Map io.EOF errors to io.ErrUnexpectedEOF

io.EOF is never expected during valid usage. In addition, database/sql
uses io.EOF as a sentinal value that all rows from a query have been
received.

See https://github.com/jackc/pgx/issues/662.

frontend_test.go

@@ -1,10 +1,11 @@
 package pgproto3_test
 
 import (
-	"errors"
+	"io"
 	"testing"
 
 	"github.com/jackc/pgproto3/v2"
+	"github.com/stretchr/testify/assert"
 )
 
 type interruptReader struct {
@@ -13,7 +14,7 @@ type interruptReader struct {
 
 func (ir *interruptReader) Read(p []byte) (n int, err error) {
 	if len(ir.chunks) == 0 {
-		return 0, errors.New("no data")
+		return 0, io.EOF
 	}
 
 	n = copy(p, ir.chunks[0])
@@ -56,3 +57,24 @@ func TestFrontendReceiveInterrupted(t *testing.T) {
 		t.Fatalf("unexpected msg: %v", msg)
 	}
 }
+
+func TestFrontendReceiveUnexpectedEOF(t *testing.T) {
+	t.Parallel()
+
+	server := &interruptReader{}
+	server.push([]byte{'Z', 0, 0, 0, 5})
+
+	frontend := pgproto3.NewFrontend(pgproto3.NewChunkReader(server), nil)
+
+	msg, err := frontend.Receive()
+	if err == nil {
+		t.Fatal("expected err")
+	}
+	if msg != nil {
+		t.Fatalf("did not expect msg, but %v", msg)
+	}
+
+	msg, err = frontend.Receive()
+	assert.Nil(t, msg)
+	assert.Equal(t, io.ErrUnexpectedEOF, err)
+}