windowsCreateProcessPathExt

fn windowsCreateProcessPathExt(
    arena: Allocator,
    dir_buf: *std.ArrayList(u16),
    app_buf: *std.ArrayList(u16),
    pathext: [:0]const u16,
    cmd_line_cache: *WindowsCommandLineCache,
    env_block: ?process.Environ.WindowsBlock,
    cwd_ptr: ?[*:0]u16,
    flags: windows.CreateProcessFlags,
    lpStartupInfo: *windows.STARTUPINFOW,
    lpProcessInformation: *windows.PROCESS.INFORMATION,
) !void {
    const app_name_len = app_buf.items.len;
    const dir_path_len = dir_buf.items.len;

    if (app_name_len == 0) return error.FileNotFound;

    defer app_buf.shrinkRetainingCapacity(app_name_len);
    defer dir_buf.shrinkRetainingCapacity(dir_path_len);

    // The name of the game here is to avoid CreateProcessW calls at all costs,
    // and only ever try calling it when we have a real candidate for execution.
    // Secondarily, we want to minimize the number of syscalls used when checking
    // for each PATHEXT-appended version of the app name.
    //
    // An overview of the technique used:
    // - Open the search directory for iteration (either cwd or a path from PATH)
    // - Use NtQueryDirectoryFile with a wildcard filename of `<app name>*` to
    //   check if anything that could possibly match either the unappended version
    //   of the app name or any of the versions with a PATHEXT value appended exists.
    // - If the wildcard NtQueryDirectoryFile call found nothing, we can exit early
    //   without needing to use PATHEXT at all.
    //
    // This allows us to use a <open dir, NtQueryDirectoryFile, close dir> sequence
    // for any directory that doesn't contain any possible matches, instead of having
    // to use a separate look up for each individual filename combination (unappended +
    // each PATHEXT appended). For directories where the wildcard *does* match something,
    // we iterate the matches and take note of any that are either the unappended version,
    // or a version with a supported PATHEXT appended. We then try calling CreateProcessW
    // with the found versions in the appropriate order.
    const dir = dir: {
        // needs to be null-terminated
        try dir_buf.append(arena, 0);
        defer dir_buf.shrinkRetainingCapacity(dir_path_len);
        const dir_path_z = dir_buf.items[0 .. dir_buf.items.len - 1 :0];
        const prefixed_path = try wToPrefixedFileW(null, dir_path_z, .{});
        break :dir dirOpenDirWindows(.cwd(), prefixed_path.span(), .{
            .iterate = true,
        }) catch |err| switch (err) {
            // These errors must not be ignored because they should not be able
            // to affect which file is chosen to execute. Also `error.Canceled`
            // must never be swallowed.
            error.Canceled,
            error.SystemResources,
            error.Unexpected,
            error.ProcessFdQuotaExceeded,
            error.SystemFdQuotaExceeded,
            => |e| return e,

            error.AccessDenied,
            error.PermissionDenied,
            error.SymLinkLoop,
            error.FileNotFound,
            error.NotDir,
            error.NoDevice,
            error.NetworkNotFound,
            error.NameTooLong,
            error.BadPathName,
            => return error.FileNotFound,
        };
    };
    defer windows.CloseHandle(dir.handle);

    // Add wildcard and null-terminator
    try app_buf.append(arena, '*');
    try app_buf.append(arena, 0);
    const app_name_wildcard = app_buf.items[0 .. app_buf.items.len - 1 :0];

    // This 2048 is arbitrary, we just want it to be large enough to get multiple FILE_DIRECTORY_INFORMATION entries
    // returned per NtQueryDirectoryFile call.
    var file_information_buf: [2048]u8 align(@alignOf(windows.FILE_DIRECTORY_INFORMATION)) = undefined;
    const file_info_maximum_single_entry_size = @sizeOf(windows.FILE_DIRECTORY_INFORMATION) + (windows.NAME_MAX * 2);
    if (file_information_buf.len < file_info_maximum_single_entry_size) {
        @compileError("file_information_buf must be large enough to contain at least one maximum size FILE_DIRECTORY_INFORMATION entry");
    }
    var io_status: windows.IO_STATUS_BLOCK = undefined;

    const num_supported_pathext = @typeInfo(process.WindowsExtension).@"enum".field_names.len;
    var pathext_seen: [num_supported_pathext]bool = @splat(false);
    var any_pathext_seen = false;
    var unappended_exists = false;

    // Fully iterate the wildcard matches via NtQueryDirectoryFile and take note of all versions
    // of the app_name we should try to spawn.
    // Note: This is necessary because the order of the files returned is filesystem-dependent:
    //       On NTFS, `blah.exe*` will always return `blah.exe` first if it exists.
    //       On FAT32, it's possible for something like `blah.exe.obj` to be returned first.
    while (true) {
        // If we get nothing with the wildcard, then we can just bail out
        // as we know appending PATHEXT will not yield anything.
        switch (windows.ntdll.NtQueryDirectoryFile(
            dir.handle,
            null,
            null,
            null,
            &io_status,
            &file_information_buf,
            file_information_buf.len,
            .Directory,
            .FALSE, // single result
            &.init(app_name_wildcard),
            .FALSE, // restart iteration
        )) {
            .SUCCESS => {},
            .NO_SUCH_FILE => return error.FileNotFound,
            .NO_MORE_FILES => break,
            .ACCESS_DENIED => return error.AccessDenied,
            else => |status| return windows.unexpectedStatus(status),
        }

