Fix count interpolation with computed values

This commit enables the use computed values when interpolating
the count special variable.

This is achieved by defering the expansion of `count` during
walkInput and walkValidate as the real value isn't necessary
at this point.

The value will be resolved during walkRefresh and DynamicExpand
will work as expected.

Partially fixes hashicorp#3888.

config/config.go

@@ -82,6 +82,8 @@ type Resource struct {
 	Provider     string
 	DependsOn    []string
 	Lifecycle    ResourceLifecycle
+
+	DeferCountComputation bool
 }
 
 // Copy returns a copy of this Resource. Helpful for avoiding shared
@@ -212,6 +214,10 @@ func (r *Module) Id() string {
 
 // Count returns the count of this resource.
 func (r *Resource) Count() (int, error) {
+	if r.DeferCountComputation {
+		return 1, nil
+	}
+
 	raw := r.RawCount.Value()
 	count, ok := r.RawCount.Value().(string)
 	if !ok {

terraform/eval_context.go

@@ -81,4 +81,11 @@ type EvalContext interface {
 	// State returns the global state as well as the lock that should
 	// be used to modify that state.
 	State() (*State, *sync.RWMutex)
+
+	// True if we can ignore missing computed values when interpolating
+	// the count variable
+	//
+	// This is useful as we don't have the full tree during walkInput
+	// or walkValidate
+	CanIgnoreMissingCountExpansion() bool
 }

terraform/eval_context_builtin.go

@@ -43,6 +43,7 @@ type BuiltinEvalContext struct {
 	DiffLock            *sync.RWMutex
 	StateValue          *State
 	StateLock           *sync.RWMutex
+	IgnoreMissingCount  bool
 
 	once sync.Once
 }
@@ -56,6 +57,10 @@ func (ctx *BuiltinEvalContext) Stopped() <-chan struct{} {
 	return ctx.StopContext.Done()
 }
 
+func (ctx *BuiltinEvalContext) CanIgnoreMissingCountExpansion() bool {
+	return ctx.IgnoreMissingCount
+}
+
 func (ctx *BuiltinEvalContext) Hook(fn func(Hook) (HookAction, error)) error {
 	for _, h := range ctx.Hooks {
 		action, err := fn(h)

terraform/eval_context_mock.go

@@ -206,3 +206,7 @@ func (c *MockEvalContext) State() (*State, *sync.RWMutex) {
 	c.StateCalled = true
 	return c.StateState, c.StateLock
 }
+
+func (c *MockEvalContext) CanIgnoreMissingCountExpansion() bool {
+	return false
+}

terraform/eval_count_computed.go

@@ -6,19 +6,26 @@ import (
 	"github.com/hashicorp/terraform/config"
 )
 
-// EvalCountCheckComputed is an EvalNode that checks if a resource count
-// is computed and errors if so. This can possibly happen across a
-// module boundary and we don't yet support this.
-type EvalCountCheckComputed struct {
+// EvalCountFixComputed is an EvalNode that checks if a resource count
+// is computed and try to fix the value.
+// It can only be fixed if the walk operation is either walkInput or walkValidate
+// Otherwise it errors
+type EvalCountFixComputed struct {
 	Resource *config.Resource
 }
 
 // TODO: test
-func (n *EvalCountCheckComputed) Eval(ctx EvalContext) (interface{}, error) {
-	if n.Resource.RawCount.Value() == unknownValue() {
-		return nil, fmt.Errorf(
-			"%s: value of 'count' cannot be computed",
-			n.Resource.Id())
+func (n *EvalCountFixComputed) Eval(ctx EvalContext) (interface{}, error) {
+	if ctx.CanIgnoreMissingCountExpansion() {
+		n.Resource.DeferCountComputation = true
+	} else {
+		n.Resource.DeferCountComputation = false
+
+		if n.Resource.RawCount.Value() == unknownValue() {
+			return nil, fmt.Errorf(
+				"%s: value of 'count' cannot be computed",
+				n.Resource.Id())
+		}
 	}
 
 	return nil, nil

terraform/graph_walk_context.go

@@ -66,6 +66,11 @@ func (w *ContextGraphWalker) EnterPath(path []string) EvalContext {
 	w.interpolaterVars[key] = variables
 	w.interpolaterVarLock.Unlock()
 
+	ignoreMissingCount := false
+	if w.Operation == walkInput || w.Operation == walkValidate {
+		ignoreMissingCount = true
+	}
+
 	ctx := &BuiltinEvalContext{
 		StopContext:         w.StopContext,
 		PathValue:           path,
@@ -92,6 +97,7 @@ func (w *ContextGraphWalker) EnterPath(path []string) EvalContext {
 		},
 		InterpolaterVars:    w.interpolaterVars,
 		InterpolaterVarLock: &w.interpolaterVarLock,
+		IgnoreMissingCount:  ignoreMissingCount,
 	}
 
 	w.contexts[key] = ctx

terraform/node_resource_abstract_count.go

@@ -19,7 +19,7 @@ func (n *NodeAbstractCountResource) EvalTree() EvalNode {
 	// into more computed values.
 	var evalCountCheckComputed EvalNode
 	if !n.Validate {
-		evalCountCheckComputed = &EvalCountCheckComputed{Resource: n.Config}
+		evalCountCheckComputed = &EvalCountFixComputed{Resource: n.Config}
 	}
 
 	return &EvalSequence{