os: ExpandEnv: Missing curly bracket at end of string substitutes var name whether or not env var is set

[gfl-chris]

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

go version go1.12.9 linux/amd64

and

Go Playground (1.13.4)

Does this issue reproduce with the latest release?

Yes. (Go Playground)

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

go env Output

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ceineke/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/ceineke/.gvm/pkgsets/go1.12.9/global"
GOPROXY=""
GORACE=""
GOROOT="/home/ceineke/.gvm/gos/go1.12.9"
GOTMPDIR=""
GOTOOLDIR="/home/ceineke/.gvm/gos/go1.12.9/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build732428512=/tmp/go-build -gno-record-gcc-switches"

and

Go Playground

What did you do?

https://play.golang.org/p/GSfBKiZqj2L

What did you expect to see?

Either

Hello, ${name
Hello, ${name

or

Hello, ${name
Hello, foo

What did you see instead?

Hello, name
Hello, name

[cuonglm]

{ is an undefined environment variable, so ${ is expanded to empty string. I think it's working as intended. Environment variable is not the same as shell variable.

[gfl-chris]

It's not and it shouldn't. The code here points out that ${ is bad syntax. I guess what I'm trying to say is that it shouldn't be "eaten."

Concrete example: Running ExpandEnv over a YAML file whose last line looks like this:

  password: abcd${efgh

will, IMHO, incorrectly clobber the password.

[cuonglm]

It's not and it shouldn't. The code here points out that ${ is bad syntax. I guess what I'm trying to say is that it shouldn't be "eaten."

Concrete example: Running ExpandEnv over a YAML file whose last line looks like this:

  password: abcd${efgh

will, IMHO, incorrectly clobber the password.

Hmm, it's also not consistent, since when we can do os.Setenv("{", "foo") then os.Getenv("{") will return foo, but ExpandEnv does not.

I think it's better to just improve the document of ExpandEnv to indicate that the expansion follow the rule of shell expansion, invalid syntax will return empty string.

[toothrot]

/cc @josharian @mvdan @ianlancetaylor @bradfitz

[ianlancetaylor]

I agree that we should mention this in the documentation and not change the behavior of the function.

[ghost]

In another case Expand was fixed:
#24345
Perhaps in this case ExpandEnv should be fixed too.

[ianlancetaylor]

There is no significant difference between Expand and ExpandEnv.

See also #26135.

That said, I think you are suggesting that for ${ with no } we should keep the ${. That may be OK.

[gopherbot]

Change https://golang.org/cl/208857 mentions this issue: os: add documentation for invalid expansion syntax in ExpandEnv

[ghost]

you are suggesting that for ${ with no } we should keep the ${.

Yep. It's not even like ExpandEnv replaces ${ with a value of the { variable: https://play.golang.org/p/CzLSGzkLAWx

[fitzy101]

I'm looking at picking up an issue for a first contribution, and it looks as though this is a candidate. Is that actively being addressed by anyone? It looks like there is still a decision to be made on the expected behaviour.