Add attrs as a special attribute of the print command (#1323)

Before, there was no way to get a list of all of the attributes of a
rule.  Now print attrs will print a list containing all of the
attribute names of a given rule.

Fixes #1322.

RELNOTES: "print 'attrs' //rule" prints all attributes of the rule

buildozer/README.md

@@ -265,6 +265,7 @@ the attribute, a warning is printed on stderr.
 There are some special attributes in the `print` command:
 
   * `kind`: displays the name of the function
+  * `attrs`: displays the attribute names of the rule
   * `label`: the fully qualified label
   * `rule`: the entire rule definition
   * `startline`: the line number on which the rule begins in the BUILD file

buildozer/buildozer_test.sh

@@ -1251,6 +1251,18 @@ function test_print_version() {
   assert_output '12345'
 }
 
+function test_print_attrs() {
+  in='package()
+cc_library(
+  name = "a",
+  srcs = ["a.cc"],
+  deps = ["//foo"],
+)'
+  run "$in" 'print attrs' '//pkg:*'
+  assert_output '[]
+[name srcs deps]'
+}
+
 function test_new_cc_library() {
   in='cc_test(name = "a")
 

edit/buildozer.go

@@ -370,6 +370,10 @@ func cmdPrint(opts *Options, env CmdEnvironment) (*build.File, error) {
 			fields[i] = &apipb.Output_Record_Field{
 				Value: &apipb.Output_Record_Field_Text{Text: env.File.Path},
 			}
+		} else if str == "attrs" {
+			fields[i] = &apipb.Output_Record_Field{
+				Value: &apipb.Output_Record_Field_List{List: &apipb.RepeatedString{Strings: env.Rule.AttrKeys()}},
+			}
 		} else if value == nil {
 			fmt.Fprintf(opts.ErrWriter, "rule \"//%s:%s\" has no attribute \"%s\"\n",
 				env.Pkg, env.Rule.Name(), str)