improve dist/setup-config format mismatch detection

[hvr]

@dcoutts asked me to file this:

I was having this weird issues with the devel version of cabal where some cabal invocations would fail with the following rather non-obvious error message

cabal: Prelude.chr: bad argument: 2041223

Even with -v3 this is the very first output you get from cabal. Only strace provided the hint that setup-config was the last file read before that error message was emitted.

(the underlying problem was most likely that cabal had been compiled against an older snapshot of the Cabal lib than the Cabal lib snapshot registered in the pkg db. And those two snapshots probably disagreed on the setup-config schema)

[23Skidoo]

/cc @ttuegel

[dcoutts]

In the previous text based format we included info about the version of Cabal that wrote it and reported an appropriate message if the version did not match (and iirc, that was one of the cases where we'd automatically reconfigure).

[hvr]

fwiw, @svenpanne experienced this issue some time ago too: https://www.haskell.org/pipermail/ghc-devs/2014-October/006962.html

[23Skidoo]

@dcoutts
We still do this, but new code is probably less robust when the Cabal snapshot changes, but the version number doesn't. Maybe the header should also include Git revision ID.

[23Skidoo]

For those who come here by googling the error message: to fix this issue, just delete dist/setup-config.

[ttuegel]

The new code should be more robust to format changes, in the sense that binary returns errors explicitly rather than throwing exceptions. I think the problem here is that the header format changed; now we use a binary version header. I suspect this happens when a pre-binary-setup-config Cabal tries to read the binary header. I think we should switch back to the old header format; the new format still won't be backward compatible, but we will produce meaningful error messages. If we keep the old header format, we can be forward-compatible with the text setup-config format (eliminating the need to cabal configure explicitly when upgrading from text to binary; see #2214). The cost is breaking the format for anyone using a git checkout of Cabal (they will have to reconfigure manually; a small cost for an unreleased mistake).

[23Skidoo]

@ttuegel

I think we should switch back to the old header format;

+1 on this.

I suspect this happens when a pre-binary-setup-config Cabal tries to read the binary header.

I think I had this happen to me with a new Cabal, but I don't know how to reproduce (the problem went away after a reconfigure). It'd help if someone could attach a setup-config file that exhibits this behaviour.

@hvr

Which version of cabal-install are you using?

[hvr]

@23Skidoo

The cabal-install executable was generated from

https://launchpad.net/~hvr/+archive/ubuntu/ghc/+sourcepub/4539223/+listing-archive-extra

(i.e. 7ece59b) whereas the Cabal library gitlinked in GHC HEAD (and thus registered in the pkg-db) is currently at 6c395bb

[23Skidoo]

@hvr
Thanks. So this is a version that includes the binary setup-config patches.

[luite]

+1 on better handling of version mismatches. i like the idea of sticking with the old header format and automatically reconfiguring.

I've also been seeing this type of error a lot, working with recent cabal versions. I know how to fix it, but it's probably going to bite lots of users, in particular in mixed cabal version setups (and i'll have to keep asking ghc 7.8 users to upgrade their cabal library to use ghcjs)

[svenpanne]

I just triggered a rebuild for one of my projects, and things seem to work now: https://travis-ci.org/haskell-opengl/StateVar/jobs/40772431 @hvr: Have the Ubuntu packages been updated?

The only funny thing is the warning https://travis-ci.org/haskell-opengl/StateVar/jobs/40772431#L96 ("Warning: /tmp/pkgConf-StateVar-1.0.12205.0: Unrecognized field data-dir on line 19"). Not a show-stopper, but I really try to keep all builds warning-free, so what's going on there?

[luite]

data-dir is a new field in InstalledPackageInfo:
7fa8f88

I think the Cabal library shipping with GHC is still too old to include this change.