Fixes issue #912: Executes liftF effect twice in a product composition

When combining a Done fetch with a Blocked fetch in product/productR/map2,
the previous implementation would call product(fa, c) or map2(fa, c)(f), causing
the already-completed effect `fa` to be re-run.

The fix uses the already-computed result value instead of re-running
the entire fetch:
- product: use c.map((a, _)) instead of product(fa, c)
- productR: use c or c.as(b) instead of productR(fa)(c)
- map2: use c.map(b => f(a, b)) instead of map2(fa, c)(f)

Added unit tests to verify that side effects are executed exactly once.

Author: v.karamyshev

fetch/src/main/scala/fetch.scala

@@ -301,9 +301,9 @@ object `package` {
             case (Done(a), Done(b)) =>
               Done[F, Z](f(a, b))
             case (Done(a), Blocked(br, c)) =>
-              Blocked[F, Z](br, map2(fa, c)(f))
+              Blocked[F, Z](br, c.map(b => f(a, b)))
             case (Blocked(br, c), Done(b)) =>
-              Blocked[F, Z](br, map2(c, fb)(f))
+              Blocked[F, Z](br, c.map(a => f(a, b)))
             case (Blocked(br, c), Blocked(br2, c2)) =>
               Blocked[F, Z](combineRequestMaps(br, br2), map2(c, c2)(f))
             case (_, Throw(e)) =>
@@ -324,9 +324,9 @@ object `package` {
             case (Done(a), Done(b)) =>
               Done[F, (A, B)]((a, b))
             case (Done(a), Blocked(br, c)) =>
-              Blocked[F, (A, B)](br, product(fa, c))
+              Blocked[F, (A, B)](br, c.map((a, _)))
             case (Blocked(br, c), Done(b)) =>
-              Blocked[F, (A, B)](br, product(c, fb))
+              Blocked[F, (A, B)](br, c.map((_, b)))
             case (Blocked(br, c), Blocked(br2, c2)) =>
               Blocked[F, (A, B)](combineRequestMaps(br, br2), product(c, c2))
             case (_, Throw(e)) =>
@@ -343,9 +343,9 @@ object `package` {
             case (Done(a), Done(b)) =>
               Done[F, B](b)
             case (Done(a), Blocked(br, c)) =>
-              Blocked[F, B](br, productR(fa)(c))
+              Blocked[F, B](br, c)
             case (Blocked(br, c), Done(b)) =>
-              Blocked[F, B](br, productR(c)(fb))
+              Blocked[F, B](br, c.as(b))
             case (Blocked(br, c), Blocked(br2, c2)) =>
               Blocked[F, B](combineRequestMaps(br, br2), productR(c)(c2))
             case (_, Throw(e)) =>

fetch/src/test/scala/FetchTests.scala

@@ -942,4 +942,51 @@ class FetchTests extends FetchSpec {
 
     io.map(_ shouldEqual List(0, 1, 2, 42)).unsafeToFuture()
   }
+
+  // Side effect re-execution tests (Issue #912)
+
+  "Product should not re-execute side effects from completed fetches" in {
+    Ref[IO]
+      .of(0)
+      .flatMap { counter =>
+        val fetchWithSideEffect: Fetch[IO, Unit] = Fetch.liftF(counter.update(_ + 1))
+
+        val program = (
+          List.range(1, 10).map(one[IO]).reduce(_ >> _),
+          fetchWithSideEffect
+        ).tupled
+
+        Fetch.run[IO](program) >> counter.get.map(_ shouldEqual 1)
+      }
+      .unsafeToFuture()
+  }
+
+  "ProductR should not re-execute side effects from completed fetches" in {
+    Ref[IO]
+      .of(0)
+      .flatMap { counter =>
+        val fetchWithSideEffect: Fetch[IO, Unit] = Fetch.liftF(counter.update(_ + 1))
+
+        val program = List.range(1, 10).map(one[IO]).reduce(_ >> _) *> fetchWithSideEffect
+
+        Fetch.run[IO](program) >> counter.get.map(_ shouldEqual 1)
+      }
+      .unsafeToFuture()
+  }
+
+  "mapN should not re-execute side effects from completed fetches" in {
+    Ref[IO]
+      .of(0)
+      .flatMap { counter =>
+        val fetchWithSideEffect: Fetch[IO, Int] = Fetch.liftF(counter.updateAndGet(_ + 1))
+
+        val program = (
+          List.range(1, 10).map(one[IO]).reduce(_ >> _),
+          fetchWithSideEffect
+        ).mapN((_, n) => n)
+
+        Fetch.run[IO](program) >> counter.get.map(_ shouldEqual 1)
+      }
+      .unsafeToFuture()
+  }
 }