os: fix ForkExec() handling of envv == nil

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/1913047

src/pkg/exec/exec_test.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"io/ioutil"
 	"testing"
+	"os"
 )
 
 func TestRunCat(t *testing.T) {
@@ -84,3 +85,25 @@ func TestMergeWithStdout(t *testing.T) {
 		t.Fatal("close:", err)
 	}
 }
+
+func TestAddEnvVar(t *testing.T) {
+	err := os.Setenv("NEWVAR", "hello world")
+	if err != nil {
+		t.Fatal("setenv:", err)
+	}
+	cmd, err := Run("/bin/sh", []string{"sh", "-c", "echo $NEWVAR"}, nil, "",
+		DevNull, Pipe, DevNull)
+	if err != nil {
+		t.Fatal("run:", err)
+	}
+	buf, err := ioutil.ReadAll(cmd.Stdout)
+	if err != nil {
+		t.Fatal("read:", err)
+	}
+	if string(buf) != "hello world\n" {
+		t.Fatalf("read: got %q", buf)
+	}
+	if err = cmd.Close(); err != nil {
+		t.Fatal("close:", err)
+	}
+}

src/pkg/os/exec.go

@@ -16,6 +16,9 @@ import (
 // will cause the child to have no open file descriptor with that index.
 // If dir is not empty, the child chdirs into the directory before execing the program.
 func ForkExec(argv0 string, argv []string, envv []string, dir string, fd []*File) (pid int, err Error) {
+	if envv == nil {
+		envv = Environ()
+	}
 	// Create array of integer (system) fds.
 	intfd := make([]int, len(fd))
 	for i, f := range fd {