terraform: share graph walker's variables lock w/ interpolater

The graph walker holds a lock that's passed down to BuiltinEvalContext
and guards access to interpolation variables as they're written using
SetVariables.

The likely problem being expressed in #5733 is that the same map
reference is also passed down to the Interpolater.Variables field, which
is used for variable lookup.

Here, we plumb the same lock we're using to guard access for writes down
and acquire it before doing variable reads as well. It's not as fine
grained as perhaps it could be, but all the context tests pass and I
believe this should address #5733.

terraform/graph_walk_context.go

@@ -81,11 +81,12 @@ func (w *ContextGraphWalker) EnterPath(path []string) EvalContext {
 		StateValue:          w.Context.state,
 		StateLock:           &w.Context.stateLock,
 		Interpolater: &Interpolater{
-			Operation: w.Operation,
-			Module:    w.Context.module,
-			State:     w.Context.state,
-			StateLock: &w.Context.stateLock,
-			Variables: variables,
+			Operation:     w.Operation,
+			Module:        w.Context.module,
+			State:         w.Context.state,
+			StateLock:     &w.Context.stateLock,
+			Variables:     variables,
+			VariablesLock: &w.interpolaterVarLock,
 		},
 		InterpolaterVars:    w.interpolaterVars,
 		InterpolaterVarLock: &w.interpolaterVarLock,

terraform/interpolate.go

@@ -23,11 +23,12 @@ const (
 // Interpolater is the structure responsible for determining the values
 // for interpolations such as `aws_instance.foo.bar`.
 type Interpolater struct {
-	Operation walkOperation
-	Module    *module.Tree
-	State     *State
-	StateLock *sync.RWMutex
-	Variables map[string]string
+	Operation     walkOperation
+	Module        *module.Tree
+	State         *State
+	StateLock     *sync.RWMutex
+	Variables     map[string]string
+	VariablesLock *sync.Mutex
 }
 
 // InterpolationScope is the current scope of execution. This is required
@@ -273,6 +274,8 @@ func (i *Interpolater) valueUserVar(
 	n string,
 	v *config.UserVariable,
 	result map[string]ast.Variable) error {
+	i.VariablesLock.Lock()
+	defer i.VariablesLock.Unlock()
 	val, ok := i.Variables[v.Name]
 	if ok {
 		result[n] = ast.Variable{