DeepCopy of time.Time generate invalid code

[Radek-Pysny]

Good day!

TL;DR: When I have custom struct with at least one field of type time.Time and I use goderive to generate DeepCopy implementation of that struct, the generated code tries to access private types of time package. This leads into code that does not compile!

If I made anything wrong, please ping me. Perhaps, I missed something important in documetation, or it is misuse... Thanks in advance

How to get to this state

Start Go module from scratch and add goderive as tool for Go module

go get -tool github.com/awalterschulze/goderive

Now we have in go.mod file:

module 15-goderive-issue

go 1.24

tool github.com/awalterschulze/goderive

require (
	github.com/awalterschulze/goderive v0.5.1 // indirect
	github.com/kisielk/gotool v1.0.0 // indirect
	golang.org/x/sys v0.5.0 // indirect
	golang.org/x/tools v0.6.0 // indirect
)

And let us introduce a small example with Xyz structure having field of type time.Time, so deep copy can be generated:

// main.go

package main

import (
	"fmt"
	"time"
)

type Xyz struct {
	timestamp time.Time
}

func (xyz *Xyz) Clone() *Xyz {
	if xyz == nil {
		return nil
	}

	var clone Xyz
	deriveDeepCopyXyz(&clone, xyz)

	return &clone
}

func main() {
	xyz := &Xyz{}
	fmt.Println(xyz)
}

Generate the "deep copy" using:

go tool goderive ./...

Generated code

Whole generated file is:

// Code generated by goderive DO NOT EDIT.

package main

import (
	"reflect"
	"time"
	"unsafe"
)

// deriveDeepCopyXyz recursively copies the contents of src into dst.
func deriveDeepCopyXyz(dst, src *Xyz) {
	func() {
		field := new(time.Time)
		deriveDeepCopy(field, &src.timestamp)
		dst.timestamp = *field
	}()
}

// deriveDeepCopy recursively copies the contents of src into dst.
func deriveDeepCopy(dst, src *time.Time) {
	src_v := reflect.Indirect(reflect.ValueOf(src))
	dst_v := reflect.Indirect(reflect.ValueOf(dst))
	*(*uint64)(unsafe.Pointer(dst_v.FieldByName("wall").UnsafeAddr())) = *(*uint64)(unsafe.Pointer(src_v.FieldByName("wall").UnsafeAddr()))
	*(*int64)(unsafe.Pointer(dst_v.FieldByName("ext").UnsafeAddr())) = *(*int64)(unsafe.Pointer(src_v.FieldByName("ext").UnsafeAddr()))
	if *(**time.Location)(unsafe.Pointer(src_v.FieldByName("loc").UnsafeAddr())) == nil {
		*(**time.Location)(unsafe.Pointer(dst_v.FieldByName("loc").UnsafeAddr())) = nil
	} else {
		*(**time.Location)(unsafe.Pointer(dst_v.FieldByName("loc").UnsafeAddr())) = new(time.Location)
		deriveDeepCopy_(*(**time.Location)(unsafe.Pointer(dst_v.FieldByName("loc").UnsafeAddr())), *(**time.Location)(unsafe.Pointer(src_v.FieldByName("loc").UnsafeAddr())))
	}
}

