param(
    [Parameter(Mandatory = $true)][string]$Url,
    [Parameter(Mandatory = $true)][string]$Out,
    [Parameter(Mandatory = $true)][string]$Lock
)

# Download notify.exe with curl to a temp file, then atomic-rename on success.
# Always remove the lock dir at the end (success or failure).
$tmp = $Out + ".downloading"
try {
    & curl.exe -fsSL $Url -o $tmp
    if ((Test-Path -LiteralPath $tmp) -and ((Get-Item -LiteralPath $tmp).Length -gt 0)) {
        Move-Item -LiteralPath $tmp -Destination $Out -Force
    }
}
finally {
    Remove-Item -LiteralPath $tmp -Force -ErrorAction SilentlyContinue
    Remove-Item -LiteralPath $Lock -Recurse -Force -ErrorAction SilentlyContinue
}