Possible Bug: load_library loses custom lib_location in error.py

[AndreasScharf]

Hi,
while working on a project I encountered a strange behavior for an implementation of the snap7.client.Client(lib_location='/path/to/lib')

Setup
So in my current setup i just a Raspberry Pi to build a executable for Raspian and later transfer it to a "target" system.
Both Raspberry Pi Trixie 32bit OS.
I wanted to have a single executable to transfer so no global /usr/lib/libsnap7.so or similar. Thats why I choose to zip it in the executable and unzip snap7.so at runtime in /tmp/_MEI... folder. The dynamic link is gave my snap7.client.Client as param lib_location.

Enviroment:

• 2x RPi 4 with Trixie OS 32bit (build-system & target)
• executable form pyinstaller, with libsnap7.so zipped
• at runtime unzipped libsnap7.so in /tmp/_MEI... folder
• following call in my source code
  s = snap7.client.Client(lib_locations=temp_location_snap7)

1. use pyinstaller to build project
2. scp transfer dist/app form build to target
3. execute ./app (no global, local or package libsnap7.so)

Expected behavior & problem
I was expecting that the custom lib_location i give snap7.client.Client will be used exclusively.

For a single successful connection this works fine. But as soon as you hit a connection error, the line 163 in error.py will call load_library with no params. So there will be no link to the snap7.so as there is neither a global, local or package installation of libsnap7.so.

Solution & change suggestion
My change suggestion for the common.py would be to "remember" the custom loaded "lib_location" in a global variable as the Client or Server will be called before the error handler.

# global variable to remember the loaded library
LIB_LOCATION = ''

@cache
def load_library(lib_location: Optional[str] = None) -> Snap7CliProtocol:
    """Loads the `snap7.dll` library.
    Returns:
        cdll: a ctypes cdll object with the snap7 shared library loaded.
    """
    # check if there is a custom location set
    # client load_libary() will called before any snap7.error is used
    global LIB_LOCATION
    if lib_location:
        LIB_LOCATION = lib_location

    if not lib_location:
        lib_location = LIB_LOCATION or _find_in_package() or find_library("snap7") or _find_locally("snap7")

    if not lib_location:
        _raise_error()

    return cast(Snap7CliProtocol, cdll.LoadLibrary(lib_location))

I know this might be a super edge rare edge case and not the pretties solution but this fix works for my case. Note i did not run any of the unittest as i did not got the test env to running in my python Raspberry Pi setup yet.

[gijzelaerr]

i'm currently working on a native python implementation that drops the dll/so requirement. This will become 3.0 and should solve your issue.

[gijzelaerr]

can you help us and have a look at this PR and see if it works for you? #569

[gijzelaerr]

This issue has been resolved by the native Python implementation (merged in #569). The pure Python client no longer uses a C shared library (libsnap7.so/snap7.dll), so there is no load_library function that can lose the custom path. The lib_location parameter is now accepted for backwards compatibility but is ignored — no external library is loaded at all. Closing this issue.