Debug Support for Any Exceptions and Uncaught Exceptions

[sgtoj]

Is it possible to debug support to enable break points for: Any Exceptions or Uncaught Exceptions?

[daviwil]

I'm not sure how this would work in the case of PowerShell. What do you think, @rkeithhill?

[rkeithhill]

I've wanted this for a while but I think PowerShell would need to be modified to support this. Who's the debugger expert on the team? Paul H?

FYI, I filed a UserVoice suggestion on this back in March - please vote it up: https://windowsserver.uservoice.com/forums/301869-powershell/suggestions/12843021-enhance-set-psbreakpoint-to-allow-us-to-set-except

[daviwil]

Yep, @PaulHigin is the PowerShell debugging expert.

[PaulHigin]

This is a fairly common request and something we have in our backlog, but is impossible to implement with our current script debugger. The reason is that the script debugger runs on the script execution thread and requires running script to work. But once an exception is thrown on the script execution thread the script is no longer running and the script debugger no longer works. At this point you need a managed/native debugger. We could have script debugger break for non-terminating errors since script execution continues and the script debugger is working. But I am not sure how useful that would be.

Jason, @lzybkr, has experimented with a mixed managed code/script debugger and it would be ideal for this kind of thing. When the managed exception is thrown this debugger could show both managed and PowerShell script stacks.

[daviwil]

The mixed code/script debugger would be extremely nice to have and would be pretty easy to surface in VS Code.

[lzybkr]

That prototype generated a pdb and code that the C# debugger could consume - it was useful, but not a PowerShell experience.

[rkeithhill]

@PaulHigin What do you think about this workaround by @nightroman? https://github.com/nightroman/PowerShelf/blob/master/Debug-Error.ps1

[PaulHigin]

Very clever. I like it. But I believe this just handles the case where PowerShell is throwing the exception (either through throw keyword or -ErrorAction Stop). @lzybkr can you confirm? I was thinking in terms of a general exception being thrown in managed code. Still this looks to be very useful and something we could support internally.

[nightroman]

Unfortunately, Debug-Error.ps1 does not show the error itself, it just breaks into the debugger. The error has not been written yet (i.e. added to $Error) by PowerShell. So you have to guess what has happened. Sometimes this is clear, sometimes not. The tool is useful in special cases, rather rare.

[andrewducker]

Any movement on this? At the moment I'm reduced to stepping through my module (and all of its dependencies) line by line trying to find where the exception is being thrown. Or adding a load of Write-Host statements to see which ones get hit.

[SeeminglyScience]

There is a way to break before the terminating error is thrown, but after it's been written to $Error. It isn't very clean though.

Set-PSBreakpoint -Variable StackTrace -Mode Write -Action {
    $null = Set-PSBreakpoint -Variable ErrorActionPreference -Mode Read
}

throw 'test'
# In debugger:
$Error[0]
# Returns:
# test
# At line:1 char:1
# + throw 'test'
# + ~~~~~~~~~~~~
#     + CategoryInfo          : OperationStopped: (test:String) [], RuntimeException
#     + FullyQualifiedErrorId : test

After $StackTrace is written to, $ErrorActionPreference is checked to see if the error should be terminating. When it checks $ErrorActionPreference it has already written to $Error regardless of the preference value.

For this to be reliable you would need to store state between the two breakpoints to ensure they fire on the same sequence point (in case the script later reads the preference directly). It would also need to handle situations where the break point already exists and be able to remove itself after the first hit.

I'll start looking into the feasibility of adding support for this.