Log error details via Display instead of Debug

[dhardy]

Compare current Debug output:

 WARN 2018-01-31T18:21:22Z: rand: OsRng failed [falling back to JitterRng]: Error { kind: Unavailable, msg: "missing", cause: None }
 WARN 2018-01-31T18:21:22Z: rand: JitterRng failed: Error { kind: Unavailable, msg: "timer jitter failed basic quality tests", cause: Some(NoTimer) }
thread 'main' panicked at 'could not initialize thread_rng: Error { kind: Unavailable, msg: "seeding a new RNG failed: both OS and Jitter entropy sources failed", cause: Some(Error { kind: Unavailable, msg: "missing", cause: None }) }', /home/dhardy/rust/rand/src/lib.rs:975:22

verses prettier Display output:

 WARN 2018-01-31T18:21:39Z: rand: OsRng failed [falling back to JitterRng]: RNG error [permanent failure]: missing
 WARN 2018-01-31T18:21:39Z: rand: JitterRng failed: RNG error [permanent failure]: timer jitter failed basic quality tests; cause: no timer available
thread 'main' panicked at 'could not initialize thread_rng: Error { kind: Unavailable, msg: "seeding a new RNG failed: both OS and Jitter entropy sources failed", cause: Some(Error { kind: Unavailable, msg: "missing", cause: None }) }', /home/dhardy/rust/rand/src/lib.rs:975:22

I checked implementations of Display for error types in the main Rust repo and all of them covered all their state I think; but none had a situation like this with optional cause. So this Display implementation seems reasonable to me.

(Note that the condition is a bit funky due to no_std not having a cause at all.)

[pitdicker]

Ah, good use of Display!

I like "permanent failure or unavailable" more then "permanent failure", but shorter is a little neater here.
I don't see much use in adding "Rng error:" to the string. If I look at std::io::Error it also does not add such a text. What do you think of "{} ({}); cause: {}" as format string?

Example result:

 WARN 2018-02-01T18:02:36Z: rand: OsRng failed [falling back to JitterRng]: permanent failure (unexpected getrandom error); cause: Operation not permitted (os error 1)
DEBUG 2018-02-01T18:02:36Z: rand::jitter: JitterRng: testing timer ...
 WARN 2018-02-01T18:02:36Z: rand: JitterRng failed: permanent failure (timer jitter failed basic quality tests); cause: no timer available

[dhardy]

Reminder to self: unwrap and expect on Result will display the error via Debug; this might want changing in a couple of places.

[dhardy]

Specifically, check:

• THREAD_RNG_KEY init uses expect
• EntropyRng::fill_bytes uses unwrap
• EntropyRng::try_fill_bytes logging

[dhardy]

Updated; this affects EntropyRng logging significantly

[pitdicker]

Look good to me! Have you made all the changes you wanted?

[dhardy]

For now, though there might be some more tweaks after #252 merges.