Fix compiling on Mac OS X 10.14 (and presumably anywhere else that LIBXML_THREAD_ENABLED is set)

[caldwell]

The problem is that LIBXML_THREAD_ENABLED changes the xmlIndentTreeOutput global from a simple global variable into a function call that returns an lvalue. The only proper way to detect and handle this is to just use the macro in C.

This patch adds c_wrappers.c and compiles it in the build.rs. I tested this with a recent nightly rust on Mac OS X 10.14 and Debian testing.

[dginev]

Hi @caldwell , thanks a lot for this PR and problem report!

Would you mind helping me check if we can solve this using the bindgen-generated low-level helpers in bindings.rs, to avoid adding new C files?

Two questions:

• Can you get the .travis.yml setup to fail in CI, by setting the env var, so that we can test this setup in the future? I can't seem to reproduce the issue locally for now on my Ubuntu setup.
• Have you tried accessing the global state from c_helpers.rs via e.g.

pub fn setIndentTreeOutput(indent: c_int) {
  unsafe {
    let mut global_state = xmlGetGlobalState();
    (*global_state).xmlIndentTreeOutput = indent;
  }
}

I would consider it a win if we can keep the repo clean of .c files, we had a range of those early on, and bindgen was a huge help in clearing them out.

Edit: Ah, I see that this is a compile-time flag for building libxml2, so it won't be at all easy to reproduce on a linux box, we'd have to rebuild the package from scratch... Hm.

[caldwell]

Hi, thanks for taking a look. I agree less C would be great. Your suggestion works on my setup, but I don't think it's quite right. In particular, after looking at the libxml2 sources, there are 2 places to get the globals. The struct returned from xmlGetGlobalState() is one of them, but there's also a plain global int defined in globals.c. They have a function __xmlIndentTreeOutput() that chooses the struct or the global int based on whether it's running in the main thread (which is always true if threads aren't enabled at all). I was hesitant to use __xmlIndentTreeOutput() because it's named as an internal function and I wasn't sure if it was always there. But after looking at the code it seems to always be defined.

So I suggest this:

pub fn setIndentTreeOutput(indent: c_int) {
  unsafe {
    let xml_indent_tree_output_ptr = __xmlIndentTreeOutput();
    (*xml_indent_tree_output_ptr) = indent;
  }
}

This works on my Mac. I want to test on my linux machine and then I'll push an update.

[dginev]

Wonderful, thank you for taking a detailed look and refining the PR. Looks good to go! I'll release a minor version in case you want to have a cargo dependency.