buildozer: add new_if_absent command

buildozer/README.md

@@ -108,6 +108,8 @@ Buildozer supports the following commands(`'command args'`):
   * `new <rule_kind> <rule_name> [(before|after) <relative_rule_name>]`: Add a
     new rule at the end of the BUILD file (before/after `<relative_rule>`). The
     identifier `__pkg__` can be used to position rules relative to package().
+  * `new_if_absent <rule_kind> <rule_name> [(before|after) <relative_rule_name>]`:
+    Same as `new`, but does nothing if a rule with the same name already exists.
   * `print <attr(s)>`
   * `remove <attr>`: Removes attribute `attr`. The wildcard `*` matches all
     attributes except `name`.

buildozer/buildozer_test.sh

@@ -1306,6 +1306,20 @@ function test_new_already_exists() {
   assert_err "rule 'a' already exists"
 }
 
+function test_new_if_absent_cc_library() {
+  in='cc_test(name = "a")'
+  run "$in" 'new_if_absent cc_library foo' '//pkg:__pkg__'
+  assert_equals 'cc_test(name = "a")
+
+cc_library(name = "foo")'
+}
+
+function test_new_if_absent_existing_rule() {
+  in='cc_library(name = "foo")'
+  ERROR=3 run "$in" 'new_if_absent cc_library foo' '//pkg:__pkg__'
+  assert_equals 'cc_library(name = "foo")'
+}
+
 function test_new_before_first() {
   in='cc_test(name = "a")'
   run "$in" 'new java_library foo before a' 'pkg/BUILD'

edit/buildozer.go

@@ -236,18 +236,16 @@ func cmdMove(opts *Options, env CmdEnvironment) (*build.File, error) {
 	return nil, nil
 }
 
-func cmdNew(opts *Options, env CmdEnvironment) (*build.File, error) {
+// createNewRule is a helper function for creating a new rule.
+// It is used by cmdNew and cmdNewIfAbsent.
+func createNewRule(env CmdEnvironment) (*build.File, error) {
 	kind := env.Args[0]
 	name := env.Args[1]
 	addAtEOF, insertionIndex, err := findInsertionIndex(env)
 	if err != nil {
 		return nil, err
 	}
 
-	if FindRuleByName(env.File, name) != nil {
-		return nil, fmt.Errorf("rule '%s' already exists", name)
-	}
-
 	call := &build.CallExpr{X: &build.Ident{Name: kind}}
 	rule := &build.Rule{Call: call, ImplicitName: ""}
 	rule.SetAttr("name", &build.StringExpr{Value: name})
@@ -260,6 +258,22 @@ func cmdNew(opts *Options, env CmdEnvironment) (*build.File, error) {
 	return env.File, nil
 }
 
+func cmdNew(opts *Options, env CmdEnvironment) (*build.File, error) {
+	name := env.Args[1]
+	if FindRuleByName(env.File, name) != nil {
+		return nil, fmt.Errorf("rule '%s' already exists", name)
+	}
+	return createNewRule(env)
+}
+
+func cmdNewIfAbsent(opts *Options, env CmdEnvironment) (*build.File, error) {
+	name := env.Args[1]
+	if FindRuleByName(env.File, name) != nil {
+		return nil, nil
+	}
+	return createNewRule(env)
+}
+
 // findInsertionIndex is used by cmdNew to find the place at which to insert the new rule.
 func findInsertionIndex(env CmdEnvironment) (bool, int, error) {
 	if len(env.Args) < 4 {
@@ -928,6 +942,7 @@ var AllCommands = map[string]CommandInfo{
 	"fix":                   {cmdFix, true, 0, -1, "<fix(es)>?"},
 	"move":                  {cmdMove, true, 3, -1, "<old_attr> <new_attr> <value(s)>"},
 	"new":                   {cmdNew, false, 2, 4, "<rule_kind> <rule_name> [(before|after) <relative_rule_name>]"},
+	"new_if_absent":         {cmdNewIfAbsent, false, 2, 4, "<rule_kind> <rule_name> [(before|after) <relative_rule_name>]"},
 	"print":                 {cmdPrint, true, 0, -1, "<attribute(s)>"},
 	"remove":                {cmdRemove, true, 1, -1, "<attr> <value(s)>"},
 	"remove_comment":        {cmdRemoveComment, true, 0, 2, "<attr>? <value>?"},

edit/buildozer_test.go

@@ -606,6 +606,54 @@ func TestCmdSubstitute(t *testing.T) {
 	}
 }
 
+var newIfAbsentTests = []struct {
+	args      []string
+	buildFile string
+	expected  string
+}{
+	{[]string{"go_library", "new_rule"},
+		``,
+		`go_library(name = "new_rule")`,
+	},
+	{[]string{"go_library", "new_rule"},
+		`go_library(name = "existing_rule")`,
+		`go_library(name = "existing_rule")
+
+go_library(name = "new_rule")`,
+	},
+	{[]string{"go_library", "existing_rule"},
+		`go_library(name = "existing_rule")`,
+		`go_library(name = "existing_rule")`,
+	},
+}
+
+func TestCmdNewIfAbsent(t *testing.T) {
+	for i, tt := range newIfAbsentTests {
+		bld, err := build.Parse("BUILD", []byte(tt.buildFile))
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		expectedBld, err := build.Parse("BUILD", []byte(tt.expected))
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		expected := strings.TrimSpace(string(build.Format(expectedBld)))
+		env := CmdEnvironment{
+			File: bld,
+			Args: tt.args,
+		}
+		if result, _ := cmdNewIfAbsent(NewOpts(), env); result != nil {
+			bld = result
+		}
+		got := strings.TrimSpace(string(build.Format(bld)))
+		if got != expected {
+			t.Errorf("cmdNewIfAbsent(%d):\ngot:\n%s\nexpected:\n%s", i, got, expected)
+		}
+	}
+}
+
 func TestCmdDictAddSet_missingColon(t *testing.T) {
 	for _, tc := range []struct {
 		name string