cmd/pack: r command create output file if not exist

Go 1.15 pack's r command creates the output file if it does not
exist. The system "ar" command does this as well. Do the same.

For bazelbuild/rules_go#2762.

Change-Id: Icd88396b5c714b735c859a29ab29851e4301f4d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/279516
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

2 files changed, 29 insertions, 3 deletions

diff --git a/src/cmd/pack/pack.go b/src/cmd/pack/pack.go
index 3dffabe5ec..412ea36d60 100644
--- a/src/cmd/pack/pack.go
+++ b/src/cmd/pack/pack.go
@@ -43,7 +43,7 @@ func main() {
 		ar = openArchive(os.Args[2], os.O_RDONLY, os.Args[3:])
 		ar.scan(ar.printContents)
 	case 'r':
-		ar = openArchive(os.Args[2], os.O_RDWR, os.Args[3:])
+		ar = openArchive(os.Args[2], os.O_RDWR|os.O_CREATE, os.Args[3:])
 		ar.addFiles()
 	case 'c':
 		ar = openArchive(os.Args[2], os.O_RDWR|os.O_TRUNC|os.O_CREATE, os.Args[3:])
@@ -124,10 +124,13 @@ func openArchive(name string, mode int, files []string) *Archive {
 		log.Fatal(err)
 	}
 	var a *archive.Archive
-	if mode&os.O_CREATE != 0 { // the c command
+	if mode&os.O_TRUNC != 0 { // the c command
 		a, err = archive.New(f)
 	} else {
 		a, err = archive.Parse(f, verbose)
+		if err != nil && mode&os.O_CREATE != 0 { // the r command
+			a, err = archive.New(f)
+		}
 	}
 	if err != nil {
 		log.Fatal(err)
diff --git a/src/cmd/pack/pack_test.go b/src/cmd/pack/pack_test.go
index 16a5135800..118376f9df 100644
--- a/src/cmd/pack/pack_test.go
+++ b/src/cmd/pack/pack_test.go
@@ -303,7 +303,7 @@ func TestIssue21703(t *testing.T) {
 }
 
 // Test the "c" command can "see through" the archive generated by the compiler.
-// This is peculiar. (See issue )
+// This is peculiar. (See issue #43271)
 func TestCreateWithCompilerObj(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 
@@ -368,6 +368,29 @@ func TestCreateWithCompilerObj(t *testing.T) {
 	}
 }
 
+// Test the "r" command creates the output file if it does not exist.
+func TestRWithNonexistentFile(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	dir := tmpDir(t)
+	defer os.RemoveAll(dir)
+	src := filepath.Join(dir, "p.go")
+	prog := "package p; var X = 42\n"
+	err := os.WriteFile(src, []byte(prog), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	run := func(args ...string) string {
+		return doRun(t, dir, args...)
+	}
+
+	goBin := testenv.GoToolPath(t)
+	run(goBin, "build", "cmd/pack") // writes pack binary to dir
+	run(goBin, "tool", "compile", "-o", "p.o", "p.go")
+	run("./pack", "r", "p.a", "p.o") // should succeed
+}
+
 // doRun runs a program in a directory and returns the output.
 func doRun(t *testing.T, dir string, args ...string) string {
 	cmd := exec.Command(args[0], args[1:]...)