Freeze environment reading after saving deps

Any reads after the deps are saved won't show up as dependencies later.
So panic if new environment variables are read after saving deps.

Change-Id: Ia51deaf750804d3b99e69c001939a104c2d8c9f2

common/config.go

@@ -48,8 +48,9 @@ type config struct {
 
 	srcDir string // the path of the root source directory
 
-	envLock sync.Mutex
+	envLock   sync.Mutex
-	envDeps map[string]string
+	envDeps   map[string]string
+	envFrozen bool
 }
 
 type jsonConfigurable interface {
@@ -178,6 +179,9 @@ func (c *config) Getenv(key string) string {
 	var exists bool
 	c.envLock.Lock()
 	if val, exists = c.envDeps[key]; !exists {
+		if c.envFrozen {
+			panic("Cannot access new environment variables after envdeps are frozen")
+		}
 		val = os.Getenv(key)
 		c.envDeps[key] = val
 	}
@@ -186,6 +190,9 @@ func (c *config) Getenv(key string) string {
 }
 
 func (c *config) EnvDeps() map[string]string {
+	c.envLock.Lock()
+	c.envFrozen = true
+	c.envLock.Unlock()
 	return c.envDeps
 }