GetConfigResourceChanges from plans

In order to find any changes related to a particular configuration
address, we need a new method to get changes to all possible instances.

plans/changes.go

@@ -52,6 +52,22 @@ func (c *Changes) ResourceInstance(addr addrs.AbsResourceInstance) *ResourceInst
 	}
 
 	return nil
+
+}
+
+// ConfigResourceInstances returns the planned change for the current objects
+// of the resource instances of the given address, if any. Returns nil if no
+// changes are planned.
+func (c *Changes) ConfigResourceInstances(addr addrs.ConfigResource) []*ResourceInstanceChangeSrc {
+	var changes []*ResourceInstanceChangeSrc
+	for _, rc := range c.Resources {
+		resAddr := rc.Addr.ContainingResource().Config()
+		if resAddr.Equal(addr) && rc.DeposedKey == states.NotDeposed {
+			changes = append(changes, rc)
+		}
+	}
+
+	return changes
 }
 
 // ResourceInstanceDeposed returns the plan change of a deposed object of

plans/changes_sync.go

@@ -62,6 +62,29 @@ func (cs *ChangesSync) GetResourceInstanceChange(addr addrs.AbsResourceInstance,
 	panic(fmt.Sprintf("unsupported generation value %#v", gen))
 }
 
+// GetConfigResourceChanges searched the set of resource instance
+// changes and returns all changes related to a given configuration address.
+// This is be used to find possible changes related to a configuration
+// reference.
+//
+// If no such changes exist, nil is returned.
+//
+// The returned objects are a deep copy of the change recorded in the plan, so
+// callers may mutate them although it's generally better (less confusing) to
+// treat planned changes as immutable after they've been initially constructed.
+func (cs *ChangesSync) GetConfigResourceChanges(addr addrs.ConfigResource) []*ResourceInstanceChangeSrc {
+	if cs == nil {
+		panic("GetConfigResourceChanges on nil ChangesSync")
+	}
+	cs.lock.Lock()
+	defer cs.lock.Unlock()
+	var changes []*ResourceInstanceChangeSrc
+	for _, c := range cs.changes.ConfigResourceInstances(addr) {
+		changes = append(changes, c.DeepCopy())
+	}
+	return changes
+}
+
 // 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.