fn: fix loop var capture and clarify naming in ParSliceErrCollect

Ensure each goroutine captures its own index/value in
ParSliceErrCollect, and rename variables for clearer intent. Avoids
closure capture bug and makes concurrency guards explicit.

Author: ffranr

fn/concurrency.go

@@ -26,7 +26,12 @@ func ParSlice[V any](ctx context.Context, s []V, f ErrFunc[V]) error {
 	errGroup.SetLimit(runtime.GOMAXPROCS(0))
 
 	for _, v := range s {
+		// Snapshot v now so the goroutine sees the intended value
+		// even if the caller mutates the slice s later. This is a
+		// shallow copy only, if V contains pointers, their contents can
+		// still change.
 		v := v
+
 		errGroup.Go(func() error {
 			return f(ctx, v)
 		})
@@ -43,25 +48,46 @@ func ParSlice[V any](ctx context.Context, s []V, f ErrFunc[V]) error {
 func ParSliceErrCollect[V any](ctx context.Context, s []V,
 	f ErrFunc[V]) (map[int]error, error) {
 
-	errGroup, ctx := errgroup.WithContext(ctx)
+	errGroup, groupCtx := errgroup.WithContext(ctx)
 	errGroup.SetLimit(runtime.GOMAXPROCS(0))
 
-	var instanceErrorsMutex sync.Mutex
-	instanceErrors := make(map[int]error, len(s))
+	var (
+		instanceErrorsMu sync.Mutex
+		instanceErrors   = make(map[int]error, len(s))
+	)
 
 	for idx := range s {
+		// Rebind idx inside the loop so that each goroutine captures
+		// a distinct copy of the index. Without this, all goroutines
+		// may observe the same final idx value due to closure capture.
+		idx := idx
+
+		// Snapshot s[idx] now so the goroutine sees the intended value
+		// even if the caller mutates the slice later. This is a shallow
+		// copy only, if V contains pointers, their contents can still
+		// change.
+		v := s[idx]
+
 		errGroup.Go(func() error {
-			err := f(ctx, s[idx])
+			// If already canceled, skip work without signaling a
+			// error.
+			select {
+			case <-groupCtx.Done():
+				return nil
+			default:
+			}
+
+			err := f(groupCtx, v)
 			if err != nil {
-				instanceErrorsMutex.Lock()
+				instanceErrorsMu.Lock()
 				instanceErrors[idx] = err
-				instanceErrorsMutex.Unlock()
+				instanceErrorsMu.Unlock()
 			}
 
 			// Avoid returning an error here, as that would cancel
 			// the errGroup and terminate all slice element
 			// processing instances. Instead, collect the error and
-			// return it later.
+			// return them later.
 			return nil
 		})
 	}
@@ -71,8 +97,7 @@ func ParSliceErrCollect[V any](ctx context.Context, s []V,
 	// The goroutines that are executing in parallel should not return an
 	// error, but the Wait call may return an error if the context is
 	// canceled or timed out.
-	err := errGroup.Wait()
-	if err != nil {
+	if err := errGroup.Wait(); err != nil {
 		return nil, fmt.Errorf("failed to wait on error group in "+
 			"ParSliceErrorCollect: %w", err)
 	}