util-app: Add App.loadServiceBindings API

Summary: Problem

Some users want compile time assurance that a specific implementation
will be used for `LoadService.apply`.

Solution

Provide a new `App.loadServiceBindings` API that allows a specific
implementation to be specified.

Result

Users can have more assurance of which implementation is used. A lint
rule is added to protect against duplicate registrations.

JIRA Issues: csl-6125

Differential Revision: https://phabricator.twitter.biz/D146554

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

@@ -21,6 +21,7 @@ trait Linters { app: App =>
     TooManyCumulativeGaugesRules() ++
       Seq(
         SchedulerBlockingRule(),
+        DuplicateLoadServiceBindings(),
         NumberOfStatsReceiversRule(),
         StackRegistryDuplicatesRule(ClientRegistry, Set.empty),
         StackRegistryDuplicatesRule(ServerRegistry, Set.empty),

server/src/main/scala/com/twitter/server/lint/DuplicateLoadServiceBindings.scala

@@ -0,0 +1,34 @@
+package com.twitter.server.lint
+
+import com.twitter.app.LoadService
+import com.twitter.util.lint.{Category, Issue, Rule}
+
+/**
+ * Lint rule for duplicate calls to `LoadService.bind`.
+ */
+object DuplicateLoadServiceBindings {
+
+  def apply(): Rule = Rule(
+    Category.Configuration,
+    "Duplicate calls to `LoadService.bind`",
+    """
+      |`LoadService.bind` allows users to specify a specific
+      |implementation. If this is getting called multiple
+      |times for the same interface, it indicates a setup/configuration
+      |issue that may cause surprises.
+    """.stripMargin
+  ) {
+    issues(LoadService.duplicateBindings)
+  }
+
+  /** exposed for testing */
+  private[lint] def issues(dupes: Set[Class[_]]): Seq[Issue] =
+    if (dupes.isEmpty) {
+      Nil
+    } else {
+      dupes.map { dupe =>
+        Issue(s"Duplicate for interface: ${dupe.getName}")
+      }.toSeq
+    }
+
+}

server/src/test/scala/com/twitter/server/lint/DuplicateLoadServiceBindingsTest.scala

@@ -0,0 +1,20 @@
+package com.twitter.server.lint
+
+import org.scalatest.FunSuite
+
+class DuplicateLoadServiceBindingsTest extends FunSuite {
+
+  private trait TestTrait
+
+  test("no duplicates") {
+    assert(DuplicateLoadServiceBindings.issues(Set.empty).isEmpty)
+  }
+
+  test("includes the names of the duplicates") {
+    val cls = classOf[TestTrait]
+    assert(cls.getName == "com.twitter.server.lint.DuplicateLoadServiceBindingsTest$TestTrait")
+    val issues = DuplicateLoadServiceBindings.issues(Set(cls))
+    assert(issues.head.details.contains(cls.getName))
+  }
+
+}