finagle-core, twitter-server: Add c.t.finagle.DtabFlags

Problem

There is currently no defined way for users of the `c.t.finagle.Dtab` API to
append to the `Dtab.base` delegation table with a supplemental value that is
passed at runtime.

Solution

Add the `c.t.finagle.DtabFlags` trait which defines a `c.t.app.Flag`, "dtab.add"
that can provide a supplemental delegation table (`Dtab`) to append to the
current server `Dtab.base`.

The trait exposes a `addDtabs()` function which applies the parsed Flag value
and appends it to the current `Dtab.base`.

Result

Users are able to augment the base delegation table (`Dtab.base`) of the server
with a value passed at runtime via the command line.

JIRA Issues: CSL-7766

Differential Revision: https://phabricator.twitter.biz/D297596

CHANGELOG.rst

@@ -12,6 +12,14 @@ Changes
 
 * Remove deprecated uses of `c.t.server.ShadowAdminServer`. ``PHAB_ID=D269149``
 
+* Mix in the `c.t.finagle.DtabFlags` to allow servers to append to the "base" `c.t.finagle.Dtab`
+  delegation table. Users can now call `c.t.finagle.DtabFlags#addDtabs()` when they want to append
+  the parsed Flag value to the `Dtab.base` delegation table. Users should note to only call this
+  method _after_ Flag parsing has occurred (which is after **init** and before **premain**).
+
+  We also update the `c.t.server.handler.DtabHandler` to always return a proper JSON response of
+  the currently configured `c.t.finagle.Dtab.base`. ``PHAB_ID=D297596``
+
 19.3.0
 ------
 

server/src/main/scala/com/twitter/server/Admin.scala

@@ -138,7 +138,7 @@ trait Admin { self: App with AdminHttpServer with Stats =>
       ),
       Route(
         path = "/admin/dtab",
-        handler = new TextBlockView().andThen(new DtabHandler),
+        handler = new DtabHandler,
         alias = "Dtab",
         group = Some(Grouping.ProcessInfo),
         includeInIndex = true
@@ -195,7 +195,7 @@ trait Admin { self: App with AdminHttpServer with Stats =>
       ),
       Route(
         path = LoadBalancersHandler.RoutePath,
-        handler = new LoadBalancersHandler(),
+        handler = new LoadBalancersHandler,
         alias = "Load Balancers",
         group = Some(Grouping.ProcessInfo),
         includeInIndex = true

server/src/main/scala/com/twitter/server/FlagResolver.scala

@@ -4,8 +4,9 @@ import com.twitter.app.GlobalFlag
 import com.twitter.finagle.{Addr, Resolver, Name}
 import com.twitter.util.Var
 
-// TODO: deprecate in favor of Wily dtabs.
-
+@deprecated(
+  "Users should prefer using Dtabs which are overridable by setting the `dtab.add` flag",
+  "2019-04-03")
 object resolverMap
     extends GlobalFlag[Map[String, String]](
       Map.empty,

server/src/main/scala/com/twitter/server/TwitterServer.scala

@@ -1,6 +1,7 @@
 package com.twitter.server
 
 import com.twitter.app.App
+import com.twitter.finagle.DtabFlags
 import com.twitter.finagle.util.DefaultTimer
 import com.twitter.util.Timer
 import com.twitter.util.logging.Logging
@@ -36,6 +37,7 @@ trait TwitterServer
     with Slf4jBridge
     with Logging
     with Linters
+    with DtabFlags
     with Hooks
     with AdminHttpServer
     with Admin

server/src/main/scala/com/twitter/server/handler/DtabHandler.scala

@@ -1,18 +1,40 @@
 package com.twitter.server.handler
 
+import com.twitter.finagle.http.{MediaType, Request, Response}
 import com.twitter.finagle.{Dtab, Service}
-import com.twitter.finagle.http.{Request, Response}
-import com.twitter.server.util.HttpUtils.newOk
+import com.twitter.io.Buf
+import com.twitter.server.util.{HttpUtils, JsonConverter}
 import com.twitter.util.Future
 
 /**
- * Dumps a simple string representation of the current Dtab.
+ * Dumps a simple JSON string representation of the current [[com.twitter.finagle.Dtab.base]].
  *
  * From the Dtab docs: A Dtab--short for delegation table--comprises a sequence
- * of delegation rules. Together, these describe how to bind a
- * path to an Addr.
+ * of delegation rules. Together, these describe how to bind a path to an Addr.
+ *
+ * {{{
+ * {
+ *   "dtab": [
+ *     "/srv => /srv#/production",
+ *     "/srv => /srv#/prod",
+ *     "/s => /srv/local",
+ *     "/$/inet => /$/nil",
+ *     "/zk => /$/nil"
+ *   ]
+ * }
+ * }}}
+ *
+ * @see [[com.twitter.finagle.Dtab]]
  */
-class DtabHandler extends Service[Request, Response] {
+final class DtabHandler extends Service[Request, Response] {
   def apply(req: Request): Future[Response] =
-    newOk(Dtab.base.toString)
+    HttpUtils.newResponse(contentType = MediaType.Json, content = Buf.Utf8(jsonResponse))
+
+  private[this] def jsonResponse: String = {
+    JsonConverter.writeToString(Map("dtab" -> formattedDtabEntries))
+  }
+
+  private[this] def formattedDtabEntries: Seq[String] = {
+    for (dentry <- Dtab.base) yield { s"${dentry.prefix.show} => ${dentry.dst.show}" }
+  }
 }