// deriveDeepCopy_ recursively copies the contents of src into dst.
func deriveDeepCopy_(dst, src *time.Location) {
	src_v := reflect.Indirect(reflect.ValueOf(src))
	dst_v := reflect.Indirect(reflect.ValueOf(dst))
	*(*string)(unsafe.Pointer(dst_v.FieldByName("name").UnsafeAddr())) = *(*string)(unsafe.Pointer(src_v.FieldByName("name").UnsafeAddr()))
	if *(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr())) == nil {
		*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())) = nil
	} else {
		if *(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())) != nil {
			if len(*(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr()))) > len(*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr()))) {
				if cap(*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr()))) >= len(*(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr()))) {
					*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())) = (*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())))[:len(*(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr())))]
				} else {
					*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())) = make([]time.zone, len(*(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr()))))
				}
			} else if len(*(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr()))) < len(*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr()))) {
				*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())) = (*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())))[:len(*(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr())))]
			}
		} else {
			*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())) = make([]time.zone, len(*(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr()))))
		}
		copy(*(*[]time.zone)(unsafe.Pointer(dst_v.FieldByName("zone").UnsafeAddr())), *(*[]time.zone)(unsafe.Pointer(src_v.FieldByName("zone").UnsafeAddr())))
	}
	if *(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr())) == nil {
		*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())) = nil
	} else {
		if *(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())) != nil {
			if len(*(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr()))) > len(*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr()))) {
				if cap(*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr()))) >= len(*(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr()))) {
					*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())) = (*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())))[:len(*(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr())))]
				} else {
					*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())) = make([]time.zoneTrans, len(*(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr()))))
				}
			} else if len(*(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr()))) < len(*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr()))) {
				*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())) = (*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())))[:len(*(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr())))]
			}
		} else {
			*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())) = make([]time.zoneTrans, len(*(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr()))))
		}
		copy(*(*[]time.zoneTrans)(unsafe.Pointer(dst_v.FieldByName("tx").UnsafeAddr())), *(*[]time.zoneTrans)(unsafe.Pointer(src_v.FieldByName("tx").UnsafeAddr())))
	}
	*(*string)(unsafe.Pointer(dst_v.FieldByName("extend").UnsafeAddr())) = *(*string)(unsafe.Pointer(src_v.FieldByName("extend").UnsafeAddr()))
	*(*int64)(unsafe.Pointer(dst_v.FieldByName("cacheStart").UnsafeAddr())) = *(*int64)(unsafe.Pointer(src_v.FieldByName("cacheStart").UnsafeAddr()))
	*(*int64)(unsafe.Pointer(dst_v.FieldByName("cacheEnd").UnsafeAddr())) = *(*int64)(unsafe.Pointer(src_v.FieldByName("cacheEnd").UnsafeAddr()))
	if *(**time.zone)(unsafe.Pointer(src_v.FieldByName("cacheZone").UnsafeAddr())) == nil {
		*(**time.zone)(unsafe.Pointer(dst_v.FieldByName("cacheZone").UnsafeAddr())) = nil
	} else {
		*(**time.zone)(unsafe.Pointer(dst_v.FieldByName("cacheZone").UnsafeAddr())) = new(time.zone)
		**(**time.zone)(unsafe.Pointer(dst_v.FieldByName("cacheZone").UnsafeAddr())) = **(**time.zone)(unsafe.Pointer(src_v.FieldByName("cacheZone").UnsafeAddr()))
	}
}

And the problematic part is in deriveDeepCopy_, where are directly accessed private structs like time.zone and time.zoneTrans. This

Build process failing

Output from build:

# 15-goderive-issue
./derived.gen.go:39:15: name zone not exported by package time
./derived.gen.go:40:13: name zone not exported by package time
./derived.gen.go:42:16: name zone not exported by package time
./derived.gen.go:43:21: name zone not exported by package time
./derived.gen.go:44:22: name zone not exported by package time
./derived.gen.go:45:16: name zone not exported by package time
./derived.gen.go:47:92: name zone not exported by package time
./derived.gen.go:49:28: name zone not exported by package time
./derived.gen.go:50:15: name zone not exported by package time
./derived.gen.go:53:90: name zone not exported by package time
./derived.gen.go:53:90: too many errors

[Radek-Pysny]

@awalterschulze Do you have any advice? I tried to have own deepCopy function for time.Time, but it is ignored. Also clone seems to generate similar issues as the entry point is the following:

// deriveCloneTicket returns a clone of the src parameter.
func deriveCloneXyz(src *Xyz) *Xyz {
	if src == nil {
		return nil
	}
	dst := new(Xyz)
	deriveDeepCopy(dst, src)
	return dst
}

[awalterschulze]

Yes this is a long standing issue with an agreed upon solution, but that solution has not been built yet, since we haven't found a volunteer yet: #47

For now I would recommend manually writing the DeepCopy method for that particular struct
This way goderive will not attempt to generate DeepCopy for that struct and rather reuse the method in the case that it is part of a recursive structure.

[Radek-Pysny]

I did not fully understand to your answer in combination to that linked #47. So far:

• I can see deriveDeepCopy* does not allow more than two arguments, so passing methods is not yet possible.
• I was not able to force goderive to use my own implementation. See the code snippet below. If this is included in the same file from where I generate goderiveDeepCopy, it is ignored. Tried different naming, but no success yet.
• Tried to use deriveClone*, but it internally uses deriveDeepCopy*, so I found just a dead end.

func deriveDeepCopyTime(dst, src *time.Time) {
	*dst = *src
}

Can you see, what am I going wrong?

[awalterschulze]

Yeah nothing in #47 is implemented yet, so the only way is to clone time currently is to write your own Clone method and for your nested structure.

type MyStruct struct {
	Int64     int64
	NestedStruct *MyStructWithTime
}

func (m *MyStruct) Clone() *MyStruct {
	if m == nil {
		return nil
	}
	n := &MyStruct{}
	deriveDeepCopy(n, m)
	return n
}

type MyStructWithTime struct {
   otherField int
   timeField time.Time
}

func (m * MyStructWithTime) Clone() * MyStructWithTime {
	// manually write this yourself
}