belauncher: Prepend install path for game_exe starting with backslash.

[PhialsBasement]

belauncher.exe fails to launch the game when a title's BELauncher.ini specifies a 64BitExe= value with a leading backslash (a Win32 "root-relative" path), e.g.:

64BitExe=\Game\Binaries\Win64\Game.exe

The existing guard in programs/belauncher/main.c only prepends the install directory when PathIsRelativeW(game_exeW) returns TRUE, however, PathIsRelativeW returns FALSE for any path whose first character is \ (per the implementation in dlls/kernelbase/path.c)

Reproduced under Proton Experimental (11.0-20260518b) with a title whose BELauncher.ini uses a leading-backslash 64BitExe=. With WINEDEBUG=+belauncher:

trace:belauncher:wWinMain PROTON_ORIG_LAUNCHER_NAME L"S:\common\<Game>\<Game>.exe".
trace:belauncher:wWinMain cmdline L"-XRRuntime=... -d3d11".
trace:belauncher:wWinMain Launching original launcher L"S:\common\<Game>\<Game>.exe".
trace:belauncher:wWinMain Launching game executable \<Game>\Binaries\Win64\<Game>.exe for BattlEye.
trace:belauncher:wWinMain game_exe L"\<Game>\Binaries\Win64\<Game>.exe".
trace:belauncher:wWinMain path L"S:\common\<Game>\", be_arg (null).
trace:belauncher:wWinMain launch_cmd L"\<Game>\Binaries\Win64\<Game>.exe -XRRuntime=... -d3d11".
err:belauncher:wWinMain CreateProcessW failed, error 2.

Note launch_cmd is missing the path prefix, even though path is known and traced one line earlier. The file at S:\common\<Game>\<Game>\Binaries\Win64\<Game>.exe exists; the path Wine actually queried (<current_drive>:\<Game>\Binaries\Win64\<Game>.exe) does not.

Editing the title's BELauncher.ini to remove the leading backslash makes the launch succeed, confirming the diagnosis.

P.S this fixes Contractors Showdown/Exfilzone which is yet another W for the steam frame

[Hi-Angel]

I think it might be useful to add a comment about game_exeW[0] == L'\\'.

I'm not a Windows dev, but I searched around and it seems leading backslash is supposed to translate to the root of the disk we're currently in.

That implies the line game_exeW[0] == L'\\' is working around BattleEye bug, where it translates the dir incorrectly (i.e. the game that works on Windows actually wasn't supposed to work there either).

So, I think it's useful to add a comment explaining the situation, i.e. why exactly we do the game_exeW[0] == L'\\' check here.

[PhialsBasement]

Ive just applied some comments to help identify that.

[Etaash-mathamsetty]

Maybe it would be better to check if the path is relative on the string + 1 to remove the \ character?

Something like

If (*str = L'\\') str++;

[PhialsBasement]

Maybe it would be better to check if the path is relative on the string + 1 to remove the \ character?

Something like

If (*str = L'\\') str++;

Good idea, done with memmove since game_exeW is an array rather than a pointer, plus a game_exeW[1] != L'\\' guard so UNC paths aren't mangled.

[Etaash-mathamsetty]

I think using memmove is a bit overkill, just define a second variable that points to the array

[PhialsBasement]

I think using memmove is a bit overkill, just define a second variable that points to the array

I'd prefer to keep the memmove, it leaves game_exeW in a single consistent state after the strip, so later code (here or any future patch) can't go for the original array and get the unstripped form.