[CmdletBinding()]
param(
    [Parameter(Mandatory = $true)]
    [ValidateSet("NoExistingFunction", "ExistingFunction")]
    [string]$Mode,
    [Parameter(Mandatory = $true)][string]$SupportPath
)

$ErrorActionPreference = "Stop"
. $SupportPath

if ($Mode -ceq "NoExistingFunction") {
    $before = Get-MinerOpsNativeGetItemState
    if ([bool]$before.FunctionExisted) {
        throw "Child process unexpectedly began with a global Get-Item function."
    }
    $observed = @(Invoke-MinerOpsNativeGetItemTrace -Action {
        Get-Item -LiteralPath $SupportPath
    })
    if ($observed -cnotcontains $SupportPath) {
        throw "Trace did not observe Get-Item."
    }
    Assert-MinerOpsNativeGetItemState -Expected $before `
        -Context "no-preexisting-function child test"
    return
}

$existing = { param($LiteralPath) "preexisting:$LiteralPath" }
Set-Item -LiteralPath "Function:\global:Get-Item" -Value $existing -Force
$before = Get-MinerOpsNativeGetItemState
$observed = @(Invoke-MinerOpsNativeGetItemTrace -Action {
    Get-Item -LiteralPath $SupportPath
})
if ($observed -cnotcontains $SupportPath) {
    throw "Trace did not observe Get-Item over an existing global function."
}
$after = Get-MinerOpsNativeGetItemState
Assert-MinerOpsNativeGetItemState -Expected $before `
    -Context "preexisting-function child test"
if ($after.FunctionScriptBlock.ToString() -cne $existing.ToString()) {
    throw "Existing Get-Item ScriptBlock was not restored exactly."
}
if (-not [object]::ReferenceEquals(
    $after.FunctionScriptBlock,
    $before.FunctionScriptBlock
)) {
    throw "Existing Get-Item ScriptBlock instance changed."
}