From ab68cdc420e46512b125d37386d177ed1c82a463 Mon Sep 17 00:00:00 2001
From: Julian Brunner <julian.brunner@gmail.com>
Date: Mon, 26 Aug 2024 05:10:19 +0200
Subject: [PATCH 1/4] avoid dropping upstream items in mergeSource

---
 conduit/src/Data/Conduit/Internal/Conduit.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/conduit/src/Data/Conduit/Internal/Conduit.hs b/conduit/src/Data/Conduit/Internal/Conduit.hs
index 9139fa353..8384e7d3e 100644
--- a/conduit/src/Data/Conduit/Internal/Conduit.hs
+++ b/conduit/src/Data/Conduit/Internal/Conduit.hs
@@ -613,7 +613,7 @@ mergeSource = loop . sealConduitT
         go a = do
           (src1, mi) <- lift $ src0 $$++ await
           case mi of
-            Nothing -> return ()
+            Nothing -> leftover a
             Just i  -> yield (i, a) >> loop src1
 
 

From a200ebd3a997d9a349519076cf1cd6b3e4557e88 Mon Sep 17 00:00:00 2001
From: Julian Brunner <julian.brunner@gmail.com>
Date: Mon, 26 Aug 2024 15:52:53 +0200
Subject: [PATCH 2/4] add test case for mergeSource item dropping

---
 conduit/test/main.hs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/conduit/test/main.hs b/conduit/test/main.hs
index c7860c622..5ee5b1cd8 100644
--- a/conduit/test/main.hs
+++ b/conduit/test/main.hs
@@ -704,6 +704,12 @@ main = hspec $ do
                 withShortAlphaIndex = CI.mergeSource (CL.sourceList ["A", "B", "C"])
             output <- runConduit $ src .| withShortAlphaIndex .| CL.consume
             output `shouldBe` [("A", 1), ("B", 2), ("C", 3)]
+        it "does not drop upstream items" $ do
+            let num = CL.sourceList [1 .. 10 :: Int]
+            let chr = CL.sourceList ['a' .. 'c']
+            (output, remainder) <- runConduit $ num .| liftA2 (,) (CI.mergeSource chr .| CL.consume) CL.consume
+            output `shouldBe` [('a', 1), ('b', 2), ('c', 3)]
+            remainder `shouldBe` [4 .. 10]
 
     describe "passthroughSink" $ do
         it "works" $ do

From 67478550c371b283f683c843d648a8c5b98f5290 Mon Sep 17 00:00:00 2001
From: Julian Brunner <julian.brunner@gmail.com>
Date: Mon, 26 Aug 2024 15:55:29 +0200
Subject: [PATCH 3/4] add changelog entry for #513

---
 conduit/ChangeLog.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/conduit/ChangeLog.md b/conduit/ChangeLog.md
index 1f7cce1aa..33d498863 100644
--- a/conduit/ChangeLog.md
+++ b/conduit/ChangeLog.md
@@ -1,5 +1,9 @@
 # ChangeLog for conduit
 
+## 1.3.6
+
+* Avoid dropping upstream items in `mergeSource` [#513](https://github.com/snoyberg/conduit/pull/513)
+
 ## 1.3.5
 
 * Add `groupOn`

From 84c7abc10e85ad1e9c12de8efcf53d9d7da2d1f3 Mon Sep 17 00:00:00 2001
From: Julian Brunner <julian.brunner@gmail.com>
Date: Mon, 26 Aug 2024 16:38:56 +0200
Subject: [PATCH 4/4] import liftA2 for tests to work on older versions of base

---
 conduit/test/main.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/conduit/test/main.hs b/conduit/test/main.hs
index 5ee5b1cd8..2d33ea38e 100644
--- a/conduit/test/main.hs
+++ b/conduit/test/main.hs
@@ -32,7 +32,7 @@ import Control.Monad.Trans.Class (lift)
 import Control.Monad.Trans.Writer (execWriter, tell, runWriterT)
 import Control.Monad.Trans.State (evalStateT, get, put)
 import qualified Control.Monad.Writer as W
-import Control.Applicative (pure, (<$>), (<*>))
+import Control.Applicative (pure, (<$>), (<*>), liftA2)
 import qualified Control.Monad.Catch as Catch
 import Data.Functor.Identity (Identity,runIdentity)
 import Control.Monad (forever, void)