s3: refactor to use generated code instead of reflection to copy structs

backend/s3/gen_setfrom.go

@@ -0,0 +1,101 @@
+// Generate boilerplate code for setting similar structs from each other
+
+//go:build ignore
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"reflect"
+	"strings"
+
+	"github.com/aws/aws-sdk-go/service/s3"
+)
+
+// flags
+var (
+	outputFile = flag.String("o", "", "Output file name, stdout if unset")
+)
+
+// globals
+var (
+	out io.Writer = os.Stdout
+)
+
+// genSetFrom generates code to set the public members of a from b
+//
+// a and b should be pointers to structs
+//
+// a can be a different type from b
+//
+// Only the Fields which have the same name and assignable type on a
+// and b will be set.
+//
+// This is useful for copying between almost identical structures that
+// are frequently present in auto-generated code for cloud storage
+// interfaces.
+func genSetFrom(a, b interface{}) {
+	name := fmt.Sprintf("setFrom_%T_%T", a, b)
+	name = strings.Replace(name, ".", "", -1)
+	name = strings.Replace(name, "*", "", -1)
+	fmt.Fprintf(out, "\n// %s copies matching elements from a to b\n", name)
+	fmt.Fprintf(out, "func %s(a %T, b %T) {\n", name, a, b)
+	ta := reflect.TypeOf(a).Elem()
+	tb := reflect.TypeOf(b).Elem()
+	va := reflect.ValueOf(a).Elem()
+	vb := reflect.ValueOf(b).Elem()
+	for i := 0; i < tb.NumField(); i++ {
+		bField := vb.Field(i)
+		tbField := tb.Field(i)
+		name := tbField.Name
+		aField := va.FieldByName(name)
+		taField, found := ta.FieldByName(name)
+		if found && aField.IsValid() && bField.IsValid() && aField.CanSet() && tbField.Type.AssignableTo(taField.Type) {
+			fmt.Fprintf(out, "\ta.%s = b.%s\n", name, name)
+		}
+	}
+	fmt.Fprintf(out, "}\n")
+}
+
+func main() {
+	flag.Parse()
+
+	if *outputFile != "" {
+		fd, err := os.Create(*outputFile)
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer func() {
+			err := fd.Close()
+			if err != nil {
+				log.Fatal(err)
+			}
+		}()
+		out = fd
+	}
+
+	fmt.Fprintf(out, `// Code generated by "go run gen_setfrom.go"; DO NOT EDIT.
+
+package s3
+
+import "github.com/aws/aws-sdk-go/service/s3"
+`)
+
+	genSetFrom(new(s3.ListObjectsInput), new(s3.ListObjectsV2Input))
+	genSetFrom(new(s3.ListObjectsV2Output), new(s3.ListObjectsOutput))
+	genSetFrom(new(s3.ListObjectVersionsInput), new(s3.ListObjectsV2Input))
+	genSetFrom(new(s3.ObjectVersion), new(s3.DeleteMarkerEntry))
+	genSetFrom(new(s3.ListObjectsV2Output), new(s3.ListObjectVersionsOutput))
+	genSetFrom(new(s3.Object), new(s3.ObjectVersion))
+	genSetFrom(new(s3.CreateMultipartUploadInput), new(s3.HeadObjectOutput))
+	genSetFrom(new(s3.CreateMultipartUploadInput), new(s3.CopyObjectInput))
+	genSetFrom(new(s3.UploadPartCopyInput), new(s3.CopyObjectInput))
+	genSetFrom(new(s3.HeadObjectOutput), new(s3.GetObjectOutput))
+	genSetFrom(new(s3.CreateMultipartUploadInput), new(s3.PutObjectInput))
+	genSetFrom(new(s3.HeadObjectOutput), new(s3.PutObjectInput))
+}