Add UseFlex(c) for the cloud implementation.

impl/memory/context.go

Index: impl/memory/context.go
diff --git a/impl/memory/context.go b/impl/memory/context.go
index feddfda29bd9a3485ac4c86d9dbe2449c78f09e2..9d38ce1cd4e6f583dcb25743f3f624fd48566707 100644
--- a/impl/memory/context.go
+++ b/impl/memory/context.go
@@ -96,6 +96,30 @@ func Use(c context.Context) context.Context {
 	return UseWithAppID(c, "dev~app")
 }
 
+// UseInfo adds an implementation for:
+//   * github.com/luci/gae/service/info
+// The application id wil be set to 'aid', and will not be modifiable in this
+// context. If 'aid' contains a "~" character, it will be treated as the
+// fully-qualified App ID and the AppID will be the string following the "~".
+func UseInfo(c context.Context, aid string) context.Context {
+	if c.Value(&memContextKey) != nil {
+		panic(errors.New("memory.Use: called twice on the same Context"))
+	}
+
+	fqAppID := aid
+	if parts := strings.SplitN(fqAppID, "~", 2); len(parts) == 2 {
+		aid = parts[1]
+	}
+
+	memctx := newMemContext(fqAppID)

[Vadim Sh., 2017/04/20 00:12:46]

this thing has mentions of datastore and task queues. "Info" service is,
supposedly, unrelated. I'm not sure what will happen if we return
half-initialized mem context.

I don't know this code very well... I think Robbie should review.

+	c = context.WithValue(c, &memContextKey, memctx)
+
+	return useGI(useGID(c, func(mod *globalInfoData) {
+		mod.appID = aid
+		mod.fqAppID = fqAppID
+	}))
+}
+
 // UseWithAppID adds implementations for the following gae services to the
 // context:
 //   * github.com/luci/gae/service/datastore
@@ -117,23 +141,9 @@ func Use(c context.Context) context.Context {
 //
 // Using this more than once per context.Context will cause a panic.
 func UseWithAppID(c context.Context, aid string) context.Context {
-	if c.Value(&memContextKey) != nil {
-		panic(errors.New("memory.Use: called twice on the same Context"))
-	}
 	c = memlogger.Use(c)
-
-	fqAppID := aid
-	if parts := strings.SplitN(fqAppID, "~", 2); len(parts) == 2 {
-		aid = parts[1]
-	}
-
-	memctx := newMemContext(fqAppID)
-	c = context.WithValue(c, &memContextKey, memctx)
-	c = useGID(c, func(mod *globalInfoData) {
-		mod.appID = aid
-		mod.fqAppID = fqAppID
-	})
-	return useMod(useMail(useUser(useTQ(useRDS(useMC(useGI(c)))))))
+	c = UseInfo(c, aid) // Panics if UseWithAppID is called twice.
+	return useMod(useMail(useUser(useTQ(useRDS(useMC(c))))))
 }
 
 func cur(c context.Context) (*memContext, bool) {