Prevent Hotswap#get from acquiring a lock when there is no Resource present

std/shared/src/main/scala/cats/effect/std/Hotswap.scala

@@ -73,8 +73,9 @@ sealed trait Hotswap[F[_], R] {
   def swap(next: Resource[F, R]): F[R]
 
   /**
-   * Gets the current resource, if it exists. The returned resource is guaranteed to be
-   * available for the duration of the returned resource.
+   * Acquires a shared lock to retrieve the current resource, if it exists. The returned
+   * resource is guaranteed to be available for its duration. The lock is released if the
+   * current resource does not exist.
    */
   def get: Resource[F, Option[R]]
 
@@ -121,8 +122,6 @@ object Hotswap {
       def raise(message: String): F[Unit] =
         F.raiseError[Unit](new RuntimeException(message))
 
-      def shared: Resource[F, Unit] = semaphore.permit
-
       def exclusive: Resource[F, Unit] =
         Resource.makeFull[F, Unit](poll => poll(semaphore.acquireN(Long.MaxValue)))(_ =>
           semaphore.releaseN(Long.MaxValue))
@@ -141,12 +140,13 @@ object Hotswap {
             }
 
           override def get: Resource[F, Option[R]] =
-            shared.evalMap { _ =>
-              state.get.map {
-                case Acquired(r, _) => Some(r)
-                case _ => None
-              }
-            }
+            Resource.makeFull[F, Option[R]] { poll =>
+              poll(semaphore.acquire) *>
+                state.get.flatMap {
+                  case Acquired(r, _) => F.pure(Some(r))
+                  case _ => semaphore.release.as(None)
+                }
+            } { r => if (r.isDefined) semaphore.release else F.unit }
 
           override def clear: F[Unit] =
             exclusive.surround(swapFinalizer(Cleared).uncancelable)

tests/shared/src/test/scala/cats/effect/std/HotswapSpec.scala

@@ -165,6 +165,16 @@ class HotswapSpec extends BaseSpec { outer =>
 
       TestControl.executeEmbed(go, IORuntimeConfig(1, 2)).replicateA_(1000) must completeAs(())
     }
-  }
 
+    "get should not acquire a lock when there is no resource present" in ticked {
+      implicit ticker =>
+        val go = Hotswap.create[IO, Unit].use { hs =>
+          hs.get.useForever.start *>
+            hs.swap(IO.sleep(1.second).toResource) *>
+            IO.sleep(2.seconds) *>
+            hs.get.use_.timeout(1.second).void
+        }
+        go must completeAs(())
+    }
+  }
 }