)
             try:
                 lib = lib_type(library_path)
                 cls._define_library_prototypes(lib)

Can you please try that?

[sbraz]

I've filed a PR for the doc problem: python/cpython#29976

[anjos]

Thanks for the doc patch - it will help. For the actual patch you propose, it seems that it works well under OSX (arm, M1), but not for Windows, Linux or OSX (intel): https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=424833&view=logs&jobId=4f922444-fdfe-5dcf-b824-02f86439ef14&j=4f922444-fdfe-5dcf-b824-02f86439ef14&t=b2a8456a-fb11-5506-ca32-5ccd32538dc0

[anjos]

Note: In the case of conda-forge, the library is installed together with other DLLs, in an environment-wide installation. To create a setup to work with, you can do the following:

1. Install miniforge or, better, mambaforge from https://github.com/conda-forge/miniforge
2. Create an environment to work with: conda create -n test python=3.8 mediainfo
3. Clone your package in a work directory, then pip install it in editable mode: pip install -e .
4. Run the tests

I hope it helps.

[sbraz]

You're right, my patch is not ideal because on Linux we don't always provide a full path.

I'm really not keen on setting winmode=0 as it might have security implications. Have you considered using os.add_dll_directory() so that your system-wide DLL location is properly recognised? Surely this would benefit other packages besides pymediainfo.

[anjos]

There is one curious thing: the patch at conda-forge is only applied if sys.version = (3, 8). We can then speculate that this problem is actually temporary and will be auto-fixed when python 3.8 is deprecated. I propose we leave it as is in this case.