Update on missing SHA

[snoyberg]

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

OK, this is a bit involved. Ignoring cabal file revisions for a moment, consider you run stack build in a project using lts-7.15, and the project uses foldl-1.2.2. Suppose you had last updated your package index when foldl-1.2.2 wasn't available. Stack will automatically update your index before trying to build so that it can get the .cabal file for foldl-1.2.2.

Now consider a similar situation: your project uses yesod-core, which for nightly-2017-01-10 is version 1.4.30, with cabal file revision #1. If your index is missing that revision, previously you would get an error message about a missing revision. With this patch, you'll instead get the behavior that the index will automatically update.

In addition, there are commits here to improve the error messages delivered (to indicate whether the lookup occurred in a Git repo or in the tarball), and to standardize logic around choosing Git vs HTTP index methods.

Testing

In order to test, I first created a 00-index.tar file with the most recent content (by running stack update), and then pruned all recent .cabal files (including the first revision of yesod-core-1.4.30) with the following program run from ~/.stack/indices/Hackage:

#!/usr/bin/env stack
{- stack --resolver lts-7.14 --install-ghc runghc
    --package tar
    --package classy-prelude-conduit
-}
{-# LANGUAGE NoImplicitPrelude, OverloadedStrings #-}
import ClassyPrelude.Conduit
import qualified Codec.Archive.Tar as Tar
import qualified Codec.Archive.Tar.Entry as Tar
import Codec.Compression.GZip (decompress)
import System.Directory

main :: IO ()
main = do
    void $ tryIO $ removeFile "01-index.cache"
    void $ tryIO $ removeFile "01-index.tar"
    lbs <- readFile "01-index.tar.gz"
    let entries = Tar.read $ decompress lbs
    writeFile "01-index.tar" $ Tar.write $ go entries
  where
    go (Tar.Next e es)
        | toKeep e = e : go es
        | otherwise = go es
    go Tar.Done = []
    go (Tar.Fail e) = impureThrow e

    toKeep e = Tar.entryTime e < 1483747200

Then, I ran the following:

$ rm -rf yesod-core-1.4.30/ && stack --resolver nightly-2017-01-10 unpack yesod-core

Before this change: I would get an error message about a missing Git SHA. After this change, I get the following:

$ rm -rf yesod-core-1.4.30/ && stack --resolver nightly-2017-01-10 unpack yesod-core
Missing some cabal revision files, updating indices
Downloading package index from https://s3.amazonaws.com/hackage.fpcomplete.com/01-index.tar.gz                  
Populated index cache.                                                                                          
Unpacked yesod-core-1.4.30 to /Users/michael/Desktop/yesod-core-1.4.30/

I also confirmed that the output yesod-core-1.4.30/yesod-core.cabal file contains:

x-revision: 1

[borsboom]

This looks good to me.

[borsboom]

The behaviour described in the FIXME does seem more expected to me. I wonder how many people rely on the current behaviour.

[snoyberg]

I'll open up a new Github issue about this then, but changing the behavior of this doesn't seem like it should block the current PR.