terraform: new apply resource node supports data sources

This enables the new apply graph's resource node to apply data sources.
Data sources appear to only be tested for "refresh" which is likely
where they're set but they've also been implemented (not my code, not
trying to edit code) within the "apply" operation as well.

This adds an apply test to ensure data sources work, and then modifies
the new apply node to support data sources.

terraform/node_resource_apply.go

@@ -2,6 +2,8 @@ package terraform
 
 import (
 	"fmt"
+
+	"github.com/hashicorp/terraform/config"
 )
 
 // NodeApplyableResource represents a resource that is "applyable":
@@ -49,6 +51,106 @@ func (n *NodeApplyableResource) EvalTree() EvalNode {
 		stateDeps = oldN.StateDependencies()
 	}
 
+	// Eval info is different depending on what kind of resource this is
+	switch n.Config.Mode {
+	case config.ManagedResourceMode:
+		return n.evalTreeManagedResource(
+			stateId, info, resource, stateDeps,
+		)
+	case config.DataResourceMode:
+		return n.evalTreeDataResource(
+			stateId, info, resource, stateDeps)
+	default:
+		panic(fmt.Errorf("unsupported resource mode %s", n.Config.Mode))
+	}
+}
+
+func (n *NodeApplyableResource) evalTreeDataResource(
+	stateId string, info *InstanceInfo,
+	resource *Resource, stateDeps []string) EvalNode {
+	var provider ResourceProvider
+	var config *ResourceConfig
+	var diff *InstanceDiff
+	var state *InstanceState
+
+	return &EvalSequence{
+		Nodes: []EvalNode{
+			// Get the saved diff for apply
+			&EvalReadDiff{
+				Name: stateId,
+				Diff: &diff,
+			},
+
+			// Stop here if we don't actually have a diff
+			&EvalIf{
+				If: func(ctx EvalContext) (bool, error) {
+					if diff == nil {
+						return true, EvalEarlyExitError{}
+					}
+
+					if diff.GetAttributesLen() == 0 {
+						return true, EvalEarlyExitError{}
+					}
+
+					return true, nil
+				},
+				Then: EvalNoop{},
+			},
+
+			// We need to re-interpolate the config here, rather than
+			// just using the diff's values directly, because we've
+			// potentially learned more variable values during the
+			// apply pass that weren't known when the diff was produced.
+			&EvalInterpolate{
+				Config:   n.Config.RawConfig.Copy(),
+				Resource: resource,
+				Output:   &config,
+			},
+
+			&EvalGetProvider{
+				Name:   n.ProvidedBy()[0],
+				Output: &provider,
+			},
+
+			// Make a new diff with our newly-interpolated config.
+			&EvalReadDataDiff{
+				Info:     info,
+				Config:   &config,
+				Previous: &diff,
+				Provider: &provider,
+				Output:   &diff,
+			},
+
+			&EvalReadDataApply{
+				Info:     info,
+				Diff:     &diff,
+				Provider: &provider,
+				Output:   &state,
+			},
+
+			&EvalWriteState{
+				Name:         stateId,
+				ResourceType: n.Config.Type,
+				Provider:     n.Config.Provider,
+				Dependencies: stateDeps,
+				State:        &state,
+			},
+
+			// Clear the diff now that we've applied it, so
+			// later nodes won't see a diff that's now a no-op.
+			&EvalWriteDiff{
+				Name: stateId,
+				Diff: nil,
+			},
+
+			&EvalUpdateStateHook{},
+		},
+	}
+}
+
+func (n *NodeApplyableResource) evalTreeManagedResource(
+	stateId string, info *InstanceInfo,
+	resource *Resource, stateDeps []string) EvalNode {
 	// Declare a bunch of variables that are used for state during
 	// evaluation. Most of this are written to by-address below.
 	var provider ResourceProvider