[ccache] Caching failed compilations

[Joel Rosdahl]

Hi,

I did have a look at how feasible it is, and basically I think it can be
> done.


Yes, caching failures (from the compiler, not the preprocessor) would be
feasible and I think that it's a good idea, at least as an optional feature.

I'm not very tempted to add a new kind of file for storing the exit code in
the cache, though. Instead I think that we should switch to store only one
file per result in the cache (except the manifest) and store everything
(exit code, stderr, object file, dependency file, etc.) in it. The downside
would be that hard_link mode wouldn't be possible anymore but the upside is
that fewer i-nodes will be used, which should improve performance in
theory. (Today at least two i-nodes are used per cached result plus one for
the manifest.)

-- Joel