make sure ReadState returns an error

ReadState would hide any errors, assuming that it was an empty state.
This can mask errors on Windows, where the OS enforces read locks on the
state file.

terraform/state.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"io/ioutil"
 	"log"
+	"os"
 	"reflect"
 	"sort"
 	"strconv"
@@ -1876,13 +1877,21 @@ var ErrNoState = errors.New("no state")
 // ReadState reads a state structure out of a reader in the format that
 // was written by WriteState.
 func ReadState(src io.Reader) (*State, error) {
-	buf := bufio.NewReader(src)
-	if _, err := buf.Peek(1); err != nil {
-		// the error is either io.EOF or "invalid argument", and both are from
-		// an empty state.
+	// check for a nil file specifically, since that produces a platform
+	// specific error if we try to use it in a bufio.Reader.
+	if f, ok := src.(*os.File); ok && f == nil {
 		return nil, ErrNoState
 	}
 
+	buf := bufio.NewReader(src)
+
+	if _, err := buf.Peek(1); err != nil {
+		if err == io.EOF {
+			return nil, ErrNoState
+		}
+		return nil, err
+	}
+
 	if err := testForV0State(buf); err != nil {
 		return nil, err
 	}