tools: handle paths like ~/ or $HOME/.

tilde should be located at the beginning of line.

Change-Id: I271ba5220da3c483838d1741d908755aee8e081e
Reviewed-on: https://go-review.googlesource.com/46430
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/go-contrib-init/contrib.go

@@ -87,16 +87,30 @@ func expandUser(s string) string {
 	} else if runtime.GOOS == "plan9" {
 		env = "home"
 	}
-	if home := os.Getenv(env); home != "" {
-		return strings.Replace(s, "~", home, 1)
+	home := os.Getenv(env)
+	if home == "" {
+		return s
 	}
-	return s
+
+	if len(s) >= 2 && s[0] == '~' && os.IsPathSeparator(s[1]) {
+		if runtime.GOOS == "windows" {
+			s = filepath.ToSlash(filepath.Join(home, s[2:]))
+		} else {
+			s = filepath.Join(home, s[2:])
+		}
+	}
+	return os.Expand(s, func(env string) string {
+		if env == "HOME" {
+			return home
+		}
+		return os.Getenv(env)
+	})
 }
 
 func cookiesFile() string {
 	out, _ := exec.Command("git", "config", "http.cookiefile").Output()
 	if s := strings.TrimSpace(string(out)); s != "" {
-		if strings.Contains(s, "~") {
+		if strings.HasPrefix(s, "~") {
 			s = expandUser(s)
 		}
 		return s

cmd/go-contrib-init/contrib_test.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"os"
+	"runtime"
+	"testing"
+)
+
+func TestExpandUser(t *testing.T) {
+	env := "HOME"
+	if runtime.GOOS == "windows" {
+		env = "USERPROFILE"
+	} else if runtime.GOOS == "plan9" {
+		env = "home"
+	}
+
+	oldenv := os.Getenv(env)
+	os.Setenv(env, "/home/gopher")
+	defer os.Setenv(env, oldenv)
+
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{input: "~/foo", want: "/home/gopher/foo"},
+		{input: "${HOME}/foo", want: "/home/gopher/foo"},
+		{input: "/~/foo", want: "/~/foo"},
+	}
+	for _, tt := range tests {
+		got := expandUser(tt.input)
+		if got != tt.want {
+			t.Fatalf("want %q, but %q", tt.want, got)
+		}
+	}
+}