Fix bug in Resource.flatMap when acquire throws an exception

Author: japgolly

library/testUtil/src/main/scala/japgolly/scalajs/react/test/internal/Resource.scala

@@ -19,19 +19,23 @@ class Resource[F[_]: Effect, A](private val acquire: F[A], private val release:
     var aOpt: Option[A] = None
     var bReleaseOpt: Option[B => F[Unit]] = None
 
-    Resource.make(
-      F.flatMap(acquire){ a =>
-        aOpt = Some(a)
-        val other: Resource[F, B] = f(a)
-        bReleaseOpt = Some(other.release)
-        other.acquire
-      },
-      b =>
-        (aOpt, bReleaseOpt) match {
-          case (Some(a), Some(bRelease)) => F.finallyRun(bRelease(b), release(a))
-          case _ => F.throwException(new IllegalStateException("Resource.flatMap: release attempted without acquire being invoked") )
+    val newAcquire: F[B] = F.flatMap(acquire) { a =>
+      aOpt = Some(a)
+      val acquireB: F[B] =
+        F.flatMap(F.delay(f(a))) { otherResource =>
+          bReleaseOpt = Some(otherResource.release)
+          otherResource.acquire
         }
-    )
+      F.onError(acquireB, _ => release(a))
+    }
+
+    val newRelease: B => F[Unit] = b =>
+      (aOpt, bReleaseOpt) match {
+        case (Some(a), Some(bRelease)) => F.finallyRun(bRelease(b), release(a))
+        case _ => F.throwException(new IllegalStateException("Resource.flatMap: release attempted without a successful acquire"))
+      }
+
+    Resource.make[F, B](newAcquire, newRelease)
   }
 
   def map[B](f: A => B): Resource[F, B] =

library/tests/src/test/scala/japgolly/scalajs/react/test/internal/ResourceTest.scala

@@ -0,0 +1,22 @@
+package japgolly.scalajs.react.test.internal
+
+import japgolly.scalajs.react._
+import japgolly.microlibs.testutil.TestUtil._
+import utest._
+
+object ResourceTest extends TestSuite {
+
+  override def tests = Tests {
+
+    "error_in_acquire" - {
+      var n = 0
+      val nRes = Resource.make[CallbackTo, Unit](Callback { n += 1 }, _ => Callback { n -= 1 })
+      val errRes = Resource.make[CallbackTo, Int](CallbackTo { ??? : Int }, _ => Callback { n -= 100 })
+      val r = nRes.flatMap(_ => nRes.flatMap(_ => errRes))
+      val t = r.use_(_ => ()).attemptTry.runNow()
+      assert(t.isFailure)
+      assertEq(n, 0)
+    }
+
+  }
+}

library/util/src/main/scala/japgolly/scalajs/react/util/Effect.scala

@@ -17,6 +17,9 @@ trait Effect[F[_]] extends Monad[F] {
 
   @inline final def throwException[A](t: Throwable): F[A] =
     delay(throw t)
+
+  def onError[A, B](fa: => F[A], f: Throwable => F[B]): F[A] =
+    handleError[A, A](fa)(t => finallyRun(throwException[A](t), f(t)))
 }
 
 object Effect extends EffectFallbacks {