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encoding/xml: marshaller panics in reflect.Value.CanInterface #43595

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d-neal opened this issue Jan 8, 2021 · 3 comments
Closed

encoding/xml: marshaller panics in reflect.Value.CanInterface #43595

d-neal opened this issue Jan 8, 2021 · 3 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
Milestone
Unplanned

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@d-neal
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d-neal commented Jan 8, 2021

What version of Go are you using (go version)?

$ go version
go version go1.15.6 darwin/amd64

Does this issue reproduce with the latest release?

YES

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/d-neal/Library/Caches/go-build"
GOENV="/Users/d-neal/Library/Application Support/go/env"
GOEXE=""
GOFLAGS="-mod=vendor"
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/d-neal/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/d-neal/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.15.6/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.15.6/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/d-neal/repos/goroute/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/0d/6gm66r353ynfl6d90m65tlmr0000gn/T/go-build360898696=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://play.golang.org/p/1txISuTviPw

The problem is not reproducible with golang 1.14
It is also not reproducible if I just remove the XML tags from Foo/Bar attributes

What did you expect to see?

<Bar b="b"></Bar>

What did you see instead?

panic: reflect: call of reflect.Value.CanInterface on zero Value

goroutine 1 [running]:
reflect.Value.CanInterface(...)
	/usr/local/go-faketime/src/reflect/value.go:1005
encoding/xml.(*printer).marshalAttr(0xc00010e000, 0xc000068dd0, 0x0, 0x0, 0x4bec8b, 0x1, 0x0, 0x0, 0x0, 0xc000068d78, ...)
	/usr/local/go-faketime/src/encoding/xml/marshal.go:551 +0x1465
encoding/xml.(*printer).marshalValue(0xc00010e000, 0x4c0aa0, 0xc00000c0e0, 0x16, 0x0, 0x0, 0x90, 0x4cb820)
	/usr/local/go-faketime/src/encoding/xml/marshal.go:517 +0x5f0
encoding/xml.(*Encoder).Encode(0xc00010e000, 0x4c0aa0, 0xc00000c0e0, 0xc00007e000, 0x20)
	/usr/local/go-faketime/src/encoding/xml/marshal.go:162 +0xb6
encoding/xml.MarshalIndent(0x4c0aa0, 0xc00000c0e0, 0x0, 0x0, 0x4e0092, 0x2, 0xc000032778, 0xc000068f78, 0x4060e5, 0xc00005e058, ...)
	/usr/local/go-faketime/src/encoding/xml/marshal.go:129 +0x159
main.main()
	/tmp/sandbox825762527/prog.go:22 +0x7b
@mvdan mvdan added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jan 8, 2021
@bradleypeabody
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I ran into this same issue. Interestingly enough, if you remove the pointer on the embedded Foo field, it works:

type Bar struct {
	Foo
	...

https://play.golang.org/p/e561Lk2yaM8

It also works if Foo (pointer) is populated with an empty instance (non-nil pointer):

	bar := &Bar{
		Foo: &Foo{},
		...

https://play.golang.org/p/Ar2OZfBapRw

With just a quick scan I didn't see anything in the docs for encoding/xml that mentions embedded structs either way. But I think it's reasonable to assume that if it works as field Foo then *Foo should work as well and have the same behavior as if Foo had a zero value. Most likely somewhere in the internals it is reading the value of each field and expecting to get a zero value for a struct and try to recursively traverse it, and instead gets a nil pointer and chokes on that. (That's just a guess, but a reasonable one. The error reflect.Value.CanInterface on zero Value is exactly what would be encountered after calling https://golang.org/pkg/reflect/#Value.Elem on a nil pointer and then trying to use the resulting reflect.Value.)

@seankhliao seankhliao changed the title XML marshaller panics in reflect.Value.CanInterface encoding/xml: marshaller panics in reflect.Value.CanInterface Jun 18, 2021
@hopehook
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hopehook commented Mar 6, 2022 •
edited

I think this issue is the same as issue #50164, and should be closed by CL https://go.dev/cl/385514.

/cc @seankhliao

@seankhliao seankhliao added this to the Unplanned milestone Aug 20, 2022
@hopehook
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hopehook commented Sep 1, 2022

Already fixed.

@hopehook hopehook closed this as completed Sep 1, 2022
@golang golang locked and limited conversation to collaborators Sep 1, 2023
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@bradleypeabody @mvdan @hopehook @gopherbot @seankhliao @d-neal