SchemaStore.dispose() writes to an already-disposed output channel: "Channel has been closed" on every shutdown

[guanzhousongmicrosoft]

Reproduced against the CI-built vscode-documentdb-0.10.0.vsix from #883 (artifact of run 31513450303).

Summary

Every extension-host shutdown logs an unhandled error while disposing the extension's subscriptions. SchemaStore.dispose() calls logStats(), which writes to the shared output channel — but that channel has already been disposed by then, so the write throws.

[error] An error occurred when disposing the subscriptions for extension 'ms-azuretools.vscode-documentdb':
[error] Error: Channel has been closed
    at AzExtLogOutputChannel.appendLine (main.js:2:362407)
    at AzExtLogOutputChannel.appendLog  (main.js:2:362726)
    at SchemaStore.logStats            (main.js:2:2120687)
    at SchemaStore.dispose             (main.js:2:2123726)

Observed in 4 of 6 windows that reached a clean shutdown. It is the only extension-level error in any of those runs — everything else is clean.

Cause

The ordering is deterministic, not a race. activate() registers the output channel into context.subscriptions before it registers SchemaStore:

ext.outputChannel            -> subscriptions[i]
…
SchemaStore.getInstance()    -> subscriptions[j]     where j > i

VS Code disposes context.subscriptions in registration order, so the channel at i is closed before SchemaStore at j runs its own dispose(). logStats() then writes to a closed channel.

Worth noting the ext.outputChannel?. optional chaining in that path reads as a guard but isn't one — the object still exists, it is merely closed, so ?. passes and appendLine throws.

Steps to reproduce

1. Install vscode-documentdb-0.10.0.vsix.
2. Open a window and let the extension activate.
3. Close the window.
4. Check logs/<session>/window1/exthost/exthost.log.

Expected: shutdown completes without errors.
Actual: the stack above, on most shutdowns.

Impact

Not user-visible — it is log noise at shutdown, and no functionality is lost. Two reasons it is still worth fixing:

• It fires on every window close, reload, and extension update, so it is persistent noise in any log a user attaches to a bug report.
• VS Code aborts the remainder of that subscription-disposal pass when a disposable throws, so a genuine cleanup failure registered after SchemaStore could be masked by this one.

Suggested fix

Any of:

• Register SchemaStore before ext.outputChannel so it disposes first; or
• Have logStats() bail when the channel is closed (track a disposed flag rather than relying on ?.); or
• Move the stats logging out of dispose() entirely — emitting telemetry at shutdown is fine, but writing to a channel that is itself a subscription is inherently order-dependent.

The first is the smallest change; the second is the most robust if anything else ever logs during disposal.

Suggested milestone

0.10.1 — not a release blocker for 0.10.0 unless the reorder turns out to be a confident one-liner.