BA requests get dropped when UI is shown on engine thread

[rseanhall]

Bugs

If this issue is a bug:

• Which version of WiX are you building with?

latest v3 and v4

• Which version of Visual Studio are you building with (if any)?

N/A

• Which version of the WiX Toolset Visual Studio Extension are you building with (if any)?

N/A

• Which version of .NET are you building with?

N/A

• If the problem occurs when installing your packages built with WiX, what is the version of Windows the package is running on?

N/A

• Describe the problem and the steps to reproduce it.

Burn uses PostThreadMessage to queue up the BA requests for actions like Detect, Plan, and Apply. If the BA shows UI in a callback, then those messages will be thrown away. This also applies when the engine shows the UAC prompt for the elevated engine. To reproduce:

1. Launch the bundle unelevated.
2. Have the BA call Elevate.
3. Have the BA call Plan inside OnElevate (OnElevateBegin in v4).
4. The UAC prompt should come up, click Yes.

• Describe the behavior you expected and how it differed from the actual behavior.

Expected: The engine processes the plan request after elevation completes.
Actual: The engine doesn't plan.

Raymond Chen post - Why do messages posted by PostThreadMessage disappear?

[BMurri]

I'd like to fix this

[rseanhall]

For implementing our own queue, I like how Raymond Chen built a linked list in co_await awaitable signal part 4.

[BMurri]

Last time I had time to wrap this up, I couldn't build v4. I'll be able to try again this weekend, so this should be in PR soon.

[barnson]

@BMurri we're targeting 4.0-preview.1 for Halloween so if you're going to implement this, it would need to be in sometime next week or so.

[rseanhall]

I wanted to make sure this got into v4 so I went ahead and did it.