"unknown" function names on armv7-linux-androideabi, dl_iterate_phdr is not used.

[artemii235]

Android has "dl_iterate_phdr" since API version 21, but it's explicitly disabled in backtrace-sys build.rs
So when I run the code

use backtrace::Backtrace;
fn main() {
    println!("{:?}", Backtrace::new());
}

using backtrace 0.3.32 I get following output:

stack backtrace:
   0: <unknown> (0xad95e4e8)
   1: <unknown> (0xad95e4c6)
   2: <unknown> (0xad95d7b8)
   3: <unknown> (0xad95d770)
   4: <unknown> (0xad95c718)
   5: <unknown> (0xad95c67e)
   6: <unknown> (0xad96bb22)
   7: <unknown> (0xad96c77a)
   8: <unknown> (0xad96c056)
   9: <unknown> (0xad95c670)
  10: __libc_init (0xea95978a)

This commit fixes the issue and I get proper backtrace, however this won't build for API versions < 21 /target/armv7-linux-androideabi/debug/deps/libbacktrace_sys-88b2f68ba9256347.rlib(elf.o):elf.c:function __rbt_backtrace_initialize: error: undefined reference to 'dl_iterate_phdr'. I've not submitted a PR yet as someone's build might be broken.

I wonder if this is possible to detect the used android API version and set the build.define("HAVE_DL_ITERATE_PHDR", "1"); accordingly.
Or maybe it's fine to mention that >=21 version is only supported since it's used on CI builds?
Please advice.

[alexcrichton]

Thanks for the report! This definitely makes sense and probably explains #151 as a long-lost issue as well.

I think that we could probably fix this for this crate, but we likely can't fix it for libstd for now. That's probably better than nothing though!

My guess as to the best fix is what you're thinking, somehow detect in the build script what the Android API version is and go from there. For the standard library we don't want to embed references to dl_iterate_phdr since that would change the binary compatibility and we're not quite ready to do that, but for the crate on crates.io we can customize to the toolchain we happen to find.

The perfect solution to this would be to update the C code to use dlsym to dynamically determine if dl_iterate_phdr is available (or somehow like that determine dynamically), but updating the C code isn't really feasible at this point. We should fix this in the findshlibs crate at some point, but that's a problem for later!