scrooge-core: Define union field info getter for union class

Problem

Some code in Schemer and Scalacheck is using reflection to
retrieve the fieldInfo field from union member classes.

Solution

Add a supported API, `ThriftUnion.fieldInfoForUnionClass`
that works the same as `unionStructFieldInfo` method on an
instance, but it takes the class as a parameter and thus
doesn't require instantiation of the class.

JIRA Issues: CSL-11926

Differential Revision: https://phabricator.twitter.biz/D871986

Author: szegedi

CHANGELOG.rst

@@ -7,6 +7,13 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+New Features
+~~~~~~~~~~~~
+
+* scrooge-core: `c.t.scrooge.ThriftUnion.fieldInfoForUnionClass` API for retrieving
+  `ThriftStructFieldInfo` for a `ThriftUnion` member class without having to instantiate
+  it. ``PHAB_ID=D871986``
+
 22.4.0
 ------
 

scrooge-core/src/main/scala/com/twitter/scrooge/ThriftUnion.scala

@@ -1,5 +1,7 @@
 package com.twitter.scrooge
 
+import com.twitter.util.Memoize
+
 /**
  * Unions are tagged with this trait.
  */
@@ -28,3 +30,27 @@ trait ThriftUnion extends ThriftStruct {
    */
   def unionStructFieldInfo: Option[ThriftStructFieldInfo]
 }
+
+object ThriftUnion {
+  private[this] val fieldInfos = Memoize.classValue { c =>
+    if (!classOf[ThriftUnion].isAssignableFrom(c)) {
+      throw new IllegalArgumentException(s"${c.getName} does not extend ThriftUnion")
+    }
+    ThriftStructMetaData
+      .forStructClass(c.asInstanceOf[Class[ThriftUnion]])
+      .unionFields
+      .collectFirst {
+        case f if f.fieldClassTag.runtimeClass == c => f.structFieldInfo
+      }
+  }
+
+  /**
+   * Given a specific union member class, returns the struct field info for it.
+   * It returns the same value as invoking `unionStructFieldInfo` on an instance
+   * of the class, without having to create an instance of it.
+   * @param c the union member class
+   * @return the struct field info for the class, or None
+   */
+  def fieldInfoForUnionClass(c: Class[_ <: ThriftUnion]): Option[ThriftStructFieldInfo] =
+    fieldInfos(c)
+}

scrooge-generator-tests/src/test/scala/com/twitter/scrooge/backend/UnionFieldRetrievalSpec.scala

@@ -0,0 +1,27 @@
+package com.twitter.scrooge.backend
+
+import com.twitter.scrooge._
+import com.twitter.scrooge.testutil.EvalHelper
+import com.twitter.scrooge.testutil.JMockSpec
+import includes.a.thriftscala._
+
+class UnionFieldRetrievalSpec extends JMockSpec with EvalHelper {
+  "UnionFieldRetrievalSpec" should {
+    "retrieve field info for a union member class" in { _ =>
+      ThriftUnion.fieldInfoForUnionClass(classOf[City.CityState]) must be(
+        Some(City.CityState.fieldInfo))
+      ThriftUnion.fieldInfoForUnionClass(classOf[City.Zipcode]) must be(
+        Some(City.Zipcode.fieldInfo))
+    }
+
+    "retrieve no field info for a top-level union class" in { _ =>
+      ThriftUnion.fieldInfoForUnionClass(classOf[City]) must be(None)
+    }
+
+    "fail to retrieve field info for a non-union class" in { _ =>
+      assertThrows[IllegalArgumentException] {
+        ThriftUnion.fieldInfoForUnionClass(classOf[Address].asInstanceOf[Class[ThriftUnion]])
+      }
+    }
+  }
+}