#14862: Switching to Cabal 2.2 makes ghc unusable -------------------------------------+------------------------------------- Reporter: alpmestan | Owner: alpmestan Type: bug | Status: new Priority: high | Milestone: Component: None | Version: 8.4.1-alpha3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by alpmestan): Alright, so gdb hasn't been all that helpful as it seems that single- stepping somehow makes me follow a codepath that just loops between the scheduler and the top of the haskell stack without really ever making progress. I have not managed to find a breakpoint that's "far enough" so that single-stepping would just lead me right to the crash. However, in the meantime hvr saw that: {{{#!hs import Data.Typeable data Foo = Foo deriving Eq data Bar = Bar deriving Eq main = print (Just Foo == cast Bar) }}} prints `True`, while it should obviously crash (because we'd expect `cast` to return `Nothing` here). What's more: {{{#!hs *Main Data.Typeable λ> typeRepFingerprint (typeRep (Proxy :: Proxy Foo)) 00000000000000000000000000000000 *Main Data.Typeable λ> typeRepFingerprint (typeRep (Proxy :: Proxy Bar)) 00000000000000000000000000000000 }}} hvr and I then looked at the code that generates those fingerprints. We end up calling `fingerprintFingerprints` for both, so I added a `print` to `fingerprintFingerprints` to see what the inputs to the fingerprinting code are in both cases. After digging some more for a while, we ended up realizing that `SIZEOF_STRUCT_MD5CONTEXT` is detected to be... 0! Which cause the md5 context & the digest to overlap in memory, and since we memset the md5 context to 0 at the end of the md5 computation in the C code, we end up with an all-0 digest, whatever the inputs! hvr has reported a fix in [https://phabricator.haskell.org/D4453#123717 the aforementionned diff]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14862#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler