Fail the parallel run when a worker returns no result

src/functions/Pester.Parallel.ps1

@@ -478,6 +478,24 @@ function Invoke-TestInParallel {
     # Keep only well-formed worker results (defensive against stray pipeline output).
     $results = @($results | & $SafeCommands['Where-Object'] { $_ -is [System.Management.Automation.PSCustomObject] -and $null -ne $_.PSObject.Properties['Containers'] })
 
+    # The filter above cannot tell stray output from a worker that died before it returned its
+    # result object, so on its own it would drop a whole test file and let the run report success
+    # with fewer files than it was given. Losing results silently is worse than failing, so compare
+    # what came back against what was sent and name the files that went missing. The worker's own
+    # error was already surfaced by Invoke-InRunspacePool, this says which file it cost us.
+    if ($results.Count -ne $work.Count) {
+        $returnedPaths = @{}
+        foreach ($r in $results) {
+            if ($null -ne $r.PSObject.Properties['Path']) { $returnedPaths[$r.Path] = $true }
+        }
+
+        $missing = @(foreach ($w in $work) {
+                if (-not $returnedPaths.ContainsKey($w.Path)) { $w.Path }
+            })
+
+        throw "Parallel run lost the results of $($missing.Count) of $($work.Count) file(s), the worker(s) running them did not return a result. See the errors above for why. Lost: $($missing -join ', ')"
+    }
+
     # Restore the original discovery order so replay and the merged run are deterministic
     # regardless of which worker finished first.
     $order = @{}

tst/Pester.RSpec.Parallel.ts.ps1

@@ -317,6 +317,57 @@ Describe 'Second' {
         }
     }
 
+    b "Lost worker results" {
+        t "fails the run instead of returning fewer files than it was given" {
+            # A worker that dies before returning its result object leaves nothing for the
+            # well-formed-result filter to keep, so without the count check the run would report
+            # success for the files that did come back and never mention the one that did not.
+            # Losing a test file silently is worse than failing, so this must throw and name it.
+            $folder = Join-Path ([IO.Path]::GetTempPath()) ([Guid]::NewGuid().Guid)
+            $null = New-Item -ItemType Directory -Path $folder -Force
+            foreach ($name in 'A', 'B', 'C') {
+                Set-Content -Path (Join-Path $folder "$name.Tests.ps1") -Value "Describe '$name' { It 'i' { 1 | Should -Be 1 } }"
+            }
+
+            try {
+                # Stand in for a worker that died: hand Invoke-TestInParallel a runspace-pool
+                # runner that drops B's result on the floor and returns the other two.
+                $err = & (Get-Module Pester) {
+                    param ($Root)
+
+                    $original = ${function:Invoke-InRunspacePool}
+                    ${function:Invoke-InRunspacePool} = {
+                        param ($InputObject, $ScriptBlock, $ThrottleLimit, $ItemParameterName = 'item', $Parameters = @{})
+                        foreach ($i in $InputObject) {
+                            if ($i.Path -like '*B.Tests.ps1') { continue }
+                            [PSCustomObject]@{ Path = $i.Path; Containers = @(); Tape = @(); Coverage = $null }
+                        }
+                    }
+
+                    try {
+                        $c = [PesterConfiguration]::Default
+                        $c.Run.Path = $Root
+                        $c.Run.Parallel = $true
+                        $c.Output.Verbosity = 'None'
+                        $containers = @(Find-File -Path $Root -Extension '.Tests.ps1' | ForEach-Object { New-BlockContainerObject -File $_ })
+
+                        try {
+                            $null = Invoke-TestInParallel -BlockContainer $containers -Configuration $c
+                            $null
+                        }
+                        catch { $_.Exception.Message }
+                    }
+                    finally { ${function:Invoke-InRunspacePool} = $original }
+                } $folder
+
+                $err | Verify-NotNull
+                $err | Verify-Like '*lost the results of 1 of 3 file(s)*'
+                $err | Verify-Like '*B.Tests.ps1*'
+            }
+            finally { Remove-Item -Path $folder -Recurse -Force }
+        }
+    }
+
     b "Invoke-InRunspacePool" {
         # The parallelism primitive Run.Parallel is built on. It replaces ForEach-Object -Parallel,
         # which does not exist on Windows PowerShell 5.1, so these run on both editions and are the