[llvm] r263742 - Bitcode: Error out instead of crashing on corrupt metadata

[Rafael Espíndola via llvm-commits]

> I could commit an 800 byte binary blob where I generated some IR then
> manually flipped a bit in a hex editor to hit one of the five error
> checks I added here. I could conceivably figure out how to generate
> similar tests for the other four checks with enough time and effort. Of
> course, since the bitcode's invalid it won't be meaningfully upgradeable
> and will probably end up hitting some other error path very soon after
> and no longer test this at all. When it does start erroring for whatever
> reason someone will find that it isn't doing anything interesting and
> probably delete it. IMO, This kind of test is useless.
>
> The case where I hit this in the real world involved a truncated bitcode
> file from an out of tree front end. This also wouldn't be terribly
> useful to commit.
>
> I think if we want to test this kind of change meaningfully we probably
> need to expose the BitcodeReader type to unit tests, then call
> individual parse functions with blobs of data that are short enough for
> a human to understand. This could end up being messy anyway though, I'm
> not really sure.


My take is that if it is worth coding a "fancy" error handling, then
it is worth testing it.

Checking the 800 byte binary would be just perfect for this I think.

Cheers,
Rafael