backend/local: Show resource instance drift as part of the plan

backend/local/backend_apply.go

@@ -70,6 +70,8 @@ func (b *Local) opApply(
 
 	// If we weren't given a plan, then we refresh/plan
 	if op.PlanFile == nil {
+		initialState := tfCtx.State().DeepCopy()
+
 		// Perform the plan
 		log.Printf("[INFO] backend/local: apply calling Plan")
 		plan, planDiags := tfCtx.Plan()
@@ -104,7 +106,7 @@ func (b *Local) opApply(
 
 			if !trivialPlan {
 				// Display the plan of what we are going to apply/destroy.
-				b.renderPlan(plan, runningOp.State, tfCtx.Schemas())
+				b.renderPlan(plan, initialState, plan.State, tfCtx.Schemas())
 				b.CLI.Output("")
 			}
 

backend/local/backend_plan.go

@@ -83,6 +83,7 @@ func (b *Local) opPlan(
 		}
 	}()
 
+	initialState := tfCtx.State().DeepCopy()
 	runningOp.State = tfCtx.State()
 
 	// Perform the plan in a goroutine so we can be interrupted
@@ -155,7 +156,7 @@ func (b *Local) opPlan(
 			return
 		}
 
-		b.renderPlan(plan, plan.State, schemas)
+		b.renderPlan(plan, initialState, plan.State, schemas)
 
 		// If we've accumulated any warnings along the way then we'll show them
 		// here just before we show the summary and next steps. If we encountered
@@ -182,8 +183,8 @@ func (b *Local) opPlan(
 	}
 }
 
-func (b *Local) renderPlan(plan *plans.Plan, baseState *states.State, schemas *terraform.Schemas) {
-	RenderPlan(plan, baseState, schemas, b.CLI, b.Colorize())
+func (b *Local) renderPlan(plan *plans.Plan, baseState, priorState *states.State, schemas *terraform.Schemas) {
+	RenderPlan(plan, baseState, priorState, schemas, b.CLI, b.Colorize())
 }
 
 // RenderPlan renders the given plan to the given UI.
@@ -206,7 +207,12 @@ func (b *Local) renderPlan(plan *plans.Plan, baseState *states.State, schemas *t
 // output values will not currently be rendered because their prior values
 // are currently stored only in the prior state. (see the docstring for
 // func planHasSideEffects for why this is and when that might change)
-func RenderPlan(plan *plans.Plan, baseState *states.State, schemas *terraform.Schemas, ui cli.Ui, colorize *colorstring.Colorize) {
+func RenderPlan(plan *plans.Plan, baseState, priorState *states.State, schemas *terraform.Schemas, ui cli.Ui, colorize *colorstring.Colorize) {
+	if statesShowDrift(baseState, priorState) {
+		// We've detected some drift by refreshing during our planning, then.
+		renderDrift(baseState, priorState, schemas, ui, colorize)
+	}
+
 	counts := map[plans.Action]int{}
 	var rChanges []*plans.ResourceInstanceChangeSrc
 	for _, change := range plan.Changes.Resources {
@@ -329,6 +335,91 @@ func RenderPlan(plan *plans.Plan, baseState *states.State, schemas *terraform.Sc
 	}
 }
 
