Merge pull request #320 from fsprojects/repo-assist/perf-fix-pairwise-splitat-20260425-0ce5ee841dd9747c

[Repo Assist] perf+fix: optimize pairwise, fix splitAt/tryTail enumerator disposal on exception

Author: dsyme

RELEASE_NOTES.md

@@ -1,3 +1,8 @@
+### 4.17.0
+
+* Performance: Optimised `AsyncSeq.pairwise` to use a `hasPrev` flag and a direct `mutable` field instead of wrapping the previous element in `Some`. Previously, each iteration allocated a new `'T option` object on the heap; the new implementation eliminates that allocation entirely, reducing GC pressure for long sequences.
+* Bug fix: `AsyncSeq.splitAt` and `AsyncSeq.tryTail` now correctly dispose the underlying enumerator when an exception or cancellation occurs during the initial `MoveNext` call. Previously the enumerator could leak if the source sequence threw during the first few steps.
+
 ### 4.16.0
 
 * Performance: Replaced `ref` cells with `mutable` locals in the `ofSeq`, `tryWith`, and `tryFinally` enumerator state machines. Each call to `ofSeq` (or any async CE block using `try...with` / `try...finally` / `use`) previously heap-allocated a `Ref<T>` wrapper object per enumerator; it now uses a direct mutable field in the generated class, reducing GC pressure. The change is equivalent to the `mutable`-for-`ref` improvement introduced in 4.11.0 for other enumerators.

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -1306,13 +1306,14 @@ module AsyncSeq =
       use ie = source.GetEnumerator()
       let! v = ie.MoveNext()
       let mutable b = v
-      let mutable prev = None
+      // Use a flag + mutable field instead of Option to avoid per-element heap allocation
+      let mutable hasPrev = false
+      let mutable prev = Unchecked.defaultof<'T>
       while b.IsSome do
           let v = b.Value
-          match prev with
-          | None -> ()
-          | Some p -> yield (p, v)
-          prev <- Some v
+          if hasPrev then yield (prev, v)
+          hasPrev <- true
+          prev <- v
           let! moven = ie.MoveNext()
           b <- moven }
 
@@ -2152,51 +2153,59 @@ module AsyncSeq =
 
   let tryTail (source: AsyncSeq<'T>) : Async<AsyncSeq<'T> option> = async {
     let ie = source.GetEnumerator()
-    let! first = ie.MoveNext()
-    match first with
-    | None ->
+    try
+        let! first = ie.MoveNext()
+        match first with
+        | None ->
+            ie.Dispose()
+            return None
+        | Some _ ->
+            return Some (asyncSeq {
+                try
+                    let! next = ie.MoveNext()
+                    let mutable b = next
+                    while b.IsSome do
+                        yield b.Value
+                        let! moven = ie.MoveNext()
+                        b <- moven
+                finally
+                    ie.Dispose() })
+    with ex ->
         ie.Dispose()
-        return None
-    | Some _ ->
-        return Some (asyncSeq {
-            try
-                let! next = ie.MoveNext()
-                let mutable b = next
-                while b.IsSome do
-                    yield b.Value
-                    let! moven = ie.MoveNext()
-                    b <- moven
-            finally
-                ie.Dispose() }) }
+        return raise ex }
 
   /// Splits an async sequence at the given index, returning the first `count` elements as an array
   /// and the remaining elements as a new AsyncSeq. The source is enumerated once.
   let splitAt (count: int) (source: AsyncSeq<'T>) : Async<'T array * AsyncSeq<'T>> = async {
       if count < 0 then invalidArg "count" "must be non-negative"
       let ie = source.GetEnumerator()
-      let ra = ResizeArray<'T>()
-      let! m = ie.MoveNext()
-      let mutable b = m
-      while b.IsSome && ra.Count < count do
-          ra.Add b.Value
-          let! next = ie.MoveNext()
-          b <- next
-      let first = ra.ToArray()
-      let rest =
-          if b.IsNone then
-              ie.Dispose()
-              empty<'T>
-          else
-              let cur = ref b
-              asyncSeq {
-                  try
-                      while cur.Value.IsSome do
-                          yield cur.Value.Value
-                          let! next = ie.MoveNext()
-                          cur.Value <- next
-                  finally
-                      ie.Dispose() }
-      return first, rest }
+      try
+          let ra = ResizeArray<'T>()
+          let! m = ie.MoveNext()
+          let mutable b = m
+          while b.IsSome && ra.Count < count do
+              ra.Add b.Value
+              let! next = ie.MoveNext()
+              b <- next
+          let first = ra.ToArray()
+          let rest =
+              if b.IsNone then
+                  ie.Dispose()
+                  empty<'T>
+              else
+                  let mutable cur = b
+                  asyncSeq {
+                      try
+                          while cur.IsSome do
+                              yield cur.Value
+                              let! next = ie.MoveNext()
+                              cur <- next
+                      finally
+                          ie.Dispose() }
+          return first, rest
+      with ex ->
+          ie.Dispose()
+          return raise ex }
 
