Fix msys2 dll paths in GdkPixbuf loaders.cache gen

[coolcatco888]

When compiling a program that depends on GTK and GdkPixbuf loaders in msys2 Windows, the loaders.cache generates dlls with an absolute path that is local to the machine it was generated on.

When the resulting compiled program is run on another Windows machine, the local paths of the dlls do not exist and the program crashes.

Example loader.cache snippet

"C:/tools/msys64/mingw64/bin/../lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.dll"
"png" 5 "gdk-pixbuf" "PNG" "LGPL"
"image/png" ""
"png" ""
"\211PNG\r\n\032\n" "" 100

The correct output should be

"lib\\gdk-pixbuf\\loaders\\libpixbufloader-png.dll"
"png" 5 "gdk-pixbuf" "PNG" "LGPL"
"image/png" ""
"png" ""
"\211PNG\r\n\032\n" "" 100

The root cause of the bug lies the libdir variable.
libdir = C:/tools/msys64/mingw64/bin

The GdkPixbuf hook generates a loaders.cache file with absolute paths and uses prefix matching to replace the dll paths with relative paths that the program can find. However, this will match none of the prefixes and therefore, the absolute path will not be changed to the relative path in the loaders.cache.

In the code example below, we see that it will try to match the following prefixes:
prefix = "C:/tools/msys64/mingw64/bin/gdk-pixbuf-2.0/2.10.0
win_prefix "\\lib\\gdk-pixbuf-2.0\\2.10.0

This will not work given the line from cachedata
"C:/tools/msys64/mingw64/bin/../lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.dll"

    output_lines = []
    prefix = '"' + os.path.join(libdir, 'gdk-pixbuf-2.0', '2.10.0')
    plen = len(prefix)

    win_prefix = '"' + '\\\\'.join(['lib', 'gdk-pixbuf-2.0', '2.10.0'])
    win_plen = len(win_prefix)

    # For each line in the updated loader cache...
    for line in cachedata.splitlines():
        if line.startswith('#'):
            continue
        if line.startswith(prefix):
            line = '"@executable_path/' + LOADER_CACHE_DEST_PATH + line[plen:]
        elif line.startswith(win_prefix):
            line = '"' + LOADER_CACHE_DEST_PATH.replace('/', '\\\\') + line[win_plen:]
        output_lines.append(line)

This fix addresses this by accounting for the dll prefix for msys2.

My solution will simplify the line path to be:
"C:/tools/msys64/mingw64/lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.dll"

which will match my new prefix:
msys2_prefix = "C:/tools/msys64/mingw64/lib/gdk-pixbuf-2.0/2.10.0

And convert the line to be:
line = "lib\\gdk-pixbuf\\loaders\\libpixbufloader-png.dll"

fixes #7838

[rokm]

Hmm, I think long-term, all that prefix matching should be replaced with a bit more advanced loaders.config parser that identifies the lines that contain path to library (seems to be first line in a block, where blocks are separated by empty lines), and processes them using path-utility functions. I.e., strips the quote character, takes the base name, prepends the collection path (anchored at @executable_path for non-Windows), and adds the quotes. Then it won't matter what the original path was.

But for the time being, the proposed fix should do the trick. Please add a news fragment for the changelog, then we should be ready to merge.

[coolcatco888]

I added the news fragment. I also noticed that the previous test run failed this test:

tests (3.12-dev, ubuntu-20.04)

Would I still be able to merge?

[coolcatco888]

Hi @rokm,

I have noticed that this test fails: tests (3.12-dev, ubuntu-20.04). I don't think the failure is related to my change since I saw a couple of pull requests get merged without this test passing.

Would you be able to merge my change? Do not have permissions to merge this pull request without all checks passing.