Store source generators' states

Previously, Bloop would not store the state of source generators after
their run, which would cause them to always be re-run, even when there
are no changes.

Author: Duhemm

frontend/src/main/scala/bloop/engine/caches/SourceGeneratorCache.scala

@@ -9,22 +9,25 @@ import bloop.logging.Logger
 import bloop.task.Task
 
 final class SourceGeneratorCache private (
-    cache: ConcurrentHashMap[SourceGenerator, SourceGenerator.Run]
+    cache: ConcurrentHashMap[SourceGenerator, Task[SourceGenerator.Run]]
 ) {
   def update(
       sourceGenerator: SourceGenerator,
       logger: Logger,
       opts: CommonOptions
   ): Task[List[AbsolutePath]] = {
-    val previous = getStateFor(sourceGenerator)
-    sourceGenerator.update(previous, logger, opts).map {
-      case SourceGenerator.NoRun => Nil
-      case SourceGenerator.PreviousRun(_, outputs) => outputs.keys.toList.sortBy(_.syntax)
-    }
-  }
-
-  private def getStateFor(sourceGenerator: SourceGenerator): SourceGenerator.Run = {
-    Option(cache.get(sourceGenerator)).getOrElse(SourceGenerator.NoRun)
+    cache
+      .compute(
+        sourceGenerator,
+        { (_, prev) =>
+          val previous = Option(prev).getOrElse(Task.now(SourceGenerator.NoRun))
+          previous.flatMap(sourceGenerator.update(_, logger, opts)).memoize
+        }
+      )
+      .map {
+        case SourceGenerator.NoRun => Nil
+        case SourceGenerator.PreviousRun(_, outputs) => outputs.keys.toList.sortBy(_.syntax)
+      }
   }
 }
 

frontend/src/test/scala/bloop/SourceGeneratorSpec.scala

@@ -209,6 +209,25 @@ object SourceGeneratorSpec extends bloop.testing.BaseSuite {
     }
   }
 
+  test("source generator is not re-run when nothing has changed") {
+    singleProjectWithSourceGenerator("glob:*.in" :: Nil) { (workspace, project, state) =>
+      val generatorOutput = project.config.sourceGenerators
+        .flatMap(_.headOption)
+        .map(p => AbsolutePath(p.outputDirectory))
+      writeFile(workspace.resolve("hello.in"), "hello")
+      writeFile(project.srcFor("test.scala", exists = false), assertNInputs(n = 1))
+      val compiledState1 = state.compile(project)
+      val origHash = sourceHashFor("NameLengths_1.scala", project, compiledState1)
+      assertExitStatus(compiledState1, ExitStatus.Ok)
+
+      val compiledState2 = compiledState1.compile(project)
+      assertExitStatus(compiledState2, ExitStatus.Ok)
+
+      val newHash = sourceHashFor("NameLengths_1.scala", project, compiledState2)
+      assertEquals(origHash, newHash)
+    }
+  }
+
   private def assertNInputs(n: Int): String =
     s"""class Foo {
        |  val length = generated.NameLengths.args_${n}