How do I debug a failing cargo test in GDB?

Question

I have a failing Cargo test:

$ cargo test
[snip]
    Running target/gunzip-c62d8688496249d8

running 2 tests
test test_extract_failure ... FAILED
test test_extract_success ... ok

failures:

---- test_extract_failure stdout ----
        task 'test_extract_failure' panicked at 'assertion failed: result.is_err()', /home/dhardy/other/flate2-rs/tests/gunzip.rs:19



failures:
    test_extract_failure

test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured

task '<main>' panicked at 'Some tests failed', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libtest/lib.rs:250

How do I launch the failing test in a debugger like GDB?

This should be a general question, but for those wanting to retrace my steps, install a recent nightly Rust build and:

git clone https://github.com/dhardy/flate2-rs.git
git checkout 24979640a880
cd flate2-rs
cargo test

Answer 1

You can get a test binary to filter the tests it runs by passing additional arguments to it; Cargo exposes this directly, too. Thus, cargo test test_extract_failure will just run that specific case. (This is convenient if you have other tests that panic and are expected to fail, so that they won’t call the rust_panic function I am about to mention, leaving only the offending call there.)

In order to use gdb, you’ll need to run the test binary directly (if you use Cargo it runs in a subprocess and thus gdb won’t catch panics inside it). Cargo helpfully tells you the file name, target/gunzip-c62d8688496249d8. You can run this directly with --test to make it a test run:

$ target/gunzip-c62d8688496249d8 --test test_extract_failure
running 1 test
test test_extract_failure ... FAILED

failures:

---- test_extract_failure stdout ----
        task 'test_extract_failure' panicked at 'assertion failed: result.is_err()', /home/dhardy/other/flate2-rs/tests/gunzip.rs:19



failures:
    test_extract_failure

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured

task '<main>' panicked at 'Some tests failed', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libtest/lib.rs:250

Now to hook it up with gdb. There is a convenient function for which you can insert a breakpoint, rust_panic. Once in gdb, break rust_panic means it will pause whenever something triggers a panic, before actually doing the unwinding.

Here’s what a session might end up looking like:

$ gdb target/demo-92d91e26f6ebc557
…
Reading symbols from target/demo-92d91e26f6ebc557...done.
(gdb) break rust_panic
Breakpoint 1 at 0xccb60
(gdb) run --test test_extract_failure
Starting program: /tmp/demo/target/demo-92d91e26f6ebc557 --test test_extract_failure
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".

running 1 test
[New Thread 0x7ffff6ef4700 (LWP 14254)]
[New Thread 0x7ffff5fff700 (LWP 14255)]
[Switching to Thread 0x7ffff5fff700 (LWP 14255)]

Breakpoint 1, 0x0000555555620b60 in rust_panic ()
(gdb) bt
#0  0x0000555555620b60 in rust_panic ()
#1  0x0000555555621274 in unwind::begin_unwind_inner::hb821324209c8ed246Qc ()
#2  0x000055555556bb6d in unwind::begin_unwind::h7834652822578025936 ()
#3  0x000055555556b9fd in demo::do_something () at <std macros>:8
#4  0x000055555556b98e in demo::test_extract_failure () at src/lib.rs:3
#5  0x000055555559aa4b in task::TaskBuilder::try_future::closure.8077 ()
#6  0x000055555560fd03 in task::TaskBuilder::spawn_internal::closure.30919 ()
#7  0x000055555561f672 in task::Task::spawn::closure.5759 ()
#8  0x0000555555621cac in rust_try_inner ()
#9  0x0000555555621c96 in rust_try ()
#10 0x000055555561f713 in unwind::try::ha8078a6ae9b50ccepFc ()
#11 0x000055555561f51c in task::Task::run::hdb5fabf381084abafOb ()
#12 0x000055555561f168 in task::Task::spawn::closure.5735 ()
#13 0x0000555555620595 in thread::thread_start::h4d73784c295273b3i6b ()
#14 0x00007ffff79c2314 in start_thread () from /usr/lib/libpthread.so.0
#15 0x00007ffff72e25bd in clone () from /usr/lib/libc.so.6
(gdb) 

In that particular case, #0–#2 and #5–#15 are noise, #3 and #4 are the signal we want.