expand: support ~ on Windows without $HOME

And add a test, which failed under Wine:

	--- FAIL: TestFields (0.00s)
	    --- FAIL: TestFields/10 (0.00s)
		expand_test.go:122:
		    want: ["/my/home"]
		    got:  [""]

While at it, rewrite interp.New's HOME fallback to use os.UserHomeDir,
since that doesn't rely on cgo.

Fixes #410.

expand/expand.go

@@ -11,6 +11,7 @@ import (
 	"os/user"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strconv"
 	"strings"
 
@@ -619,8 +620,26 @@ func (cfg *Config) expandUser(field string) (prefix, rest string) {
 		name = name[:i]
 	}
 	if name == "" {
-		return cfg.Env.Get("HOME").String(), rest
+		// Current user; try via "HOME", otherwise fall back to the
+		// system's appropriate home dir env var. Don't use os/user, as
+		// that's overkill. We can't use os.UserHomeDir, because we want
+		// to use cfg.Env, and we always want to check "HOME" first.
+
+		if vr := cfg.Env.Get("HOME"); vr.IsSet() {
+			return vr.String(), rest
+		}
+
+		if runtime.GOOS == "windows" {
+			if vr := cfg.Env.Get("USERPROFILE"); vr.IsSet() {
+				return vr.String(), rest
+			}
+		}
+		return "", field
 	}
+
+	// Not the current user; try via "HOME <name>", otherwise fall back to
+	// os/user. There isn't a way to lookup user home dirs without cgo.
+
 	if vr := cfg.Env.Get("HOME " + name); vr.IsSet() {
 		return vr.String(), rest
 	}

interp/interp.go

@@ -11,7 +11,6 @@ import (
 	"io/ioutil"
 	"math"
 	"os"
-	"os/user"
 	"path/filepath"
 	"regexp"
 	"runtime"
@@ -568,8 +567,8 @@ func (r *Runner) Reset() {
 		}
 	}
 	if vr := r.Env.Get("HOME"); !vr.IsSet() {
-		u, _ := user.Current()
-		r.Vars["HOME"] = expand.Variable{Kind: expand.String, Str: u.HomeDir}
+		home, _ := os.UserHomeDir()
+		r.Vars["HOME"] = expand.Variable{Kind: expand.String, Str: home}
 	}
 	r.Vars["PWD"] = expand.Variable{Kind: expand.String, Str: r.Dir}
 	r.Vars["IFS"] = expand.Variable{Kind: expand.String, Str: " \t\n"}

shell/expand_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"reflect"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -91,6 +92,20 @@ var fieldsTests = []struct {
 	{"~/foo/bar", strEnviron("HOME=/my/home"), []string{"/my/home/foo/bar"}},
 	{"~foo/file", strEnviron("HOME foo=/bar"), []string{"/bar/file"}},
 	{"*.go", nil, []string{"*.go"}},
+
+	{"~", func(name string) string {
+		switch runtime.GOOS {
+		case "windows":
+			if name == "USERPROFILE" {
+				return "/my/home"
+			}
+		default:
+			if name == "HOME" {
+				return "/my/home"
+			}
+		}
+		return ""
+	}, []string{"/my/home"}},
 }
 
 func TestFields(t *testing.T) {