macOS symbolication failure

[ehuss]

I tried this code:

fn main() {
    panic!();
}

starting with cdb50c6 (nightly-2020-03-26, 1.44), when run with:

RUST_BACKTRACE=1 cargo run

results in:

   Compiling foo v0.1.0 (/Users/eric/Proj/rust/cargo/scratch/foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.34s
     Running `target/debug/foo`
thread 'main' panicked at 'explicit panic', src/main.rs:2:5
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::begin_panic
   7: foo::main
   8: std::rt::lang_start::{{closure}}
   9: std::rt::lang_start_internal
  10: std::rt::lang_start
  11: main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

when run with the previous commit, I correctly get the line numbers:

   Compiling foo v0.1.0 (/Users/eric/Proj/rust/cargo/scratch/foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.61s
     Running `target/debug/foo`
thread 'main' panicked at 'explicit panic', src/main.rs:2:5
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /Users/runner/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/backtrace-0.3.45/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /Users/runner/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/backtrace-0.3.45/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1439
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:511
  11: std::panicking::begin_panic
  12: foo::main
             at src/main.rs:2
  13: std::rt::lang_start::{{closure}}
             at /rustc/02046a5d402c789c006d0da7662f800fe3c45faf/src/libstd/rt.rs:67
  14: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  15: std::panicking::try::do_call
             at src/libstd/panicking.rs:331
  16: std::panicking::try
             at src/libstd/panicking.rs:274
  17: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  18: std::rt::lang_start_internal
             at src/libstd/rt.rs:51
  19: std::rt::lang_start
             at /rustc/02046a5d402c789c006d0da7662f800fe3c45faf/src/libstd/rt.rs:67
  20: foo::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

What's interesting, is if you directly run the executable with the metadata hash (like RUST_BACKTRACE=1 target/debug/deps/foo-f5700d28f7af5060), the backtrace looks correct. I tried adjusting the dSYM symlink to include the hash, but that didn't seem to help.

I would assume this is due to #70361. cc @tmiasko @alexcrichton

If the filename mismatch doesn't ring any bells, I can maybe try to dig in and see what changed. Although the changes to backtrace-sys look pretty huge, and I'm unfamiliar with it.

[JohnTitor]

Assigning P-medium as discussed as part of the Prioritization Working Group process.

[tmiasko]

After reviewing changes to libbacktrace: the new implementation of Mach-O makes
no attempt to handle the case where an executable is renamed. The previous one
was more robust in that respect.

[alexcrichton]

This is almost surely caused by rust-lang/backtrace-rs#299 which was updated to handle fixes like ianlancetaylor/libbacktrace#32.

TBH I can't really keep track of the bugginess and bugfixes in libbacktrace any more. I'm totally lost in what's happening over time. I'm hoping that rust-lang/backtrace-rs#328 can put the nail in this coffin.

[ehuss]

@tmiasko Have you narrowed it down to a specific commit or change? Do you have a sense of how difficult it would be to fix?

I ask because an alternative would be to change Cargo so that it does not include the hash in the filename for macOS. It does this already for Windows for similar reasons (Windows embeds the path of the PDB in the executable which causes problems if it has a hash). It's only a minor loss (such as switching between features can cause rebuilds).

It sounds like switching to gimli would be great, but probably not going to happen soon?

I'm uncertain which approach would be the best approach.

[tmiasko]

@ehuss this upgrade switched from Mach-O implementation contributed by John
Calanduoni, but never successfully upstreamed, to a completely new one now
available upstream.

[ehuss]

@alexcrichton The issue is that the filename inside the dSYM directory includes the hash (target/debug/foo.dSYM/Contents/Resources/DWARF/foo-64db4e4bf99c12dd). The old implementation would use readdir to scan for a .dSYM directory, and then use readdir again to try to load anything inside the DWARF directory.

The new implementation does not use readdir, and instead expects to find a file matching the executable name. But since the filename inside .dSYM is foo-64db4e4bf99c12dd, it doesn't find a file named foo.

Does that seem worth porting the old logic? I'm reluctant to do so, primarily because I'd also like to fix this for 1.45 (far too late for 1.44), and there's only 6 weeks till the next release.

I tried with gimli (I assume that's the default for backtrace with no features set?), and it seems to work, though I didn't dig into the source for where that is handled.

I don't really have a sense of how annoying hashless executables are (I don't use Windows often enough to have experience with that). Another option is that Cargo could put each executable inside a different directory (target/debug/deps/bin-foo-64db4e4bf99c12dd/foo). So I'm still a bit uncertain which is the best approach.

[alexcrichton]

Ah ok that definitely makes sense. I think it's best to go with a Cargo fix for this rather than a backtrace one. This would involve editing libbacktrace, which I personally have very little confidence in doing. FWIW the dsym loading for gimli is here.

We should probably update gimli to at least only have one level of readdir, not two, but that's a side issue. I think it's fine to avoid hashes on more platforms for Cargo. I agree as well that we can fix this with separate output directories if it becomes a problem.