sql/schemachanger/scgraph, scplan: fixed a bug when drawing dep graph

Previously, we define all stauses an element can be in in the
declarative schema changer in the scpb package. We removed one status
(TXN_DROPPED) previously from that list and leave its enum number as a
reserved number. However, some logic in scgraph incorrectly made the
assumption that all enum numbers are active and we can just iterate
from 0 to len(enum_list)-1 in order to iterate over all possible status,
part of the logic to draw the dep graph. This is problematic because as
we continue to add more status in that enum list, such way of iteration
will be incorrect to draw the dep graph. This PR fixes that.

This PR also spotted and fixed an panic recover bug where we forget to
correctly update the return error, causing a situation where if a panic
happens and the recover catches it, we will return with a nil error.

Release note (bug fix): Fixed a bug internal to drawing dependency
graph of a DDL statement under the declarative schema changer.

pkg/sql/schemachanger/scplan/internal/scgraph/iteration.go

@@ -23,7 +23,7 @@ type NodeIterator func(n *screl.Node) error
 // ForEachNode iterates the nodes in the graph.
 func (g *Graph) ForEachNode(it NodeIterator) error {
 	for _, m := range g.targetNodes {
-		for i := 0; i < scpb.NumStatus; i++ {
+		for i := range scpb.Status_name {
 			if ts, ok := m[scpb.Status(i)]; ok {
 				if err := it(ts); err != nil {
 					return iterutil.Map(err)

pkg/sql/schemachanger/scplan/plan_explain.go

@@ -30,7 +30,7 @@ import (
 )
 
 // DecorateErrorWithPlanDetails adds plan graphviz URLs as error details.
-func (p Plan) DecorateErrorWithPlanDetails(err error) error {
+func (p Plan) DecorateErrorWithPlanDetails(err error) (retErr error) {
 	if err == nil {
 		return nil
 	}
@@ -40,7 +40,7 @@ func (p Plan) DecorateErrorWithPlanDetails(err error) error {
 			if !ok {
 				rAsErr = errors.Errorf("panic during scplan.DecorateErrorWithPlanDetails: %v", r)
 			}
-			err = errors.CombineErrors(err, rAsErr)
+			retErr = errors.CombineErrors(err, rAsErr)
 		}
 	}()