statemgr: Helper funcs RefreshAndRead and WriteAndPersist

In practice these pairs of functions are often used together when working
with a "full" statemgr, so these helper wrappers allow us to do that more
conveniently.

This also introduces a new interface statemgr.Storage, which represents
a state manager that has all of the storage capabilities but does not
necessarily support locking. In practice callers will usually just use
statemgr.Full, but these more-specific interfaces allow us to reflect
in APIs which subset of the statemgr functionality each function depends
on.

states/statemgr/helper.go

@@ -0,0 +1,41 @@
+package statemgr
+
+// The functions in this file are helper wrappers for common sequences of
+// operations done against full state managers.
+
+import (
+	"github.com/hashicorp/terraform/states"
+)
+
+// RefreshAndRead refreshes the persistent snapshot in the given state manager
+// and then returns it.
+//
+// This is a wrapper around calling RefreshState and then State on the given
+// manager.
+func RefreshAndRead(mgr Storage) (*states.State, error) {
+	err := mgr.RefreshState()
+	if err != nil {
+		return nil, err
+	}
+	return mgr.State(), nil
+}
+
+// WriteAndPersist writes a snapshot of the given state to the given state
+// manager's transient store and then immediately persists it.
+//
+// The caller must ensure that the given state is not concurrently modified
+// while this function is running, but it is safe to modify it after this
+// function has returned.
+//
+// If an error is returned, it is undefined whether the state has been saved
+// to the transient store or not, and so the only safe response is to bail
+// out quickly with a user-facing error. In situations where more control
+// is required, call WriteState and PersistState on the state manager directly
+// and handle their errors.
+func WriteAndPersist(mgr Storage, state *states.State) error {
+	err := mgr.WriteState(state)
+	if err != nil {
+		return err
+	}
+	return mgr.PersistState()
+}

states/statemgr/statemgr.go

@@ -1,5 +1,16 @@
 package statemgr
 
+// Storage is the union of Transient and Persistent, for state managers that
+// have both transient and persistent storage.
+//
+// Types implementing this interface coordinate between their Transient
+// and Persistent implementations so that the persistent operations read
+// or write the transient store.
+type Storage interface {
+	Transient
+	Persistent
+}
+
 // Full is the union of all of the more-specific state interfaces.
 //
 // This interface may grow over time, so state implementations aiming to
@@ -10,7 +21,6 @@ package statemgr
 //
 //     var _ statemgr.Full = (*ImplementingType)(nil)
 type Full interface {
-	Transient
+	Storage
-	Persistent
 	Locker
 }