Reject lossy ETS projections

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: MariusStorhaug

src/functions/private/Add-YamlDictionaryEntry.ps1

@@ -0,0 +1,37 @@
+function Add-YamlDictionaryEntry {
+    <#
+        .SYNOPSIS
+        Adds a projected mapping entry with a YAML-classified collision error.
+    #>
+    [Diagnostics.CodeAnalysis.SuppressMessageAttribute(
+        'PSUseShouldProcessForStateChangingFunctions', '',
+        Justification = 'Mutates an internal projection dictionary.'
+    )]
+    [CmdletBinding()]
+    param (
+        [Parameter(Mandatory)]
+        [System.Collections.IDictionary] $Dictionary,
+
+        [Parameter(Mandatory)]
+        [object] $Key,
+
+        [Parameter(Mandatory)]
+        [AllowNull()]
+        [object] $Value,
+
+        [Parameter(Mandatory)]
+        [pscustomobject] $KeyNode
+    )
+
+    try {
+        $Dictionary.Add($Key, $Value)
+    } catch [System.Management.Automation.MethodInvocationException] {
+        if ($_.Exception.InnerException -isnot [System.ArgumentException]) {
+            throw
+        }
+        throw (New-YamlException -Start $KeyNode.Start -End $KeyNode.End `
+                -ErrorId 'YamlProjectionKeyCollision' -Message (
+                'Distinct YAML mapping keys cannot be projected distinctly into a PowerShell dictionary.'
+            ))
+    }
+}

src/functions/private/ConvertFrom-YamlNode.ps1

@@ -154,7 +154,9 @@ function ConvertFrom-YamlNode {
             continue
         }
         if ($frame.State -eq 'OmapValue') {
-            $frame.Result.Add($frame.Key, $frame.Child.PSObject.Properties['Value'].Value)
+            Add-YamlDictionaryEntry -Dictionary $frame.Result -Key $frame.Key `
+                -Value $frame.Child.PSObject.Properties['Value'].Value `
+                -KeyNode $frame.Node.Items[$frame.Index].Entries[0].Key
             $frame.Index++
             $frame.State = 'OmapKey'
             continue
@@ -188,7 +190,8 @@ function ConvertFrom-YamlNode {
                 $frame.Key = $keyValue
             }
             if ($frame.Node.Tag -ceq 'tag:yaml.org,2002:set') {
-                $frame.Result.Add($frame.Key, $null)
+                Add-YamlDictionaryEntry -Dictionary $frame.Result -Key $frame.Key -Value $null `
+                    -KeyNode $frame.Node.Entries[$frame.Index].Key
                 $frame.Index++
                 $frame.State = 'DictionaryKey'
                 continue
@@ -209,7 +212,9 @@ function ConvertFrom-YamlNode {
             continue
         }
         if ($frame.State -eq 'DictionaryValue') {
-            $frame.Result.Add($frame.Key, $frame.Child.PSObject.Properties['Value'].Value)
+            Add-YamlDictionaryEntry -Dictionary $frame.Result -Key $frame.Key `
+                -Value $frame.Child.PSObject.Properties['Value'].Value `
+                -KeyNode $frame.Node.Entries[$frame.Index].Key
             $frame.Index++
             $frame.State = 'DictionaryKey'
             continue
@@ -244,6 +249,13 @@ function ConvertFrom-YamlNode {
                         'This mapping key cannot be represented as a PSCustomObject property. Use -AsHashtable.'
                     ))
             }
+            if (Test-YamlReservedPropertyName -Name $frame.Key) {
+                $keyNode = $frame.Node.Entries[$frame.Index].Key
+                throw (New-YamlException -Start $keyNode.Start -End $keyNode.End `
+                        -ErrorId 'YamlPropertyNameReserved' -Message (
+                        "The mapping key '$($frame.Key)' is reserved by PowerShell ETS. Use -AsHashtable."
+                    ))
+            }
             if (-not $frame.Names.Add($frame.Key)) {
                 $keyNode = $frame.Node.Entries[$frame.Index].Key
                 throw (New-YamlException -Start $keyNode.Start -End $keyNode.End `