   let toArrayAsync (source : AsyncSeq<'T>) : Async<'T[]> = async {
       let ra = (new ResizeArray<_>())

src/FSharp.Control.AsyncSeq/FSharp.Control.AsyncSeq.fsproj

@@ -24,7 +24,7 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Update="FSharp.Core" Version="4.7.2" />
-    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="10.0.6" />
+    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="10.0.7" />
     <PackageReference Include="System.Threading.Channels" Version="*" />
     <Content Include="*.fsproj; **\*.fs; **\*.fsi;" PackagePath="fable\" />
   </ItemGroup>

tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs

@@ -2816,6 +2816,13 @@ let ``AsyncSeq.pairwise with three elements should produce two pairs`` () =
   let result = AsyncSeq.pairwise source |> AsyncSeq.toListSynchronously
   Assert.AreEqual([(1, 2); (2, 3)], result)
 
+[<Test>]
+let ``AsyncSeq.pairwise with many elements produces correct pairs`` () =
+  let source = AsyncSeq.ofSeq [1..10]
+  let result = AsyncSeq.pairwise source |> AsyncSeq.toListSynchronously
+  let expected = [(1,2); (2,3); (3,4); (4,5); (5,6); (6,7); (7,8); (8,9); (9,10)]
+  Assert.AreEqual(expected, result)
+
 [<Test>]
 let ``AsyncSeq.windowed empty sequence returns empty`` () =
   let result = AsyncSeq.windowed 3 AsyncSeq.empty<int> |> AsyncSeq.toListSynchronously
@@ -3746,6 +3753,19 @@ let ``AsyncSeq.splitAt with negative count throws ArgumentException`` () =
   Assert.Throws<System.ArgumentException>(fun () ->
     AsyncSeq.splitAt -1 AsyncSeq.empty<int> |> Async.RunSynchronously |> ignore) |> ignore
 
+[<Test>]
+let ``AsyncSeq.splitAt disposes enumerator when source throws during collection`` () =
+  let mutable disposed = false
+  let source = asyncSeq {
+    use _ = { new System.IDisposable with member _.Dispose() = disposed <- true }
+    yield 1
+    yield 2
+    failwith "source error"
+  }
+  try AsyncSeq.splitAt 10 source |> Async.RunSynchronously |> ignore
+  with _ -> ()
+  Assert.IsTrue(disposed, "enumerator should be disposed after exception during collection")
+
 // ===== removeAt =====
 
 [<Test>]
@@ -4190,6 +4210,20 @@ let ``AsyncSeq.tryTail returns all-but-first elements`` () =
   let tail = result.Value |> AsyncSeq.toListAsync |> Async.RunSynchronously
   Assert.AreEqual([2;3;4;5], tail)
 
+[<Test>]
+let ``AsyncSeq.tryTail disposes enumerator when source throws on first MoveNext`` () =
+  let mutable disposed = false
+  // Use a pre-failed task so the exception occurs during MoveNext() (async), not during GetEnumerator()
+  let failedTask = System.Threading.Tasks.Task.FromException<unit>(System.Exception("source error"))
+  let source = asyncSeq {
+    use _ = { new System.IDisposable with member _.Dispose() = disposed <- true }
+    let! _ = failedTask |> Async.AwaitTask
+    yield 1
+  }
+  try AsyncSeq.tryTail source |> Async.RunSynchronously |> ignore
+  with _ -> ()
+  Assert.IsTrue(disposed, "enumerator should be disposed after exception on first MoveNext")
+
 [<Test>]
 let ``AsyncSeq.where is alias for filter`` () =
   let result =