+// statesShowDrift reports whether the two given states have any differences
+// that suggest "drift", in the sense of managed resources whose state
+// data has changed. The result makes sense only if priorState is the result
+// of running refresh operations against baseState.
+func statesShowDrift(baseState, priorState *states.State) bool {
+	if baseState == nil || priorState == nil {
+		return false
+	}
+	for _, bms := range baseState.Modules {
+		for _, brs := range bms.Resources {
+			if brs.Addr.Resource.Mode != addrs.ManagedResourceMode {
+				continue // only managed resources can "drift"
+			}
+			prs := priorState.Resource(brs.Addr)
+			if prs == nil {
+				// Refreshing detected that the remote object has been deleted
+				return true
+			}
+			if !prs.Equal(brs) {
+				// Refreshing detected that the remote object has changed.
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func renderDrift(baseState, priorState *states.State, schemas *terraform.Schemas, ui cli.Ui, colorize *colorstring.Colorize) {
+	// The cyan "Note:" here is following the color we use for the <= "read"
+	// action in the plan output, because this is reporting the result of
+	// all of the resource reading we've done. This isn't a "warning" because
+	// intentionally creating drift is an intentional part of some workflows
+	// (albeit less common ones, and some that may not be so advisable).
+	// The aim is just to draw attention to the changes as an explanation
+	// for possible unexpected changes in the plan, not to assume or suggest
+	// that such changes are "bad".
+	ui.Output("\n------------------------------------------------------------------------")
+	ui.Output(colorize.Color("\n[reset][bold][cyan]Note:[reset][bold] Objects have changed outside of Terraform[reset]"))
+	ui.Output("\nTerraform detected the following changes made outside Terraform since\nthe most recent \"terraform apply\":\n")
+
+	for _, bms := range baseState.Modules {
+		for _, brs := range bms.Resources {
+			if brs.Addr.Resource.Mode != addrs.ManagedResourceMode {
+				continue // only managed resources can "drift"
+			}
+			addr := brs.Addr
+			prs := priorState.Resource(brs.Addr)
+
+			provider := brs.ProviderConfig.Provider
+			providerSchema := schemas.ProviderSchema(provider)
+			if providerSchema == nil {
+				// Should never happen
+				ui.Output(fmt.Sprintf("(schema missing for %s)\n", provider))
+				continue
+			}
+			rSchema, _ := providerSchema.SchemaForResourceAddr(addr.Resource)
+			if rSchema == nil {
+				// Should never happen
+				ui.Output(fmt.Sprintf("(schema missing for %s)\n", addr))
+				continue
+			}
+
+			for key, bis := range brs.Instances {
+				var pis *states.ResourceInstance
+				if prs != nil {
+					pis = prs.Instance(key)
+				}
+
+				diff := format.ResourceInstanceDrift(
+					addr.Instance(key),
+					bis, pis,
+					rSchema,
+					colorize,
+				)
+				if diff != "" {
+					ui.Output(diff)
+				}
+			}
+		}
+	}
+
+	ui.Output("Unless you have made equivalent changes to your configuration, or\nignored the relevant attributes using ignore_changes, the following plan\nmay include actions to undo or respond to these changes.")
+	ui.Output("\n------------------------------------------------------------------------")
+}
+
 const planHeaderIntro = `
 An execution plan has been generated and is shown below.
 Resource actions are indicated with the following symbols:

