DISPLAY=:0.1 with just one X11 "screen" makes xproto.Setup(...).DefaultScreen panic

[tv42]

It seems setup.DefaultScreen does not check before assuming that DISPLAY is valid.

    X, err := xgb.NewConn()
    if err != nil {
        log.Fatal(err)
    }

    setup := xproto.Setup(X)
    screen := setup.DefaultScreen(X)
    fmt.Printf("%dx%d\n", screen.WidthInPixels, screen.HeightInPixels)

$ DISPLAY=:0.1 x11-screen-pixels 
panic: runtime error: index out of range

goroutine 16 [running]:
runtime.panic(0x593140, 0x6a9c1c)
    /home/tv/src/go/src/pkg/runtime/panic.c:279 +0xf5
main.main()
    /home/tv/go/src/github.com/tv42/x11-screen-pixels/main.go:42 +0x2eb

goroutine 19 [finalizer wait]:
runtime.park(0x413890, 0x6ad358, 0x6abec9)
    /home/tv/src/go/src/pkg/runtime/proc.c:1369 +0x89
runtime.parkunlock(0x6ad358, 0x6abec9)
    /home/tv/src/go/src/pkg/runtime/proc.c:1385 +0x3b
runfinq()
    /home/tv/src/go/src/pkg/runtime/mgc0.c:2644 +0xcf
runtime.goexit()
    /home/tv/src/go/src/pkg/runtime/proc.c:1445

goroutine 20 [runnable]:
github.com/BurntSushi/xgb.(*Conn).generateXIds(0xc2080580a0)
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:200
created by github.com/BurntSushi/xgb.NewConnDisplay
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:110 +0x1bb

goroutine 21 [runnable]:
github.com/BurntSushi/xgb.(*Conn).generateSeqIds(0xc2080580a0)
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:253
created by github.com/BurntSushi/xgb.NewConnDisplay
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:111 +0x1d3

goroutine 22 [runnable]:
github.com/BurntSushi/xgb.(*Conn).sendRequests(0xc2080580a0)
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:300
created by github.com/BurntSushi/xgb.NewConnDisplay
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:112 +0x1eb

goroutine 23 [runnable]:
github.com/BurntSushi/xgb.(*Conn).readResponses(0xc2080580a0)
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:349
created by github.com/BurntSushi/xgb.NewConnDisplay
    /home/tv/go/src/github.com/BurntSushi/xgb/xgb.go:113 +0x203

[BurntSushi]

That's fair. What is the expected behavior?

[tv42]

Good question! The only place that currently returns errors is NewConn, which may be too early. Maybe DefaultScreen needs an error return?

[BurntSushi]

That would be a breaking change, unfortunately. I haven't really implemented any versioning scheme yet, but XGB has been stable for long enough that I really don't want to introduce any breaking changes without really good reason.

[tv42]

Validate it in Conn.connect? I don't know if that would add round-trips.