reflect: StructTag.Get does not return 1st tag of dynamic structure created with StructOf

[ivasylenko]

Go version

go1.22.0 linux/amd64

Output of go env in your module/workspace:

GO111MODULE='on'
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/ivasylen/.cache/go-build'
GOENV='/home/ivasylen/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/ivasylen/golang/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/ivasylen/guardicore/management/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/ivasylen/guardicore/management/c2c/go/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1023271956=/tmp/go-build -gno-record-gcc-switches'

What did you do?

I am working with reflection package, creating dynamic structures with reflect.StructOf.
Later on I inspect these structure tags and 1st tag is always missing for any field.
I have simple UT

func TestDynamicStructTags(t *testing.T) {
	field := reflect.StructField{
		Name: "Field",
		Type: reflect.TypeOf(""),
		Tag:  reflect.StructTag("`k1:\"v1\" k2:\"v2\" k3:\"v3\"`"),
	}
	stType := reflect.StructOf([]reflect.StructField{field})
	for i := 0; i < stType.NumField(); i++ {
		var (
			f = stType.Field(i)
		)
		tagsFmt := fmt.Sprintf("%s, %s, %s, %s", f.Tag, f.Tag.Get("k1"), f.Tag.Get("k2"), f.Tag.Get("k3"))
		//assert.Equal(t, "`k1:\"v1\" k2:\"v2\" k3:\"v3\"`, , v2, v3", tagsFmt)
		assert.Equal(t, "`k1:\"v1\" k2:\"v2\" k3:\"v3\"`, v1, v2, v3", tagsFmt)
	}
}

What did you see happen?

In above test, k1 is missing

// k1 is missing
// expected: "`k1:\"v1\" k2:\"v2\" k3:\"v3\"`, v1, v2, v3"
// actual  : "`k1:\"v1\" k2:\"v2\" k3:\"v3\"`, , v2, v3"

What did you expect to see?

I would expect all tags returned, including k1

[callthingsoff]

package main

import (
	"fmt"
	"reflect"
)

func main() {
	fmt.Println(reflect.StructTag("k1:\"v1\"").Get("k1"))
	fmt.Println(reflect.StructTag(`k1:"v1"`).Get("k1"))
	fmt.Println(reflect.StructTag("`k1:\"v1\"`").Get("k1"))
	fmt.Println(reflect.StructTag("`k1:\"v1\"`").Get("`k1"))
}

v1
v1

v1

I think maybe you can change the StructTag key.

[dmitshur]

Thanks for reporting. https://pkg.go.dev/reflect#StructTag documentation says:

By convention, tag strings are a concatenation of optionally space-separated key:"value" pairs. Each key is a non-empty string consisting of non-control characters other than space (U+0020 ' '), quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using U+0022 '"' characters and Go string literal syntax.

It looks like the provided code is inadvertently defining a key named `k1 rather than k1 as intended due to extra backticks at the start and end of the string, and as callthingsoff's snippet demonstrates, it is possible to access the first key (even if it is named `k1).

Closing since there doesn't appear to be a problem in the reflect package.

CC @golang/runtime.