x/sys/windows: clarify safety of Proc.Call, LazyProc.Call

[bradfitz]

https://godoc.org/golang.org/x/sys/windows#LazyProc.Call and https://godoc.org/golang.org/x/sys/windows#Proc.Call are syscall.SysCall wrappers but take a bunch of uintptrs.

I assume that those wrappers are not covered by the uintptr(unsafe.Pointer) conversion exception number (4) from https://golang.org/pkg/unsafe/#Pointer, correct?

Could we document that either way in x/sys/windows?

I assume that many callers will require runtime.KeepAlive?

/cc @ianlancetaylor @mdempsky @alexbrainman @josharian @crawshaw

[mdempsky]

I think this is Q1 of #34684.

[mdempsky]

Also, runtime.KeepAlive would only guarantee that the actual pointer is still live. It doesn't guarantee that uintptr conversion stays valid. Calling Proc.Call might require growing the stack, which could invalidate a pointer to stack memory.

[bradfitz]

Yeah, this could be closed as a dup of #34684 I suppose.

But I'm not exactly sure the status of that bug. It sure sounds like {Lazy,}Proc.Call just aren't safe to use today, at least for arguments involving pointers.

Should we clarify that in the docs for now while we wait for something to happen with #34684?

[mdempsky]

Direct calls to {Lazy,}Proc.Call should be fine, at least since b3bd7ab (first included in Go 1.14).

Either way, I'm happy to have the docs clarified.

[bradfitz]

Ah! I had just missed the //go:uintptrescapes lines in that file. Okay, that makes me feel much better.

I'll send some docs, thanks!