add dbg nullable support

src/PSParallelPipeline/Commands/InvokeParallelCommand.cs

@@ -77,11 +77,7 @@ protected override void BeginProcessing()
 
     protected override void ProcessRecord()
     {
-        if (_worker is null)
-        {
-            return;
-        }
-
+        Dbg.Assert(_worker is not null);
         this.ThrowIfInputObjectIsScriptBlock(InputObject);
 
         try
@@ -103,16 +99,13 @@ protected override void ProcessRecord()
         }
         catch (Exception exception)
         {
-            exception.WriteProcessOutputError(this);
+            exception.WriteUnspecifiedError(this);
         }
     }
 
     protected override void EndProcessing()
     {
-        if (_worker is null)
-        {
-            return;
-        }
+        Dbg.Assert(_worker is not null);
 
         try
         {
@@ -135,7 +128,7 @@ protected override void EndProcessing()
         }
         catch (Exception exception)
         {
-            exception.WriteEndProcessingError(this);
+            exception.WriteUnspecifiedError(this);
         }
     }
 

src/PSParallelPipeline/Dbg/Dbg.cs

@@ -0,0 +1,11 @@
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+namespace PSParallelPipeline;
+
+internal static class Dbg
+{
+    [Conditional("DEBUG")]
+    public static void Assert([DoesNotReturnIf(false)] bool condition) =>
+        Debug.Assert(condition);
+}

src/PSParallelPipeline/Dbg/Nullable.cs

@@ -0,0 +1,150 @@
+#if !NETCOREAPP
+
+namespace System.Diagnostics.CodeAnalysis;
+
+/// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
+[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class AllowNullAttribute : Attribute { }
+
+/// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary>
+[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class DisallowNullAttribute : Attribute { }
+
+/// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class MaybeNullAttribute : Attribute { }
+
+/// <summary>Specifies that an output will not be null even if the corresponding type allows it. Specifies that an input argument was not null when the call returns.</summary>
+[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class NotNullAttribute : Attribute { }
+
+/// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class MaybeNullWhenAttribute : Attribute
+{
+    /// <summary>Initializes the attribute with the specified return value condition.</summary>
+    /// <param name="returnValue">
+    /// The return value condition. If the method returns this value, the associated parameter may be null.
+    /// </param>
+    public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+    /// <summary>Gets the return value condition.</summary>
+    public bool ReturnValue { get; }
+}
+
+/// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
+[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class NotNullWhenAttribute : Attribute
+{
+    /// <summary>Initializes the attribute with the specified return value condition.</summary>
+    /// <param name="returnValue">
+    /// The return value condition. If the method returns this value, the associated parameter will not be null.
+    /// </param>
+    public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+    /// <summary>Gets the return value condition.</summary>
+    public bool ReturnValue { get; }
+}
+
+/// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class NotNullIfNotNullAttribute : Attribute
+{
+    /// <summary>Initializes the attribute with the associated parameter name.</summary>
+    /// <param name="parameterName">
+    /// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+    /// </param>
+    public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+    /// <summary>Gets the associated parameter name.</summary>
+    public string ParameterName { get; }
+}
+
+/// <summary>Applied to a method that will never return under any circumstance.</summary>
+[AttributeUsage(AttributeTargets.Method, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class DoesNotReturnAttribute : Attribute { }
+
+/// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary>
+[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class DoesNotReturnIfAttribute : Attribute
+{
+    /// <summary>Initializes the attribute with the specified parameter value.</summary>
+    /// <param name="parameterValue">
+    /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+    /// the associated parameter matches this value.
+    /// </param>
+    public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+    /// <summary>Gets the condition parameter value.</summary>
+    public bool ParameterValue { get; }
+}
+
+/// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values.</summary>
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+[ExcludeFromCodeCoverage]
+internal sealed class MemberNotNullAttribute : Attribute
+{
+    /// <summary>Initializes the attribute with a field or property member.</summary>
+    /// <param name="member">
+    /// The field or property member that is promised to be not-null.
+    /// </param>
+    public MemberNotNullAttribute(string member) => Members = new[] { member };
+
+    /// <summary>Initializes the attribute with the list of field and property members.</summary>
+    /// <param name="members">
+    /// The list of field and property members that are promised to be not-null.
+    /// </param>
+    public MemberNotNullAttribute(params string[] members) => Members = members;
+
+    /// <summary>Gets field or property member names.</summary>
+    public string[] Members { get; }
+}
+
+/// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values when returning with the specified return value condition.</summary>
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+[ExcludeFromCodeCoverage]
+internal sealed class MemberNotNullWhenAttribute : Attribute
+{
+    /// <summary>Initializes the attribute with the specified return value condition and a field or property member.</summary>
+    /// <param name="returnValue">
+    /// The return value condition. If the method returns this value, the associated parameter will not be null.
+    /// </param>
+    /// <param name="member">
+    /// The field or property member that is promised to be not-null.
+    /// </param>
+    public MemberNotNullWhenAttribute(bool returnValue, string member)
+    {
+        ReturnValue = returnValue;
+        Members = new[] { member };
+    }
+
+    /// <summary>Initializes the attribute with the specified return value condition and list of field and property members.</summary>
+    /// <param name="returnValue">
+    /// The return value condition. If the method returns this value, the associated parameter will not be null.
+    /// </param>
+    /// <param name="members">
+    /// The list of field and property members that are promised to be not-null.
+    /// </param>
+    public MemberNotNullWhenAttribute(bool returnValue, params string[] members)
+    {
+        ReturnValue = returnValue;
+        Members = members;
+    }
+
+    /// <summary>Gets the return value condition.</summary>
+    public bool ReturnValue { get; }
+
+    /// <summary>Gets field or property member names.</summary>
+    public string[] Members { get; }
+}
+
+#endif

src/PSParallelPipeline/ExceptionHelpers.cs

@@ -15,13 +15,13 @@ internal static void WriteTimeoutError(this Exception exception, PSCmdlet cmdlet
             ErrorCategory.OperationTimeout,
             cmdlet));
 
-    internal static void WriteEndProcessingError(this Exception exception, PSCmdlet cmdlet) =>
-        cmdlet.WriteError(new ErrorRecord(
-            exception, "EndProcessingOutput", ErrorCategory.NotSpecified, cmdlet));
+    // internal static void WriteEndProcessingError(this Exception exception, PSCmdlet cmdlet) =>
+    //     cmdlet.WriteError(new ErrorRecord(
+    //         exception, "EndProcessingOutput", ErrorCategory.NotSpecified, cmdlet));
 
-    internal static void WriteProcessOutputError(this Exception exception, PSCmdlet cmdlet) =>
+    internal static void WriteUnspecifiedError(this Exception exception, PSCmdlet cmdlet) =>
         cmdlet.WriteError(new ErrorRecord(
-            exception, "ProcessingOutput", ErrorCategory.NotSpecified, cmdlet));
+            exception, "UnspecifiedCmdletError", ErrorCategory.NotSpecified, cmdlet));
 
     internal static PSOutputData CreateProcessingTaskError(
         this Exception exception, object context) =>