x/mobile: os.Executable() panics on iOS

[adambabik]

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

go version go1.9.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/adamb/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.9.2/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.9.2/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/g2/km2y9qqj4bbf329f2bt1xqc80000gn/T/go-build521592560=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

I have a function which calls os.Executable in my Go source file. I bind it with gomobile like this:
gomobile bind --target ios --tags ios -v and use the output framework in my iOS project in Xcode 9. Finally, I just call my Go function from ObjC.

What did you expect to see?

An executable path or error.

What did you see instead?

A panic:

panic: runtime error: index out of range

goroutine 17 [running, locked to thread]:
os.executable(...)
	/usr/local/Cellar/go/1.9.2/libexec/src/os/executable_darwin.go:13
os.Executable(...)
	/usr/local/Cellar/go/1.9.2/libexec/src/os/executable.go:21

[empijei]

This is likely caused by this that does not check if the path is at least 1 character long.

This function expects the path to be set by runtime/os_darwin.go as commented here, but for some reasons this is currently not working or not called.

[eliasnaur]

CC @minux, the author of https://golang.org/cl/16551 .

[josharian]

Related is #24394, which occurs on macOS.

We should obviously gracefully handle the empty string case, but it'd be good to also get it to be non-empty when possible. :)

[empijei]

I was thinking about changing the current implementation of executable() that does not check for the empty string:

func executable() (string, error) {
	ep := executablePath
	if ep[0] != '/' {
		if initCwdErr != nil {
			return ep, initCwdErr
		}

to something like:

func executable() (string, error) {
	ep := executablePath
	if len(ep) == 0 {
		return ep, errors.New("cannot find executable path")
	}
	if ep[0] != '/' {
		if initCwdErr != nil {
			return ep, initCwdErr
		}

CL here

[gopherbot]

Change https://golang.org/cl/127546 mentions this issue: os: add check for empty executable path on darwin