errorsbp: Update for go 1.20

Go 1.20 is going to add native support for batched/multi errors, and we
will be able to eventually deprecate errorsbp.Batch, but we can only do
so after we dropped support for go 1.19.

Before that, make errorsbp.Batch play nicer with go 1.20 by:

* Implement the new `Unwrap() []error` optional interface.
* Make errorsbp.BatchSize also support other errors implementing the new
  optional interface.

errorsbp/batch.go

@@ -10,14 +10,23 @@ import (
 var (
 	_ error = Batch{}
 	_ error = (*Batch)(nil)
+
+	_ batchUnwrapper = Batch{}
+	_ batchUnwrapper = (*Batch)(nil)
 )
 
+type batchUnwrapper interface {
+	Unwrap() []error
+}
+
 // Batch is an error that can contain multiple errors.
 //
 // The zero value of Batch is valid (with no errors) and ready to use.
 //
 // This type is not thread-safe.
 // The same batch should not be operated on different goroutines concurrently.
+//
+// To be deprecated when we drop support for go 1.19.
 type Batch struct {
 	errors []error
 }
@@ -185,10 +194,19 @@ func (be Batch) GetErrors() []error {
 	return errors
 }
 
+// Unwrap implements the optional interface defined in go 1.20.
+//
+// It's an alias to GetErrors.
+func (be Batch) Unwrap() []error {
+	return be.GetErrors()
+}
+
 // BatchSize returns the size of the batch for error err.
 //
 // If err is either errorsbp.Batch or *errorsbp.Batch,
 // this function returns its Len().
+// If err implements `Unwrap() []error` (optional interface defined in go 1.20),
+// it returns the length of its Unwrap return.
 // Otherwise, it returns 1 if err is non-nil, and 0 if err is nil.
 //
 // It's useful in tests,
@@ -202,6 +220,9 @@ func BatchSize(err error) int {
 	if errors.As(err, &be) {
 		return be.Len()
 	}
+	if unwrapper, ok := err.(batchUnwrapper); ok {
+		return len(unwrapper.Unwrap())
+	}
 	// single, non-batch error.
 	return 1
 }

errorsbp/batch_test.go

@@ -2,6 +2,7 @@ package errorsbp_test
 
 import (
 	"errors"
+	"fmt"
 	"reflect"
 	"testing"
 
@@ -265,3 +266,59 @@ func TestAddPrefix(t *testing.T) {
 		}
 	}
 }
+
+func TestBatchSize(t *testing.T) {
+	for _, c := range []struct {
+		label string
+		err   error
+		want  int
+	}{
+		{
+			label: "nil",
+			err:   nil,
+			want:  0,
+		},
+		{
+			label: "errors.New",
+			err:   errors.New("foo"),
+			want:  1,
+		},
+		{
+			label: "fmt.Errorf-wrap-single",
+			err:   fmt.Errorf("bar: %w", errors.New("foo")),
+			want:  1,
+		},
+		{
+			label: "batch-0",
+			err:   new(errorsbp.Batch),
+			want:  0,
+		},
+		{
+			label: "batch-1",
+			want:  1,
+			err: func() error {
+				var batch errorsbp.Batch
+				batch.Add(errors.New("foo"))
+				return batch
+			}(),
+		},
+		{
+			label: "batch-2",
+			want:  2,
+			err: func() error {
+				var batch errorsbp.Batch
+				batch.Add(errors.New("foo"))
+				batch.Add(errors.New("bar"))
+				return batch
+			}(),
+		},
+		// TODO: Add cases from errors.Join and fmt.Errorf once we drop support for
+		// go 1.19.
+	} {
+		t.Run(c.label, func(t *testing.T) {
+			if got := errorsbp.BatchSize(c.err); got != c.want {
+				t.Errorf("errorsbp.BatchSize(%#v) got %v want %v", c.err, got, c.want)
+			}
+		})
+	}
+}