Remove 0 counts from flatmap during MergeDiff

When a InstanceState is merged with an InstanceDiff, any maps arrays or
sets that no longer exist are shown as empty with a count of 0. If these
are left in the flatmap structure, they will cause errors during
expansion because their existing in the map affects the counts for
parent structures.

terraform/state.go

@@ -9,6 +9,7 @@ import (
 	"io/ioutil"
 	"log"
 	"reflect"
+	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -1664,6 +1665,25 @@ func (s *InstanceState) MergeDiff(d *InstanceDiff) *InstanceState {
 		}
 	}
 
+	// Remove any now empty array, maps or sets because a parent structure
+	// won't include these entries in the count value.
+	isCount := regexp.MustCompile(`\.[%#]$`).MatchString
+	for k, v := range result.Attributes {
+		if isCount(k) && v == "0" {
+			delete(result.Attributes, k)
+
+			// Sanity check for invalid structures.
+			// If we removed the primary count key, there should have been no
+			// other keys left with this prefix.
+			base := k[:len(k)-2]
+			for k, _ := range result.Attributes {
+				if strings.HasPrefix(k, base) {
+					panic(fmt.Sprintf("empty structure %q has entry %q", base, k))
+				}
+			}
+		}
+	}
+
 	return result
 }
 

terraform/state_test.go

@@ -1392,6 +1392,56 @@ func TestInstanceState_MergeDiff(t *testing.T) {
 	}
 }
 
+// Make sure we don't leave empty maps or arrays in the flatmapped Attributes,
+// since those may affect the counts of a parent structure.
+func TestInstanceState_MergeDiffRemoveCounts(t *testing.T) {
+	is := InstanceState{
+		ID: "foo",
+		Attributes: map[string]string{
+			"all.#":        "3",
+			"all.1111":     "x",
+			"all.1234.#":   "1",
+			"all.1234.0":   "a",
+			"all.5678.%":   "1",
+			"all.5678.key": "val",
+		},
+	}
+
+	diff := &InstanceDiff{
+		Attributes: map[string]*ResourceAttrDiff{
+			"all.#": &ResourceAttrDiff{
+				Old: "3",
+				New: "1",
+			},
+			"all.1234.0": &ResourceAttrDiff{
+				NewRemoved: true,
+			},
+			"all.1234.#": &ResourceAttrDiff{
+				Old: "1",
+				New: "0",
+			},
+			"all.5678.key": &ResourceAttrDiff{
+				NewRemoved: true,
+			},
+			"all.5678.%": &ResourceAttrDiff{
+				Old: "1",
+				New: "0",
+			},
+		},
+	}
+
+	is2 := is.MergeDiff(diff)
+
+	expected := map[string]string{
+		"all.#":    "1",
+		"all.1111": "x",
+	}
+
+	if !reflect.DeepEqual(expected, is2.Attributes) {
+		t.Fatalf("bad: %#v", is2.Attributes)
+	}
+}
+
 func TestInstanceState_MergeDiff_nil(t *testing.T) {
 	var is *InstanceState