command/format/diff.go

@@ -143,6 +143,137 @@ func ResourceChange(
 	return buf.String()
 }
 
+// ResourceInstanceDrift returns a string representation of a change to a
+// particular resource instance that was made outside of Terraform, for
+// reporting a change that has already happened rather than one that is planned.
+//
+// The the two resource instances have equal current objects then the result
+// will be an empty string to indicate that there is no drift to render.
+//
+// The resource schema must be provided along with the change so that the
+// formatted change can reflect the configuration structure for the associated
+// resource.
+//
+// If "color" is non-nil, it will be used to color the result. Otherwise,
+// no color codes will be included.
+func ResourceInstanceDrift(
+	addr addrs.AbsResourceInstance,
+	before, after *states.ResourceInstance,
+	schema *configschema.Block,
+	color *colorstring.Colorize,
+) string {
+	var buf bytes.Buffer
+
+	if color == nil {
+		color = &colorstring.Colorize{
+			Colors:  colorstring.DefaultColors,
+			Disable: true,
+			Reset:   false,
+		}
+	}
+
+	dispAddr := addr.String()
+	action := plans.Update
+
+	switch {
+	case after == nil || after.Current == nil:
+		// The object was deleted
+		buf.WriteString(color.Color(fmt.Sprintf("[bold]  # %s[reset] has been deleted", dispAddr)))
+		action = plans.Delete
+	default:
+		// The object was changed
+		buf.WriteString(color.Color(fmt.Sprintf("[bold]  # %s[reset] has been changed", dispAddr)))
+	}
+
+	buf.WriteString(color.Color("[reset]\n"))
+
+	buf.WriteString(color.Color(DiffActionSymbol(action)) + " ")
+
+	switch addr.Resource.Resource.Mode {
+	case addrs.ManagedResourceMode:
+		buf.WriteString(fmt.Sprintf(
+			"resource %q %q",
+			addr.Resource.Resource.Type,
+			addr.Resource.Resource.Name,
+		))
+	case addrs.DataResourceMode:
+		buf.WriteString(fmt.Sprintf(
+			"data %q %q ",
+			addr.Resource.Resource.Type,
+			addr.Resource.Resource.Name,
+		))
+	default:
+		// should never happen, since the above is exhaustive
+		buf.WriteString(addr.String())
+	}
+
+	buf.WriteString(" {")
+
+	p := blockBodyDiffPrinter{
+		buf:     &buf,
+		color:   color,
+		action:  action,
+		concise: experiment.Enabled(experiment.X_concise_diff),
+	}
+
+	// Most commonly-used resources have nested blocks that result in us
+	// going at least three traversals deep while we recurse here, so we'll
+	// start with that much capacity and then grow as needed for deeper
+	// structures.
+	path := make(cty.Path, 0, 3)
+
+	ty := schema.ImpliedType()
+
+	var err error
+	var oldObj, newObj *states.ResourceInstanceObject
+	oldObj, err = before.Current.Decode(ty)
+	if err != nil {
+		// Should never happen in here, since we've already been through
+		// loads of layers of encode/decode of the planned changes before now.
+		panic(fmt.Sprintf("failed to decode old object for %s while rendering diff: %s", addr, err))
+	}
+	if after != nil && after.Current != nil {
+		newObj, err = after.Current.Decode(ty)
+		if err != nil {
+			// Should never happen in here, since we've already been through
+			// loads of layers of encode/decode of the planned changes before now.
+			panic(fmt.Sprintf("failed to decode new object for %s while rendering diff: %s", addr, err))
+		}
+	}
+
+	oldVal := oldObj.Value
+	var newVal cty.Value
+	if newObj != nil {
+		newVal = newObj.Value
+	} else {
+		newVal = cty.NullVal(ty)
+	}
+
+	if newVal.RawEquals(oldVal) {
+		// Nothing to show, then.
+		return ""
+	}
+
+	// We currently have an opt-out that permits the legacy SDK to return values
+	// that defy our usual conventions around handling of nesting blocks. To
+	// avoid the rendering code from needing to handle all of these, we'll
+	// normalize first.
+	// (Ideally we'd do this as part of the SDK opt-out implementation in core,
+	// but we've added it here for now to reduce risk of unexpected impacts
+	// on other code in core.)
+	oldVal = objchange.NormalizeObjectFromLegacySDK(oldVal, schema)
+	newVal = objchange.NormalizeObjectFromLegacySDK(newVal, schema)
+
+	result := p.writeBlockBodyDiff(schema, oldVal, newVal, 6, path)
+	if result.bodyWritten {
+		buf.WriteString("\n")
+		buf.WriteString(strings.Repeat(" ", 4))
+	}
+	buf.WriteString("}\n")
+
+	return buf.String()
+}
+
 // OutputChanges returns a string representation of a set of changes to output
 // values for inclusion in user-facing plan output.
 //

command/show.go

@@ -163,7 +163,7 @@ func (c *ShowCommand) Run(args []string) int {
 		// package rather than in the backends themselves, but for now we're
 		// accepting this oddity because "terraform show" is a less commonly
 		// used way to render a plan than "terraform plan" is.
-		localBackend.RenderPlan(plan, stateFile.State, schemas, c.Ui, c.Colorize())
+		localBackend.RenderPlan(plan, stateFile.State, stateFile.State, schemas, c.Ui, c.Colorize())
 		return 0
 	}
 

states/state_equal.go

@@ -16,3 +16,23 @@ func (s *State) Equal(other *State) bool {
 	// more sophisticated comparisons.
 	return reflect.DeepEqual(s, other)
 }
+
+// Equal returns true if the receiver is functionally equivalent to other,
+// including any ephemeral portions of the state that would not be included
+// if the state were saved to files.
+func (s *Resource) Equal(other *Resource) bool {
+	// For the moment this is sufficient, but we may need to do something
+	// more elaborate in future if we have any portions of state that require
+	// more sophisticated comparisons.
+	return reflect.DeepEqual(s, other)
+}
+
+// Equal returns true if the receiver is functionally equivalent to other,
+// including any ephemeral portions of the state that would not be included
+// if the state were saved to files.
+func (s *ResourceInstance) Equal(other *ResourceInstance) bool {
+	// For the moment this is sufficient, but we may need to do something
+	// more elaborate in future if we have any portions of state that require
+	// more sophisticated comparisons.
+	return reflect.DeepEqual(s, other)
+}