BDN uses std input and output to synchronize the host and benchmark process (it's simple and it usually works everywhere):

• the benchmark process writes text to std output
  

  BenchmarkDotNet/src/BenchmarkDotNet/Templates/BenchmarkProgram.txt

  Line 35 in fa59841

  BenchmarkDotNet.Engines.HostExtensions.BeforeAnythingElse(host);

• host process reads it and takes an action if needed
  

  BenchmarkDotNet/src/BenchmarkDotNet/Loggers/SynchronousProcessOutputLoggerWithDiagnoser.cs

  Lines 55 to 57 in fa59841

  	else if (Engine.Signals.TryGetSignal(line, out var signal))
  	{
  	diagnoser?.Handle(signal, diagnoserActionParameters);

• host process writes "Acknowledgment" to the benchmark process input
  

  BenchmarkDotNet/src/BenchmarkDotNet/Loggers/SynchronousProcessOutputLoggerWithDiagnoser.cs

  Line 61 in fa59841

  process.StandardInput.WriteLine(Engine.Signals.Acknowledgment);

• the benchmark process waits until it reads the ack. It throws on invalid ack:
  

  BenchmarkDotNet/src/BenchmarkDotNet/Engines/ConsoleHost.cs

  Lines 32 to 34 in fa59841

  	string acknowledgment = inReader.ReadLine();
  	if (acknowledgment != Engine.Signals.Acknowledgment)
  	throw new NotSupportedException($"Unknown Acknowledgment: {acknowledgment}");

Example: benchmark process prints "// AfterAll", host process starts the disassembler, attaches to the benchmark process, gets the disassembly and writes the ack.

This causes 3 kinds of issues:

• when the benchmark itself writes to the console, it can disturb the communication like in how to debug reflection error #1933
• there can be encoding mismatch like in System.NotSupportedException: Unknown Acknowledgment: Acknowledgment when running in a github action #1713 (in theory we have fixed that)
• confusion for users who want to use the produced executable to repro a bug and need to manually write "Acknowledgment" after each signal (cc @AndyAyersMS, @kunalspathak, @Maoni0)

With this PR the blocking acknowledgments will be enabled only for users who use a diagnoser that actually needs it. And users will be able to run the produced exe without providing any input: