Query Store, History and Overview can report a cancelled query as an error

[erikdarlingdata]

Claude posting for Erik Darling

Found while working on #625 (PR #626). This comes from a SqlClient probe and from reading the code. I did not reproduce it in the app.

What happens

Microsoft.Data.SqlClient 7.1.0 does not always report a cancel as OperationCanceledException. A probe against SQL Server 2022 gave these results:

When the token is cancelled	What SqlClient throws
The server is still running the query	SqlException: "A severe error occurred on the current command. The results, if any, should be discarded. Operation cancelled by user."
Rows are streaming through reader.ReadAsync(ct)	TaskCanceledException

The first case is the usual one. It happens when a user cancels a slow query, and when a newer fetch replaces an older one. #626 fixed the three plan-capture paths. Their handlers now treat any exception as a cancel when the run's token was cancelled:

catch (Exception ex) when (ex is OperationCanceledException || ct.IsCancellationRequested)

The handlers below still decide by the exception type alone. A cancel that arrives as a SqlException goes to the error branch.

Where	On OperationCanceledException	On any other exception
QueryStoreGridControl.Fetch.cs:87 (time slicer load)	status "Cancelled."	status shows ex.Message
QueryStoreGridControl.Fetch.cs:140 (plan fetch)	status "Cancelled."	status shows ex.Message
QueryStoreGridControl.Sort.cs:68	nothing	status shows ex.Message
QueryStoreGridControl.WaitStats.cs:41	rethrows	status shows "Slicer: " and ex.Message
QueryStoreHistoryControl.Fetch.cs:78	status "Cancelled."	status shows ex.Message
QueryStoreOverviewControl.axaml.cs:167 and :395	nothing	the "Last refresh failed" badge (ShowRefreshError)
QueryStoreOverviewControl.axaml.cs:262	nothing	goes to QuerySessionControl.Views.cs:162, which shows it in the status strip or the badge unless a newer load replaced it
QuerySessionControl.Editor.cs:464 (SetStatusFromException)	nothing	red status with ex.Message

SetStatusFromException has no token. Its callers in QuerySessionControl.QueryStore.cs, QuerySessionControl.Schema.cs and QuerySessionControl.Views.cs must pass one, or check the token themselves.

Two handlers do not have the problem. QueryStoreGridControl.axaml.cs:218 checks the token in its general catch. QuerySessionControl.Connection.cs:226 ignores every exception.

Expected

A cancelled fetch shows "Cancelled." or nothing, the same as an OperationCanceledException does today. A real failure still shows its error.

Probe

A scratch console project that references PlanViewer.Core ran WAITFOR DELAY '00:00:10' against SQL2022 and cancelled the token after 1.5 seconds. A second run cancelled the token while a large result set streamed through ReadAsync.

[erikdarlingdata]

Fixed by #635, which is merged to dev. A cancelled Query Store, History or Overview fetch now shows "Cancelled." or nothing, and a real failure still shows its error. The fix ships in the next release.