src/functions/private/Get-YamlSerializationShape.ps1

@@ -57,13 +57,22 @@ function Get-YamlSerializationShape {
         -not $isByteArray
     )
     $isCustomObject = $Value -is [System.Management.Automation.PSCustomObject]
+    $isPurePropertyBag = $isCustomObject -or (
+        $dataProperties.Count -gt 0 -and $Value.GetType() -eq [object]
+    )
     if ($dataProperties.Count -gt 0 -and ($isDictionary -or $isSequence -or $isByteArray)) {
         throw (New-YamlSerializationException -Kind NotSupported `
                 -ErrorId 'YamlMixedObjectSemantics' -Message (
                 "Type '$($Value.GetType().FullName)' combines collection data with attached note properties and cannot be represented without loss."
             ))
     }
-    $isPropertyBag = $isCustomObject -or $dataProperties.Count -gt 0
+    if ($dataProperties.Count -gt 0 -and -not $isPurePropertyBag) {
+        throw (New-YamlSerializationException -Kind NotSupported `
+                -ErrorId 'YamlMixedObjectSemantics' -Message (
+                "Type '$($Value.GetType().FullName)' combines scalar data with attached note properties and cannot be represented without loss."
+            ))
+    }
+    $isPropertyBag = $isPurePropertyBag
 
     if (-not $isPropertyBag -and ($Value -is [string] -or $Value -is [char])) {
         $scalar.Value = [string] $Value

src/functions/private/Test-YamlReservedPropertyName.ps1

@@ -0,0 +1,25 @@
+function Test-YamlReservedPropertyName {
+    <#
+        .SYNOPSIS
+        Tests whether a property name is reserved by PowerShell ETS.
+    #>
+    [CmdletBinding()]
+    [OutputType([bool])]
+    param (
+        [Parameter(Mandatory)]
+        [string] $Name
+    )
+
+    foreach ($reservedName in @(
+            'PSObject',
+            'PSTypeNames',
+            'PSBase',
+            'PSAdapted',
+            'PSExtended'
+        )) {
+        if ($Name.Equals($reservedName, [System.StringComparison]::OrdinalIgnoreCase)) {
+            return $true
+        }
+    }
+    return $false
+}

tests/ConvertFrom-Yaml.Tests.ps1

@@ -125,6 +125,23 @@ folded: >
             $result['name'] | Should -Be 'two'
         }
 
+        It 'classifies ETS-reserved property names and preserves them with AsHashtable' {
+            foreach ($name in @(
+                    'PSObject',
+                    'psobject',
+                    'PSTypeNames',
+                    'PSBase',
+                    'PSAdapted',
+                    'PSExtended'
+                )) {
+                { "$name`: value" | ConvertFrom-Yaml } |
+                    Should -Throw -ExpectedMessage '*Use -AsHashtable*'
+
+                $result = "$name`: value" | ConvertFrom-Yaml -AsHashtable
+                $result[$name] | Should -Be 'value'
+            }
+        }
+
         It 'enumerates only top-level sequences by default' {
             $result = "- one`n- two" | ConvertFrom-Yaml
 
@@ -346,6 +363,14 @@ copy: *source
             $result['set']['one'] | Should -BeNullOrEmpty
         }
 
+        It 'classifies projection collisions from representation-distinct tagged keys' {
+            $yaml = "!foo x: one`n!bar x: two"
+
+            ($yaml | Test-Yaml) | Should -BeTrue
+            { $yaml | ConvertFrom-Yaml -AsHashtable } |
+                Should -Throw -ExpectedMessage '*cannot be projected distinctly*'
+        }
+
         It 'uses deterministic UTC semantics for zone-less explicit timestamps' {
             $result = @'
 offset: !!timestamp 2001-12-14T21:59:43.1+5:30

tests/ConvertTo-Yaml.Tests.ps1

@@ -322,6 +322,14 @@ Describe 'ConvertTo-Yaml' {
                 Should -Throw -ExpectedMessage '*combines collection data with attached note properties*'
         }
 
+        It 'rejects attached note properties on scalar values' {
+            $value = [psobject] 42
+            Add-Member -InputObject $value -MemberType NoteProperty -Name metadata -Value 'lossy'
+
+            { ConvertTo-Yaml -InputObject $value } |
+                Should -Throw -ExpectedMessage '*combines scalar data with attached note properties*'
+        }
+
         It 'preserves the sign of IEEE negative zero across serialization' {
             $negativeZero = [BitConverter]::Int64BitsToDouble([long]::MinValue)
             $yaml = ConvertTo-Yaml -InputObject $negativeZero