cargo-rr

A light wrapper around rr, the time-travelling debugger.

Do you find yourself running the same test over and over in the debugger, trying to figure out how the code got in a bad state? We have a tool for you! Easy to install and setup, it will record an execution trace, and that gives magical new powers to gdb. Step backwards, run backwards, see where variables changed their value or when a function was last called on an object (using conditional breakpoints). (source)

Suppose we ran a test and got a failure. We first re-run the test under rr to record the entire execution (including everything else on your system the test interacts with).

> cargo rr test my_test

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', tests/tests.rs:100

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2 filtered out; finished in 0.06s

Let's replay the recording.

> cargo rr replay

(rr) continue

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `42`', tests/tests.rs:100

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2 filtered out; finished in 0.06s

We'll go backwards until we return to the failed assertion so we can have a look at a and b.

(rr) break tests/tests.rs:100
(rr) reverse-continue
Continuing.

Breakpoint 1, tests::main () at tests/tests.rs:100
4           assert_eq!(a, b);

(rr) print a
$1 = 1

(rr) print b
$2 = 42

I wonder where a is set to 1?

(rr) watch -l a
(rr) reverse-continue

Old value = 1
New value = -1992923200
0x000055dcac485ed9 in tests::main () at tests/tests.rs:30
3           let a = some_calculation();

Note that since we're going in reverse the old and new values are backwards. -1992923200 is whatever used to be at the address 0x000055dcac485ed9 before it was used for a.

cargo-rr 0.1.0

cargo-rr