[new-build] Use canonical path to compiler in config hash

[edsko]

Changing the path to ghc from /opt/ghc/8.0.2/bin/ghc to /opt/ghc/8.0.2//bin/ghc causes cabal to consider the compiler to have changed, leading to a lot of unnecessary recompilation.

[hvr]

which normalisation recipe do you recommend?

[edsko]

I don't have one in mind :) But it should have the property that if P and Q point to the same file they should result in the same hash? Perhaps better yet, don't include the hash of the path but instead the hash of the binary? (May be too expensive I suppose.)

[hvr]

@edsko the problem with the hash of the binary is that you'd be hashing the wrapper shell script ghc :-)

One could as a heuristic also use the inode number of the file object ghc points to.

[edsko]

Perfect, ought to be much cheaper to compute then 😁

[hvr]

Btw, there is a dual ticket to this one (to un-differentiate ghcs which seem different but are the same), #5116 for the opposite need (i.e. to better differentiate ghcs which seem to be the same but aren't)