replace WaitGroup with errgroup to allow error handling and propagation

The program now exits with a non-zero code when errors are encountered
while reading and/or writing files.

This now also introduces Go Modules for dependency management

.travis.yml

@@ -1,13 +1,13 @@
-dist: xenial
+dist: bionic
 git:
   depth: 3
 language: go
 go:
-  - "1.x"
-  - "1.10.x"
+  - 1.13.x
 before_script:
   - go get golang.org/x/lint/golint
 script:
+  - go mod tidy
   - gofmt -d -e -l -s .
   - golint -set_exit_status ./...
   - go test -v ./...

go.mod

@@ -0,0 +1,5 @@
+module github.com/google/addlicense
+
+go 1.13
+
+require golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e

go.sum

@@ -0,0 +1,2 @@
+golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e h1:vcxGaoTs7kV8m5Np9uUNQin4BrLOthgV7252N8V+FwY=
+golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=

main.go

@@ -26,8 +26,9 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"sync"
 	"time"
+
+	"golang.org/x/sync/errgroup"
 )
 
 const helpText = `Usage: addlicense [flags] pattern [pattern ...]
@@ -92,23 +93,26 @@ func main() {
 	ch := make(chan *file, 1000)
 	done := make(chan struct{})
 	go func() {
-		var wg sync.WaitGroup
+		var wg errgroup.Group
 		for f := range ch {
-			wg.Add(1)
-			go func(f *file) {
-				defer wg.Done()
+			f := f // https://golang.org/doc/faq#closures_and_goroutines
+			wg.Go(func() error {
 				modified, err := addLicense(f.path, f.mode, t, data)
 				if err != nil {
 					log.Printf("%s: %v", f.path, err)
-					return
+					return err
 				}
 				if *verbose && modified {
 					log.Printf("%s modified", f.path)
 				}
-			}(f)
+				return nil
+			})
 		}
-		wg.Wait()
+		err := wg.Wait()
 		close(done)
+		if err != nil {
+			os.Exit(1)
+		}
 	}()
 
 	for _, d := range flag.Args() {

main_test.go

@@ -87,3 +87,55 @@ func TestMultiyear(t *testing.T) {
 	}
 	run(t, "diff", samplefile, sampleLicensed)
 }
+
+func TestWriteErrors(t *testing.T) {
+	if os.Getenv("RUNME") != "" {
+		main()
+		return
+	}
+
+	tmp := tempDir(t)
+	t.Logf("tmp dir: %s", tmp)
+	samplefile := filepath.Join(tmp, "file.c")
+
+	run(t, "cp", "testdata/initial/file.c", samplefile)
+	run(t, "chmod", "0444", samplefile)
+	cmd := exec.Command(os.Args[0],
+		"-test.run=TestWriteErrors",
+		"-l", "apache", "-c", "Google LLC", "-y", "2018",
+		samplefile,
+	)
+	cmd.Env = []string{"RUNME=1"}
+	out, err := cmd.CombinedOutput()
+	if err == nil {
+		run(t, "chmod", "0644", samplefile)
+		t.Fatalf("TestWriteErrors exited with a zero exit code.\n%s", out)
+	}
+	run(t, "chmod", "0644", samplefile)
+}
+
+func TestReadErrors(t *testing.T) {
+	if os.Getenv("RUNME") != "" {
+		main()
+		return
+	}
+
+	tmp := tempDir(t)
+	t.Logf("tmp dir: %s", tmp)
+	samplefile := filepath.Join(tmp, "file.c")
+
+	run(t, "cp", "testdata/initial/file.c", samplefile)
+	run(t, "chmod", "a-r", samplefile)
+	cmd := exec.Command(os.Args[0],
+		"-test.run=TestReadErrors",
+		"-l", "apache", "-c", "Google LLC", "-y", "2018",
+		samplefile,
+	)
+	cmd.Env = []string{"RUNME=1"}
+	out, err := cmd.CombinedOutput()
+	if err == nil {
+		run(t, "chmod", "0644", samplefile)
+		t.Fatalf("TestWriteErrors exited with a zero exit code.\n%s", out)
+	}
+	run(t, "chmod", "0644", samplefile)
+}