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Hovering over very large lists makes debugging unresponsive because results are not paged #5532
Description
Describe the bug (and reproduction)
Say you have this code:
void main() {
Uint8List data = Uint8List(268435456);
foo(data);
}
void foo(Uint8List data) {
print("hello data of size ${data.length}");
}and has a breakpoint on the foo call.
On the left under "Varaibles" you'll see "Uint8List (268435456 items)" and drop-downs breaks it into smaller chunks before eventually it requests 100 elements via a variables call with option "count":100. This works well.
If instead you place your mouse over the data variable in the source code you're in for a bad time because it requests without any "count" parameter and the VM then happily does what it was asked on all 268435456 elements - eventually (on my machine) running out of memory somewhere in return elements.mapIndexed([...]).toList(); in ProtocolConverter.convertVmInstanceToVariablesList.
Until it fails (after a while) debugging is unresponsive, and the debugging session can't even be stopped (without killing the running dart debug_adapter processing).
I suspect that if we had a size that was small enough that it wouldn't cause out-of-memory on the dart side the IDE would be in for a bad time once it had to receive that data. (Okay, I couldn't help myself: Reducing it to 1mio does indeed make the IDE use lots of CPU and lots of memory. There might even be a memory leak - after doing it with 1 mio 5 times (I think - that is start a debugging session, hover, click "continue" and wait until it actually continues, start the 2nd one etc) the memory consumption of the (remote) vs code according to the chrome task manager is now 2.1+ GB...)
Expected behavior
Either it only shows the first x (100?) entries, or (preferably) it would give the same drop-downs as the variables on the left.
Please complete the following information:
Dart Code extension: 3.112.0
Flutter extension: 3.112.0 (not activated)
App: code-server
App Host: server-distro
Remote: 127.0.0.1:49363
Host Kind: server-distro
Version: linux 1.91.1
Workspace type: Dart (LSP)
Workspace name: profiling
Dart (3.9.0-edge): (my checkout)
Flutter (3.29.3): (my checkout)
Activity
DanTup commentedon Jun 4, 2025
Thanks for the detailed info! I can reproduce this, and can see from the logs that when you perf an evaluation (hover or typing into the debug console), we don't include information required for VS Code to handle paging like we do when we return the variables for scopes (the "Variables") pane.
Edit: The same is true for normal lists, not only Uint8List.
[-]Hovering a large Uint8List makes debugging unresponsive[/-][+]Hovering over very large lists makes debugging unresponsive because results are not paged[/+]DanTup commentedon Jun 4, 2025
I've a fix open at https://dart-review.googlesource.com/c/sdk/+/432941. With that change, the lists are paged like this (in both Hovers and Debug Console):
[dds/dap] Include paging data for variables returned as top-level res…
DanTup commentedon Jun 4, 2025
Fixed by dart-lang/sdk@9756ab9. The fix will show up in a future SDK release.