Add -print0 to print names of rewritten files (bazel-contrib#1213)

This CL adds a config to fix/update routine to output files being
rewrittenby gazelle when using fix mode. This will make scripting
around gazelle much easier, eg. run gazelle in a pre-commit hook then
commit changed BUILD files.

.bazelci/presubmit.yml

@@ -35,3 +35,6 @@ platforms:
     - "-//docs:all"
     # Fails to execute, apparently due to command-line length limit.
     - "-//internal:bazel_test"
+    # gazelle prints file paths with backward slashes on windows,
+    # which doesn't match the golden files generated on *nix.
+    - "-//tests:fix_mode_print0"

cmd/gazelle/fix-update.go

@@ -49,6 +49,7 @@ type updateConfig struct {
 	walkMode       walk.Mode
 	patchPath      string
 	patchBuffer    bytes.Buffer
+	print0         bool
 }
 
 type emitFunc func(c *config.Config, f *rule.File) error
@@ -81,6 +82,7 @@ func (ucr *updateConfigurer) RegisterFlags(fs *flag.FlagSet, cmd string, c *conf
 	fs.StringVar(&ucr.mode, "mode", "fix", "print: prints all of the updated BUILD files\n\tfix: rewrites all of the BUILD files in place\n\tdiff: computes the rewrite but then just does a diff")
 	fs.BoolVar(&ucr.recursive, "r", true, "when true, gazelle will update subdirectories recursively")
 	fs.StringVar(&uc.patchPath, "patch", "", "when set with -mode=diff, gazelle will write to a file instead of stdout")
+	fs.BoolVar(&uc.print0, "print0", false, "when set with -mode=fix, gazelle will print the names of rewritten files separated with \\0 (NULL)")
 	fs.Var(&gzflag.MultiFlag{Values: &ucr.knownImports}, "known_import", "import path for which external resolution is skipped (can specify multiple times)")
 	fs.StringVar(&ucr.repoConfigPath, "repo_config", "", "file where Gazelle should load repository configuration. Defaults to WORKSPACE.")
 }

cmd/gazelle/fix.go

@@ -17,6 +17,7 @@ package main
 
 import (
 	"bytes"
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -38,5 +39,8 @@ func fixFile(c *config.Config, f *rule.File) error {
 		return err
 	}
 	f.Content = newContent
+	if getUpdateConfig(c).print0 {
+		fmt.Printf("%s\x00", outPath)
+	}
 	return nil
 }

tests/fix_mode_print0/BUILD.in

@@ -0,0 +1 @@
+# gazelle:lang disabled

tests/fix_mode_print0/BUILD.out

@@ -0,0 +1 @@
+# gazelle:lang disabled

tests/fix_mode_print0/README.md

@@ -0,0 +1,3 @@
+# Fix mode with -print0
+
+Prints all fixed files with \0 as delimiter.

tests/fix_mode_print0/already_fixed/BUILD.in

@@ -0,0 +1,6 @@
+rule(
+    name = "good",
+    visibility = [
+        "test",
+    ],
+)

tests/fix_mode_print0/already_fixed/BUILD.out

@@ -0,0 +1,6 @@
+rule(
+    name = "good",
+    visibility = [
+        "test",
+    ],
+)

tests/fix_mode_print0/arguments.txt

@@ -0,0 +1 @@
+-print0

tests/fix_mode_print0/need_fix/BUILD.in

@@ -0,0 +1,10 @@
+rule( name= "really_bad_formatting",
+
+visibility=[
+
+
+
+
+  "test"
+]
+)

tests/fix_mode_print0/need_fix/BUILD.out

@@ -0,0 +1,6 @@
+rule(
+    name = "really_bad_formatting",
+    visibility = [
+        "test",
+    ],
+)