Intermittent long delay after calling showOpenDialog before extension code continues to run

[DanTup]

In the Dart extension we have a "New Project" command that asks the user for some input (such as a directory and a project name) and creates a new project for them. A while back, I noticed that between prompting for a folder (using showOpenDialog) and the project name (using an input box) there was sometimes a long delay of ~10 seconds. It doesn't happen all the time, and seems to reproduce most often if I have left VS Code open for > 10 minutes and then tried to execute the command (this might just be coincidence though, it's quite intermittent).

I reviewed the code, and there was basically nothing between those two calls that could take any time. We do some fs.exist() calls to compute a project name, however I added some console-log timing and found the delay actually occurs while at one of the awaits (such as getting the response from showOpenDialog). I assumed there was some other extension code being triggered that was blocking the extension host preventing that code from continuing for a while.

However, I was able to capture some extension host profiles while reproducing the issue, and they don't contain any code:

slow_flutter_new_project.cpuprofile.txt

So my feeling is that this delay might be occurring outside of the extension host inside one of the other processes. I was able to reproduce it while profiling from the built-in Chrome dev tools, however that also doesn't seem to show anything:

vs_code_devtools_profile-20250619T165606.json

I noticed when trying to use --prof-startup that there is mention of a "renderer" process, but I don't know if that's the same one I profiled above (--prof-startup is unfortunately broken - see #251014 - and it's not clear to me if that would keep profiling enough for me to reproduce anyway).

So, that leaves me stuck - I'm seeing a long delay, but neither of the profiles seem to provide any clue what it is. I'm hoping someone can provide some suggestions about what else I can profile to try and track down what is occurring during this delay.

I can provide steps to reproduce, but since it's quite intermittent, I'm not sure if that will help:

1. Install the Dart + Flutter extensions
2. Set up a Flutter SDK
3. Run the Flutter: New Project command
4. Choose a folder to create the project in
5. (you may see a long delay of > 10s here)
6. You'll now be prompted for a name for the project

Most of the time it is instant, but now and then it takes a long time.

My main request is here is for instructions on how to better profile this and understand what is causing a delay in code running (it may or may not be related to showOpenDialog, it could just be something else blocking the thread - but I'm struggling to profile what).

[DanTup]

@bpasero you added the help-wanted tag, but it's not clear to me how to help with this. I'm happy to do more investigation, however I think I need to know how to better profile all of the involved processes (which might be as simple as waiting for #251014 to be fixed, I'm not sure).

Right now, all of the CPU profiling I have been able to capture just doesn't show any code running.

(I guess as a fallback, I can try to repro from source, and just litter the code with prints, but I feel like there must be a better way)

[bpasero]

It is an invitation for the open source community to chime in and figure this issue out. You know the drill, a minimal extension sample is the min-bar for any engineering involvement, anything else just does not scale with the amount of issues we get.

[DanTup]

@bpasero I understand, but I'm not asking for the Code team to look at it - I'm asking for some pointers so I can debug it.

I've been unable to make an isolated repro, and I suspect others will struggle to reproduce it. I have a fairly reliable trigger for it, but I don't know how to profile it - I've followed all the profiling instructions I found and turned up nothing. I feel like this needs some pointers on how we could go about tracking it down (if instrumenting VS Code with logging and reproducing from source is the best way, then so be it, I'll do that, but I feel like perhaps it is not).

[bpasero]

I would try with the "Performance" tab in devtools as well as extension host profile support we have when you open the Running Extensions editor: