Add BUILD_USERNAME and BUILD_HOSTNAME

As part of a future change to sandbox the build on Linux, the real
username will be switching to "nobody", and the hostname will be
switching to "android-build".

The USER environment variable will reflect the sandboxed value, so for
the build properties that want the external USER, they'll need to use
BUILD_USERNAME.

Similarly, BUILD_HOSTNAME will reflect the real value, while the
`hostname` tool will return "android-build"

Bug: 122270019
Test: check build.prop
Change-Id: I99604b9488732a63690b256dc4dd7894d369a32c

ui/build/kati.go

@@ -18,6 +18,8 @@ import (
 	"crypto/md5"
 	"fmt"
 	"io/ioutil"
+	"os"
+	"os/user"
 	"path/filepath"
 	"strings"
 
@@ -96,6 +98,22 @@ func runKati(ctx Context, config Config, extraSuffix string, args []string, envF
 
 	envFunc(cmd.Environment)
 
+	if _, ok := cmd.Environment.Get("BUILD_USERNAME"); !ok {
+		u, err := user.Current()
+		if err != nil {
+			ctx.Println("Failed to get current user")
+		}
+		cmd.Environment.Set("BUILD_USERNAME", u.Username)
+	}
+
+	if _, ok := cmd.Environment.Get("BUILD_HOSTNAME"); !ok {
+		hostname, err := os.Hostname()
+		if err != nil {
+			ctx.Println("Failed to read hostname")
+		}
+		cmd.Environment.Set("BUILD_HOSTNAME", hostname)
+	}
+
 	cmd.StartOrFatal()
 	status.KatiReader(ctx.Status.StartTool(), pipe)
 	cmd.WaitOrFatal()