        // According to the docs, this can only happen if there is not enough room in the
        // buffer to write at least one complete FILE_DIRECTORY_INFORMATION entry.
        // Therefore, this condition should not be possible to hit with the buffer size we use.
        std.debug.assert(io_status.Information != 0);

        var it = windows.FileInformationIterator(windows.FILE_DIRECTORY_INFORMATION){ .buf = &file_information_buf };
        while (it.next()) |info| {
            // Skip directories
            if (info.FileAttributes.DIRECTORY) continue;
            const filename = @as([*]u16, @ptrCast(&info.FileName))[0 .. info.FileNameLength / 2];
            // Because all results start with the app_name since we're using the wildcard `app_name*`,
            // if the length is equal to app_name then this is an exact match
            if (filename.len == app_name_len) {
                // Note: We can't break early here because it's possible that the unappended version
                //       fails to spawn, in which case we still want to try the PATHEXT appended versions.
                unappended_exists = true;
            } else if (windowsCreateProcessSupportsExtension(filename[app_name_len..])) |pathext_ext| {
                pathext_seen[@backingInt(pathext_ext)] = true;
                any_pathext_seen = true;
            }
        }
    }

    const unappended_err = unappended: {
        if (unappended_exists) {
            if (dir_path_len != 0) switch (dir_buf.items[dir_buf.items.len - 1]) {
                '/', '\\' => {},
                else => try dir_buf.append(arena, Dir.path.sep),
            };
            try dir_buf.appendSlice(arena, app_buf.items[0..app_name_len]);
            try dir_buf.append(arena, 0);
            const full_app_name = dir_buf.items[0 .. dir_buf.items.len - 1 :0];

            const is_bat_or_cmd = bat_or_cmd: {
                const app_name = app_buf.items[0..app_name_len];
                const ext_start = std.mem.findScalarLast(u16, app_name, '.') orelse break :bat_or_cmd false;
                const ext = app_name[ext_start..];
                const ext_enum = windowsCreateProcessSupportsExtension(ext) orelse break :bat_or_cmd false;
                switch (ext_enum) {
                    .cmd, .bat => break :bat_or_cmd true,
                    else => break :bat_or_cmd false,
                }
            };
            const cmd_line_w = if (is_bat_or_cmd)
                try cmd_line_cache.scriptCommandLine(full_app_name)
            else
                try cmd_line_cache.commandLine();
            const app_name_w = if (is_bat_or_cmd)
                try cmd_line_cache.cmdExePath()
            else
                full_app_name;

            if (windowsCreateProcess(
                app_name_w.ptr,
                cmd_line_w.ptr,
                env_block,
                cwd_ptr,
                flags,
                lpStartupInfo,
                lpProcessInformation,
            )) |_| {
                return;
            } else |err| switch (err) {
                error.FileNotFound,
                error.AccessDenied,
                => break :unappended err,
                error.InvalidExe => {
                    // On InvalidExe, if the extension of the app name is .exe then
                    // it's treated as an unrecoverable error. Otherwise, it'll be
                    // skipped as normal.
                    const app_name = app_buf.items[0..app_name_len];
                    const ext_start = std.mem.findScalarLast(u16, app_name, '.') orelse break :unappended err;
                    const ext = app_name[ext_start..];
                    if (windows.eqlIgnoreCaseWtf16(ext, std.unicode.utf8ToUtf16LeStringLiteral(".EXE"))) {
                        return error.UnrecoverableInvalidExe;
                    }
                    break :unappended err;
                },
                else => return err,
            }
        }
        break :unappended error.FileNotFound;
    };

    if (!any_pathext_seen) return unappended_err;

    // Now try any PATHEXT appended versions that we've seen
    var ext_it = std.mem.tokenizeScalar(u16, pathext, ';');
    while (ext_it.next()) |ext| {
        const ext_enum = windowsCreateProcessSupportsExtension(ext) orelse continue;
        if (!pathext_seen[@backingInt(ext_enum)]) continue;

        dir_buf.shrinkRetainingCapacity(dir_path_len);
        if (dir_path_len != 0) switch (dir_buf.items[dir_buf.items.len - 1]) {
            '/', '\\' => {},
            else => try dir_buf.append(arena, Dir.path.sep),
        };
        try dir_buf.appendSlice(arena, app_buf.items[0..app_name_len]);
        try dir_buf.appendSlice(arena, ext);
        try dir_buf.append(arena, 0);
        const full_app_name = dir_buf.items[0 .. dir_buf.items.len - 1 :0];

        const is_bat_or_cmd = switch (ext_enum) {
            .cmd, .bat => true,
            else => false,
        };
        const cmd_line_w = if (is_bat_or_cmd)
            try cmd_line_cache.scriptCommandLine(full_app_name)
        else
            try cmd_line_cache.commandLine();
        const app_name_w = if (is_bat_or_cmd)
            try cmd_line_cache.cmdExePath()
        else
            full_app_name;

        if (windowsCreateProcess(app_name_w.ptr, cmd_line_w.ptr, env_block, cwd_ptr, flags, lpStartupInfo, lpProcessInformation)) |_| {
            return;
        } else |err| switch (err) {
            error.FileNotFound => continue,
            error.AccessDenied => continue,
            error.InvalidExe => {
                // On InvalidExe, if the extension of the app name is .exe then
                // it's treated as an unrecoverable error. Otherwise, it'll be
                // skipped as normal.
                if (windows.eqlIgnoreCaseWtf16(ext, std.unicode.utf8ToUtf16LeStringLiteral(".EXE"))) {
                    return error.UnrecoverableInvalidExe;
                }
                continue;
            },
            else => return err,
        }
    }

    return unappended_err;
}