storage: Support index for array appends

Currently the "-" character is used to append to
an array. Appends can be also be made by specifying the
array index as long as the specified index is not
greater than the number of elements in the array as
per RFC 6902.

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>

storage/inmem/inmem_test.go

@@ -87,8 +87,11 @@ func TestInMemoryWrite(t *testing.T) {
 		{"add obj (existing)", "add", "/b/v2", `"x"`, nil, "/b", `{"v1": "hello", "v2": "x"}`},
 
 		{"append arr", "add", "/a/-", `"x"`, nil, "/a", `[1,2,3,4,"x"]`},
+		{"append arr-2", "add", "/a/4", `"x"`, nil, "/a", `[1,2,3,4,"x"]`},
 		{"append obj/arr", "add", `/c/0/x/-`, `"x"`, nil, "/c/0/x", `[true,false,"foo","x"]`},
+		{"append obj/arr-2", "add", `/c/0/x/3`, `"x"`, nil, "/c/0/x", `[true,false,"foo","x"]`},
 		{"append arr/arr", "add", `/h/0/-`, `"x"`, nil, `/h/0/3`, `"x"`},
+		{"append arr/arr-2", "add", `/h/0/3`, `"x"`, nil, `/h/0/3`, `"x"`},
 		{"append err", "remove", "/c/0/x/-", "", invalidPatchError("/c/0/x/-: invalid patch path"), "", nil},
 		{"append err-2", "replace", "/c/0/x/-", "", invalidPatchError("/c/0/x/-: invalid patch path"), "", nil},
 
@@ -255,12 +258,14 @@ func TestInMemoryTxnMultipleWrites(t *testing.T) {
 		{storage.AddOp, "/a/-", "[]"},
 		{storage.AddOp, "/a/4/-", "1"},
 		{storage.AddOp, "/a/4/-", "2"},
+		{storage.AddOp, "/a/4/2", "3"},
 		{storage.AddOp, "/b/foo", "{}"},
 		{storage.AddOp, "/b/foo/bar", "{}"},
 		{storage.AddOp, "/b/foo/bar/baz", "1"},
 		{storage.AddOp, "/arr", "[]"},
 		{storage.AddOp, "/arr/-", "1"},
 		{storage.AddOp, "/arr/0", "2"},
+		{storage.AddOp, "/arr/2", "3"},
 		{storage.AddOp, "/c/0/x/-", "0"},
 		{storage.AddOp, "/_", "null"}, // introduce new txn.log head
 		{storage.AddOp, "/c/0", `"new c[0]"`},
@@ -275,9 +280,9 @@ func TestInMemoryTxnMultipleWrites(t *testing.T) {
 		path     string
 		expected string
 	}{
-		{"/a", `[1,2,3,4,[1,2]]`},
+		{"/a", `[1,2,3,4,[1,2,3]]`},
 		{"/b/foo", `{"bar": {"baz": 1}}`},
-		{"/arr", `[2,1]`},
+		{"/arr", `[2,1,3]`},
 		{"/c/0", `"new c[0]"`},
 		{"/c/1", `"new c[1]"`},
 		{"/d/f", `{"g": {"h": 0, "i": {"j": 1}}}`},
@@ -337,6 +342,7 @@ func TestInMemoryTxnWriteFailures(t *testing.T) {
 		{storage.AddOp, "/a/0/beef", "", storage.NotFoundErr},
 		{storage.AddOp, "/arr", `[1,2,3]`, ""},
 		{storage.AddOp, "/arr/0/foo", "", storage.NotFoundErr},
+		{storage.AddOp, "/arr/4", "", storage.NotFoundErr},
 	}
 
 	for _, w := range writes {

storage/inmem/txn.go

@@ -296,7 +296,7 @@ func newUpdate(data interface{}, op storage.PatchOp, path storage.Path, idx int,
 func newUpdateArray(data []interface{}, op storage.PatchOp, path storage.Path, idx int, value interface{}) (*update, error) {
 
 	if idx == len(path)-1 {
-		if path[idx] == "-" {
+		if path[idx] == "-" || path[idx] == strconv.Itoa(len(data)) {
 			if op != storage.AddOp {
 				return nil, invalidPatchError("%v: invalid patch path", path)
 			}