tracing: fix child span optimization

When given a context with a non-recording (but real) Span, we would
return the incoming context. This would lead to an extra premature
call to `Finish()` and subsequent use-after-Finish of the Span, which
can blow up and/or hang, most of the time within net/trace code.

Prior to this commit, the crash reproduced readily (within seconds) via

```
make stress PKG=./pkg/sql TESTS=TestTrace
```

and I had no issues for ~10 minutes with this commit.

Fixes #57875.

Release note: None

pkg/util/tracing/span_test.go

@@ -11,7 +11,6 @@
 package tracing
 
 import (
-	"context"
 	"regexp"
 	"strings"
 	"testing"
@@ -182,19 +181,3 @@ Span grandchild:
 `
 	require.Equal(t, exp, recToStrippedString(childRec))
 }
-
-func TestChildSpan(t *testing.T) {
-	tr := NewTracer()
-	// Set up non-recording span.
-	sp := tr.StartSpan("foo", WithForceRealSpan())
-	ctx := ContextWithSpan(context.Background(), sp)
-	// Since the parent span was not recording, we would expect the
-	// noop span back. However - we don't, we get our inputs instead.
-	// This is a performance optimization; there is a to-do in
-	// childSpan asking for its removal, but for now it's here to stay.
-	{
-		newCtx, newSp := ChildSpan(ctx, "foo")
-		require.Equal(t, ctx, newCtx)
-		require.Equal(t, sp, newSp)
-	}
-}

pkg/util/tracing/tracer.go

@@ -559,14 +559,13 @@ func childSpan(ctx context.Context, opName string, remote bool) (context.Context
 		opts.RemoteParent = sp.Meta()
 	}
 	newSpan := tr.startSpanGeneric(opName, opts)
-	if newSpan.isNoop() {
-		// Optimization: if we end up with a noop, return the inputs
-		// to avoid ContextWithSpan call below.
-		//
-		// TODO(tbg): this is unsound. We are returning the incoming
-		// context which may have a Span that could later start recording.
-		// So in effect that span may capture parts of two goroutines
-		// accidentally.
+	if sp.isNoop() && newSpan.isNoop() {
+		// Optimization: if we started and end up with a noop, we can return
+		// the original context to save on the ContextWithSpan alloc. Note
+		// that it is important that the incoming span was the noop span. If
+		// it was a real, non-recording span, it might later start recording.
+		// Besides, the caller expects to get their own span, and will
+		// .Finish() it, leading to an extra, premature call to Finish().
 		return ctx, sp
 	}
 	return ContextWithSpan(ctx, newSpan), newSpan