Allowed nxtf to signal end of data by returning nil,nil

Added some test
Improved documentation

copy_from.go

@@ -64,10 +64,10 @@ func (cts *copyFromSlice) Err() error {
 	return cts.err
 }
 
-// CopyFromCh returns a CopyFromSource interface over the provided channel.
-// FieldNames is an ordered list of field names to copy from the struct, which
-// order must match the order of the columns.
-func CopyFromFunc(nxtf func() ([]any, error)) CopyFromSource {
+// CopyFromFunc returns a CopyFromSource interface that relies on nxtf for values.
+// nxtf returns rows until it either signals an 'end of data' by returning row=nil and err=nil,
+// or it returns an error. If nxtf returns an error, the copy is aborted.
+func CopyFromFunc(nxtf func() (row []any, err error)) CopyFromSource {
 	return &copyFromFunc{next: nxtf}
 }
 
@@ -79,11 +79,12 @@ type copyFromFunc struct {
 
 func (g *copyFromFunc) Next() bool {
 	g.valueRow, g.err = g.next()
-	return g.err == nil
+	// only return true if valueRow exists and no error
+	return g.valueRow != nil && g.err == nil
 }
 
 func (g *copyFromFunc) Values() ([]any, error) {
-	return g.valueRow, nil
+	return g.valueRow, g.err
 }
 
 func (g *copyFromFunc) Err() error {

copy_from_test.go

@@ -2,7 +2,6 @@ package pgx_test
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"os"
 	"reflect"
@@ -815,7 +814,6 @@ func TestCopyFromFunc(t *testing.T) {
 	)`)
 
 	dataCh := make(chan int, 1)
-	closeChanErr := errors.New("closed channel")
 
 	const channelItems = 10
 	go func() {
@@ -829,14 +827,12 @@ func TestCopyFromFunc(t *testing.T) {
 		pgx.CopyFromFunc(func() ([]any, error) {
 			v, ok := <-dataCh
 			if !ok {
-				return nil, closeChanErr
+				return nil, nil
 			}
 			return []any{v}, nil
 		}))
 
-	fmt.Print(copyCount, err, "\n")
-
-	require.ErrorIs(t, err, closeChanErr)
+	require.ErrorIs(t, err, nil)
 	require.EqualValues(t, channelItems, copyCount)
 
 	rows, err := conn.Query(context.Background(), "select * from foo order by a")
@@ -845,5 +841,20 @@ func TestCopyFromFunc(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, []int64{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, nums)
 
+	// simulate a failure
+	copyCount, err = conn.CopyFrom(context.Background(), pgx.Identifier{"foo"}, []string{"a"},
+		pgx.CopyFromFunc(func() func() ([]any, error) {
+			x := 9
+			return func() ([]any, error) {
+				x++
+				if x > 100 {
+					return nil, fmt.Errorf("simulated error")
+				}
+				return []any{x}, nil
+			}
+		}()))
+	require.NotErrorIs(t, err, nil)
+	require.EqualValues(t, 0, copyCount) // no change, due to error
+
 	ensureConnValid(t, conn)
 }