fn: fix loop var capture and clarify naming in ParSliceErrCollect

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)
 		})
@@ -43,39 +48,58 @@ 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 {
+		// 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 an
+			// 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.
+			// Do not return an error here. If we did, errGroup
+			// would cancel the context and stop all other element
+			// processors. Instead, record the error locally
+			// and return it after all goroutines finish.
 			return nil
 		})
 	}
 
-	// Now we will wait/block for all goroutines to finish.
-	//
-	// 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 {
+	// Wait for all goroutines to finish. In this design, goroutines do not
+	// return errors, so Wait should normally return nil. We handle context
+	// cancellation separately with an explicit ctx.Err() check.
+	if err := errGroup.Wait(); err != nil {
 		return nil, fmt.Errorf("failed to wait on error group in "+
 			"ParSliceErrorCollect: %w", err)
 	}
 
+	// If the caller's context was canceled or timed out, surface that.
+	// Return whatever per-item errors were collected before cancellation.
+	if err := ctx.Err(); err != nil {
+		return instanceErrors, err
+	}
+
 	return instanceErrors, nil
 }