symblib/gosym: add single point lookup

[florianl]

Add single point lookup functionality for Go to resolve a virtual address to function information.

Example to test this new API:

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let path = std::env::args()
        .nth(1)
        .expect("Please provide path to Go executable");
    let obj = objfile::File::load(Path::new(&path))?;
    let obj = obj.parse()?;

    let runtime_info = gosym::GoRuntimeInfo::open(&obj)?;

    // Example addresses to resolve - modify these for your own tests
    let test_addresses = [
        VirtAddr::from(0xabc123_u64),
        VirtAddr::from(0x123abc_u64),
    ];

    // Try to resolve each address
    for addr in test_addresses {
        match runtime_info.find_func(addr)? {
            Some(func) => println!(
                "Found function at {:#x}: {} {:#x}",
                addr,
                func.name()?,
                func.start_addr(),
            ),
            None => println!("No function found at {:#x}", addr),
        }
    }
    Ok(())
}

[athre0z]

Admittedly I didn't set a good example with test coverage in this library in general, but I'd still like to suggest adding a test case for this particular method. I've hand-rolled binary searches on various occasions before, and I don't think that I ever managed without at least one bug. 😅

What do you think about adding a testdata directory with a hello world Go app compiled and committed into VC, then writing a test that:

• opens the demo executable
• iterates over a ~0x10000 bytes PC range, and for each PC does:
  • a dumb linear search on functions iterator (GoRuntimeInfo::funcs().find(|x| ...))
  • a binary search using your function
  • compares result?

We could (and should) expand the test suite over time, but this could be a good start? Also happy to help with this; lmk if you'd prefer me to just push this onto this branch.

[athre0z]

LGTM -- thank you! :)