kvserver: use Background() in computeChecksumPostApply goroutine

On the leaseholder, `ctx` passed to `computeChecksumPostApply` is that
of the proposal. As of #71806, this context is canceled right after the
corresponding proposal is signaled (and the client goroutine returns
from `sendWithRangeID`). This effectively prevents most consistency
checks from succeeding (they previously were not affected by
higher-level cancellation because the consistency check is triggered
from a local queue that talks directly to the replica, i.e. had
something like a minutes-long timeout).

This caused disastrous behavior in the `clearrange` suite of roachtests.
That test imports a large table. After the import, most ranges have
estimates (due to the ctx cancellation preventing the consistency
checks, which as a byproduct trigger stats adjustments) and their stats
claim that they are very small. Before recent PR #74674, `ClearRange` on
such ranges would use individual point deletions instead of the much
more efficient pebble range deletions, effectively writing a lot of data
and running the nodes out of disk.

Failures of `clearrange` with #74674 were also observed, but they did
not involve out-of-disk situations, so are possibly an alternative
failure mode (that may still be related to the newly introduced presence
of context cancellation).

Touches #68303.

Release note: None

pkg/kv/kvserver/replica_proposal.go

@@ -236,7 +236,9 @@ func (r *Replica) computeChecksumPostApply(ctx context.Context, cc kvserverpb.Co
 	}
 
 	// Compute SHA asynchronously and store it in a map by UUID.
-	if err := stopper.RunAsyncTask(ctx, "storage.Replica: computing checksum", func(ctx context.Context) {
+	// Don't use the proposal's context for this, as it likely to be canceled very
+	// soon.
+	if err := stopper.RunAsyncTask(r.AnnotateCtx(context.Background()), "storage.Replica: computing checksum", func(ctx context.Context) {
 		func() {
 			defer snap.Close()
 			var snapshot *roachpb.RaftSnapshotData
@@ -504,7 +506,7 @@ func (r *Replica) leasePostApplyLocked(
 	if iAmTheLeaseHolder {
 		// NB: run these in an async task to keep them out of the critical section
 		// (r.mu is held here).
-		_ = r.store.stopper.RunAsyncTask(ctx, "lease-triggers", func(ctx context.Context) {
+		_ = r.store.stopper.RunAsyncTask(r.AnnotateCtx(context.Background()), "lease-triggers", func(ctx context.Context) {
 			// Re-acquire the raftMu, as we are now in an async task.
 			r.raftMu.Lock()
 			defer r.raftMu.Unlock()