backupccl: put a goroutine under the stopper's control

The backup processor was spawning a naked goroutine. We don't like that
very much - see cockroachdb#58164. This patch puts that goroutine under the
Stopper. One benefit is that the goroutine gets its own span, so it's
resilient to the parent span being Finish()ed from under it (which was a
bug until the prior commit).

Release note: None

pkg/ccl/backupccl/BUILD.bazel

@@ -110,6 +110,7 @@ go_library(
         "//pkg/util/metric",
         "//pkg/util/protoutil",
         "//pkg/util/retry",
+        "//pkg/util/stop",
         "//pkg/util/syncutil",
         "//pkg/util/timeutil",
         "//pkg/util/tracing",

pkg/ccl/backupccl/backup_processor.go

@@ -38,6 +38,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/util/ctxgroup"
 	"github.com/cockroachdb/cockroach/pkg/util/hlc"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
+	"github.com/cockroachdb/cockroach/pkg/util/stop"
 	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
 	"github.com/cockroachdb/cockroach/pkg/util/tracing"
 	"github.com/cockroachdb/errors"
@@ -158,11 +159,18 @@ func (bp *backupDataProcessor) Start(ctx context.Context) {
 		for range bp.progCh {
 		}
 	}
-	go func() {
+	if err := bp.flowCtx.Stopper().RunAsyncTaskEx(ctx, stop.TaskOpts{
+		TaskName: "backup-worker",
+		SpanOpt:  stop.ChildSpan,
+	}, func(ctx context.Context) {
 		defer cancel()
 		defer close(bp.progCh)
 		bp.backupErr = runBackupProcessor(ctx, bp.flowCtx, &bp.spec, bp.progCh)
-	}()
+	}); err != nil {
+		bp.backupErr = err
+		cancel()
+		close(bp.progCh)
+	}
 }
 
 // Next is part of the RowSource interface.