sql: fix pausable portal cleanup stack behavior

We have several cleanup "stacks" for pausable portals model, but in
reality previously we executed the functions in the "forward" direction,
matching the behavior of a "queue", so this commit fixes that iterating
over functions in reverse order. Only `dispatchToExecutionEngine` stage
has multiple cleanup functions. Ensuring the correct order was necessary
for the following commit which is what prompted this one.

Additionally, we fix comments to reflect the reverse order of execution
different cleanup stacks.

Release note: None

Author: yuzefovich

pkg/sql/prepared_stmt.go

@@ -188,7 +188,8 @@ func (n *cleanupFuncStack) appendFunc(f func(context.Context)) {
 }
 
 func (n *cleanupFuncStack) run(ctx context.Context) {
-	for i := 0; i < len(n.stack); i++ {
+	// Iterate in reverse order to simulate stack behavior.
+	for i := len(n.stack) - 1; i >= 0; i-- {
 		n.stack[i](ctx)
 	}
 	*n = cleanupFuncStack{}
@@ -226,18 +227,18 @@ type portalPauseInfo struct {
 	// When closing a portal, we need to follow the reverse order of its execution,
 	// which means running the cleanup functions of the four structs in the
 	// following order:
-	//   - exhaustPortal.cleanup
-	//   - execStmtInOpenState.cleanup
-	//   - dispatchToExecutionEngine.cleanup
 	//   - resumableFlow.cleanup
+	//   - dispatchToExecutionEngine.cleanup
+	//   - execStmtInOpenState.cleanup
+	//   - exhaustPortal.cleanup
 	//
 	// If an error occurs in any of these functions, we run the cleanup functions of
 	// this layer and its children layers, and propagate the error to the parent
 	// layer. For example, if an error occurs in execStmtInOpenStateCleanup(), we
 	// run the cleanup functions in the following order:
-	//   - execStmtInOpenState.cleanup
-	//   - dispatchToExecutionEngine.cleanup
 	//   - resumableFlow.cleanup
+	//   - dispatchToExecutionEngine.cleanup
+	//   - execStmtInOpenState.cleanup
 	//
 	// When exiting connExecutor.execStmtInOpenState(), we finally run the
 	// exhaustPortal.cleanup function in connExecutor.execPortal().