Checkptr failure

[JetSetIlly]

As of version 1.14 of Go the race detector adds checkptr instrumention. This compilation option ensures that program's follow Go's unsafe.Pointer safety rules. https://golang.org/doc/go1.14#compiler

Currently, Gopher2600 fails the checkptr test because of how fonts are setup for imgui-go. To wit, the following triggers the checkptr error:

// Store our identifier
rnd.imguiIO.Fonts().SetTextureID(imgui.TextureID(rnd.fontTexture))

fontTexture has previously been generated and bound and has the value 0. imgui.TextureID casts that value to a uintptr and thus triggers the error (pointer value too low).

It is possible to turn off checkptr with a build flag (the Makefile includes a race target that does this):

go run -race -gcflags=all=-d=checkptr=0 gopher2600.go debug

This is useful because it has allowed gopher2600 to be cleaned of race errors but the checkptr error remains.

In a nutshell, I'm not sure how to fix it. As far as I can tell, the fontTexture value is not really a pointer but casting it to uintptr raises the suspicions of the checkPtrcode. I also cannot see how to alter imgui-go in such a way to avoid using uintptr.

[mdempsky]

Hi, I added the checkptr feature to the Go compiler. Are you still having this problem? If so, can you share the checkptr error message and stack trace that are printed?

[JetSetIlly]

Hi Matthew, thanks for looking at this. Complete stack trace below:

fatal error: checkptr: unsafe pointer arithmetic

goroutine 1 [running, locked to thread]:
runtime.throw(0xb7c05c, 0x23)
/home/steve/Go/go-current/src/runtime/panic.go:1114 +0x72 fp=0xc000185720 sp=0xc0001856f0 pc=0x484842
runtime.checkptrArithmetic(0x1, 0x0, 0x0, 0x0)
/home/steve/Go/go-current/src/runtime/checkptr.go:26 +0xce fp=0xc000185750 sp=0xc000185720 pc=0x45723e
github.com/inkyblackness/imgui-go/v2.TextureID.handle(...)
/home/steve/Go/go/pkg/mod/github.com/!jet!set!illy/imgui-go/v2@v2.2.1-0.20200317095507-51a6e45b93a9/TextureID.go:24
github.com/inkyblackness/imgui-go/v2.FontAtlas.SetTextureID.func1(0x1d6e580, 0x1)
/home/steve/Go/go/pkg/mod/github.com/!jet!set!illy/imgui-go/v2@v2.2.1-0.20200317095507-51a6e45b93a9/FontAtlas.go:163 +0x78 fp=0xc000185790 sp=0xc000185750 pc=0x908768
github.com/inkyblackness/imgui-go/v2.FontAtlas.SetTextureID(0x1d6e580, 0x1)
/home/steve/Go/go/pkg/mod/github.com/!jet!set!illy/imgui-go/v2@v2.2.1-0.20200317095507-51a6e45b93a9/FontAtlas.go:163 +0x43 fp=0xc0001857b0 sp=0xc000185790 pc=0x901f23
github.com/jetsetilly/gopher2600/gui/sdlimgui.(*glsl).setup(0xc0000b4000)
/home/steve/gopher2600/gui/sdlimgui/glsl.go:439 +0x1b75 fp=0xc000185b28 sp=0xc0001857b0 pc=0x955315
github.com/jetsetilly/gopher2600/gui/sdlimgui.newGlsl(0x1d6a818, 0xc0000a4000, 0x0, 0x0, 0xa57700)
/home/steve/gopher2600/gui/sdlimgui/glsl.go:92 +0x1d4 fp=0xc000185bb0 sp=0xc000185b28 pc=0x950364
github.com/jetsetilly/gopher2600/gui/sdlimgui.NewSdlImgui(0xc47c00, 0xc0001543c0, 0xc000185ed8, 0x4, 0x4)
/home/steve/gopher2600/gui/sdlimgui/sdlimgui.go:107 +0x39a fp=0xc000185d48 sp=0xc000185bb0 pc=0x95c51a
main.debug.func1(0xc000185ed8, 0xc000185df8, 0x4, 0x1)
/home/steve/gopher2600/gopher2600.go:340 +0x4f fp=0xc000185d98 sp=0xc000185d48 pc=0x9beccf
main.main()
/home/steve/gopher2600/gopher2600.go:136 +0x505 fp=0xc000185f88 sp=0xc000185d98 pc=0x9b73c5
runtime.main()
/home/steve/Go/go-current/src/runtime/proc.go:203 +0x212 fp=0xc000185fe0 sp=0xc000185f88 pc=0x486ec2
runtime.goexit()
/home/steve/Go/go-current/src/runtime/asm_amd64.s:1373 +0x1 fp=0xc000185fe8 sp=0xc000185fe0 pc=0x4b69f1

goroutine 35 [syscall]:
os/signal.signal_recv(0x4b69f1)
/home/steve/Go/go-current/src/runtime/sigqueue.go:147 +0x9c
os/signal.loop()
/home/steve/Go/go-current/src/os/signal/signal_unix.go:23 +0x30
created by os/signal.Notify.func1
/home/steve/Go/go-current/src/os/signal/signal.go:127 +0x7c

goroutine 20 [select]:
main.debug(0xc00018e200, 0xc0001204c0, 0x0, 0x0)
/home/steve/gopher2600/gopher2600.go:352 +0x9a4
main.launch(0xc0001204c0)
/home/steve/gopher2600/gopher2600.go:201 +0x5c4
created by main.main
/home/steve/gopher2600/gopher2600.go:111 +0x25f

goroutine 37 [select]:
github.com/jetsetilly/gopher2600/television.NewTelevision.func1(0xc0001543c0)
/home/steve/gopher2600/television/television.go:177 +0x2ac
created by github.com/jetsetilly/gopher2600/television.NewTelevision
/home/steve/gopher2600/television/television.go:170 +0x36e
exit status 2

[mdempsky]

Thanks. It looks like this is an issue with imgui-go.

The problem isn't that you're converting a value to uintptr. It's that imgui-go's TextureID.handle method is converting a uintptr to C.IggTextureID, where C.IggTextureID is defined as typedef void *IggTextureID in C.

Probably imgui-go should be updated to use typedef uintptr_t IggTextureId instead.

[mdempsky]

Filed inkyblackness/imgui-go#98

[JetSetIlly]

Fixed by changes described here inkyblackness/imgui-go#98