core: Remove changes from the plan after they are applied

plans/changes_sync.go

@@ -61,3 +61,29 @@ func (cs *ChangesSync) GetResourceInstanceChange(addr addrs.AbsResourceInstance,
 	}
 	panic(fmt.Sprintf("unsupported generation value %#v", gen))
 }
+
+// RemoveResourceInstanceChange searches the set of resource instance changes
+// for one matching the given address and generation, and removes it from the
+// set if it exists.
+func (cs *ChangesSync) RemoveResourceInstanceChange(addr addrs.AbsResourceInstance, gen states.Generation) {
+	if cs == nil {
+		panic("RemoveResourceInstanceChange on nil ChangesSync")
+	}
+	cs.lock.Lock()
+	defer cs.lock.Unlock()
+
+	dk := states.NotDeposed
+	if realDK, ok := gen.(states.DeposedKey); ok {
+		dk = realDK
+	}
+
+	addrStr := addr.String()
+	for i, r := range cs.changes.Resources {
+		if r.Addr.String() != addrStr || r.DeposedKey != dk {
+			continue
+		}
+		copy(cs.changes.Resources[i:], cs.changes.Resources[i+1:])
+		cs.changes.Resources = cs.changes.Resources[:len(cs.changes.Resources)-1]
+		return
+	}
+}

terraform/eval_apply.go

@@ -195,7 +195,12 @@ func (n *EvalApplyPost) Eval(ctx EvalContext) (interface{}, error) {
 
 	if resourceHasUserVisibleApply(n.Addr) {
 		absAddr := n.Addr.Absolute(ctx.Path())
-		newState := state.Value
+		var newState cty.Value
+		if state != nil {
+			newState = state.Value
+		} else {
+			newState = cty.NullVal(cty.DynamicPseudoType)
+		}
 		var err error
 		if n.Error != nil {
 			err = *n.Error

terraform/eval_diff.go

@@ -754,9 +754,21 @@ type EvalWriteDiff struct {
 
 // TODO: test
 func (n *EvalWriteDiff) Eval(ctx EvalContext) (interface{}, error) {
+	changes := ctx.Changes()
+	addr := n.Addr.Absolute(ctx.Path())
+	if n.Change == nil || *n.Change == nil {
+		// Caller sets nil to indicate that we need to remove a change from
+		// the set of changes.
+		gen := states.CurrentGen
+		if n.DeposedKey != states.NotDeposed {
+			gen = n.DeposedKey
+		}
+		changes.RemoveResourceInstanceChange(addr, gen)
+		return nil, nil
+	}
+
 	providerSchema := *n.ProviderSchema
 	change := *n.Change
-	addr := n.Addr.Absolute(ctx.Path())
 
 	if change.Addr.String() != n.Addr.String() || change.DeposedKey != n.DeposedKey {
 		// Should never happen, and indicates a bug in the caller.
@@ -774,7 +786,6 @@ func (n *EvalWriteDiff) Eval(ctx EvalContext) (interface{}, error) {
 		return nil, fmt.Errorf("failed to encode planned changes for %s: %s", addr, err)
 	}
 
-	changes := ctx.Changes()
 	changes.AppendResourceInstanceChange(csrc)
 	if n.DeposedKey == states.NotDeposed {
 		log.Printf("[TRACE] EvalWriteDiff: recorded %s change for %s", change.Action, addr)