fn: add ParSliceErrCollect

Add ParSliceErrCollect function to handle errors without halting
parallel processing. ParSliceErrCollect is similar to ParSlice but
allows processing to continue even when errors occur. Instead of halting
on the first error, it collects all errors for later handling, providing
more robust parallel execution.

Author: ffranr

fn/concurrency.go

@@ -2,7 +2,9 @@ package fn
 
 import (
 	"context"
+	"fmt"
 	"runtime"
+	"sync"
 
 	"golang.org/x/sync/errgroup"
 )
@@ -32,3 +34,48 @@ func ParSlice[V any](ctx context.Context, s []V, f ErrFunc[V]) error {
 
 	return errGroup.Wait()
 }
+
+// ParSliceErrCollect can be used to execute a function on each element of a
+// slice in parallel. This function is fully blocking and will wait for all
+// goroutines to finish (subject to context cancellation/timeout). Any errors
+// will be collected and returned as a map of slice element index to error.
+// Active goroutines limited with number of CPU.
+func ParSliceErrCollect[V any](ctx context.Context, s []V,
+	f ErrFunc[V]) (map[int]error, error) {
+
+	errGroup, ctx := errgroup.WithContext(ctx)
+	errGroup.SetLimit(runtime.NumCPU())
+
+	var instanceErrorsMutex sync.Mutex
+	instanceErrors := make(map[int]error, len(s))
+
+	for idx := range s {
+		errGroup.Go(func() error {
+			err := f(ctx, s[idx])
+			if err != nil {
+				instanceErrorsMutex.Lock()
+				instanceErrors[idx] = err
+				instanceErrorsMutex.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 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 {
+		return nil, fmt.Errorf("failed to wait on error group in "+
+			"ParSliceErrorCollect: %w", err)
+	}
+
+	return instanceErrors, nil
+}