A Zig `SHGetKnownFolderPath` implementation in the standard library

[squeek502]

EDIT: This is currently being worked on here: https://github.com/squeek502/get-known-folder-path

---

Follow up from https://github.com/ziglang/zig/pull/18091

To avoid a dependency on shell32.dll, the SHGetKnownFolderPath call within std.fs.getAppDataDir was replaced with a LOCALAPPDATA environment variable lookup. This has the potential to regress getAppDataDir behavior for certain setups and it's not foolproof to rely on LOCALAPPDATA being set.

Instead, SHGetKnownFolderPath should be reimplemented in Zig without introducing a dependency on shell32. For this, wine's implementation would likely be a good reference.

• wine's implementation
• Procmon csv output of the standalone program below on my computer
• https://github.com/ziglang/zig/blob/master/lib/std/fs/get_app_data_dir.zig

Standalone version of the previous implementation using SHGetKnownFolderPath (this is ultimately the behavior that the Zig reimplementation is looking to match, but it shouldn't be FOLDERID_LocalAppData-specific):

const std = @import("std");
const os = std.os;

pub extern "shell32" fn SHGetKnownFolderPath(
    rfid: *const os.windows.KNOWNFOLDERID,
    dwFlags: os.windows.DWORD,
    hToken: ?os.windows.HANDLE,
    ppszPath: *[*:0]os.windows.WCHAR,
) callconv(os.windows.WINAPI) os.windows.HRESULT;

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer std.debug.assert(gpa.deinit() == .ok);
    const allocator = gpa.allocator();

    var dir_path_ptr: [*:0]u16 = undefined;
    switch (SHGetKnownFolderPath(
        &os.windows.FOLDERID_LocalAppData,
        os.windows.KF_FLAG_CREATE,
        null,
        &dir_path_ptr,
    )) {
        os.windows.S_OK => {
            const global_dir = std.unicode.utf16leToUtf8Alloc(allocator, std.mem.sliceTo(dir_path_ptr, 0)) catch |err| switch (err) {
                error.UnexpectedSecondSurrogateHalf => return error.AppDataDirUnavailable,
                error.ExpectedSecondSurrogateHalf => return error.AppDataDirUnavailable,
                error.DanglingSurrogateHalf => return error.AppDataDirUnavailable,
                error.OutOfMemory => return error.OutOfMemory,
            };
            defer allocator.free(global_dir);
            std.debug.print("{s}\n", .{global_dir});
        },
        os.windows.E_OUTOFMEMORY => return error.OutOfMemory,
        else => return error.AppDataDirUnavailable,
    }
}

[matu3ba]

I'll work on this to get a stab at some more windows tooling.

[squeek502]

@matu3ba I've got an implementation mostly complete for retrieving a known path that I'll likely create a PR for soon.

Right now I'm thinking that the more complicated parts should not be supported (KF_FLAG_CREATE and KF_FLAG_INIT in particular, which would potentially need to do things like create "Library" folders, init desktop.ini files, respect various undocumented flags in FolderDescriptions registry entries, etc).

I'll go into more detail in the PR.

[squeek502]

I've decided against making a PR for now to allow for (hopefully) developing a more conformant implementation. SHGetKnownFolderPath has a lot of moving parts, and Zig only really cares about getting the path to the Local App Data directory, so %LOCALAPPDATA% seems like a decent solution since it's set by the OS. That is, this issue doesn't seem urgent enough to warrant merging in a potentially half-baked SHGetKnownFolderPath reimplementation just to stop using %LOCALAPPDATA%.

I've put my mostly-working-but-probably-has-incorrect-corner-cases implementation here with a lot of explanation in the readme:

https://github.com/squeek502/get-known-folder-path

If anyone wants to help out or has ideas about how to go about this sort of thing, it'd be appreciated. It feels like it's nearly there, just a few pieces that need to be figured out.