`GetPropertyReply::value` causes a crash when the length and format are zero

[RealKC]

Here is a reproducer: https://gist.github.com/RealKC/3f472c7baa19ba27c88af27e0abcc494. If you uncomment the innermost if, the program will happily print the atoms for the windows which have _NET_WM_STATE set on them, if you keep it commented, the program will eventually maybe crash with the following assertion only if the WM returns properties with length 0 and format 0 on them:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `32`: mismatched format of xproto::GetPropertyReply::value', /home/kc/programming/xcbbtest/target/debug/build/xcb-bb99a98c60179771/out/xproto.rs:13675:9

Note that this isn't reproducible everywhere, this happens for me on awesomewm but not to a friend who uses KDE.

Some possible fixes for the issue:

• introduce a fallible try_value that instead of asserting returns an error?
• return an empty slice if the length is 0 before checking that the formats match?

I'm not sure which of those is more elegant, but as an end-user I'd like to handle this case in some manner and I'm not sure checking the length would always work (I assume yes, since if the WM is sending 0 for format, you can't really use the reply anyway) and I don't exactly believe this is going against the EWMH spec? Some properties have type/format next to their name(at least that's how I interpret it, maybe I missed something in the spec), like _NET_WM_WINDOW_TYPE(https://specifications.freedesktop.org/wm-spec/latest/ar01s05.html#idm46476783509232) which has ATOM[]/32 but _NET_WM_STATE only has ATOM[] see https://specifications.freedesktop.org/wm-spec/latest/ar01s05.html#idm46476783496896

[rtbo]

As far as I know, format is always supposed to be either 8, 16 or 32. But I don't think I encountered zero length props already. What is the value of format when length is not zero for the same property? Is it 32?

I can easily fix this by checking the length before the assertion.

[RealKC]

What is the value of format when length is not zero for the same property? Is it 32?

Yeah it's always either 32(when length is nonzero) or 0(when length is zero) from my testing

[rtbo]

It should be OK now. Don't hesitate to ask to reopen if not.

[RealKC]

I just made the test program use xcb from git and it works! Thanks a lot :)