archive/zip: fails to encode filenames with utf-8 chars if 'Modified' is not set

[Bysmyyr]

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

➜ go version
go version go1.15.6 linux/amd64

Does this issue reproduce with the latest release?

that's almost latests

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

go env Output

➜ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/olli/.cache/go-build"
GOENV="/home/olli/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/olli/go/pkg/mod"
GONOPROXY="github.com/SharperShape"
GOOS="linux"
GOPATH="/home/olli/go"
GOPROXY="direct"
GOROOT="/usr/lib/golang"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/olli/code/ziponator/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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build503259521=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Creating a zipfile with nonASCII characters in zipped files fail to encode if Modified is not set. The behavior could be ok still but currently, it anyway checks that if utf-8 mode is needed which makes the programmer to think it should work without any tricks.

so:

w := zip.NewWriter(os.Stdout)
f, err := w.Create("asdää123ääääääend")
… write to f …

…doesn't work, but:

w := zip.NewWriter(os.Stdout)
fh := zip.FileHeader{
	Name:   "asdää123ääääääend",
	Method: zip.Deflate,
	Modified: time.Now(),
}
f, err := w.CreateHeader(&fh)

works

Problem confirmed here:
https://gophers.slack.com/archives/C029RQSEE/p1612881140479100?thread_ts=1612878897.474000&cid=C029RQSEE

What did you expect to see?

asdää123ääääääend

What did you see instead?

asd├д├д123├д├д├д├д├д├дend

[dmitshur]

CC @dsnet, @bradfitz via owners.