Spurious reinstallation of add-source'd dependencies

[bitemyapp]

I'm trying to make something like an idempotent build process that doesn't rebuild any more than it needs to.

Problem I'm running into is that even if the vendored dependencies haven't changed, running:

cabal sandbox add-source ../blah

Will force reinstallation of blah even if it hasn't changed at all.

[dcoutts]

I've seen this too, it's a right PITA. We should really track it down. It's one of the items in #2631.

@bitemyapp can you make a recipe to reproduce it reliably?

[grayjay]

I spent some time on this issue. This causes the package to be installed twice:

cabal sandbox init
cabal sandbox add-source ../blah
cabal install --only-dependencies
cabal sandbox add-source ../blah
cabal configure
cabal build

add-source sets the timestamps for ../blah to 0 in .cabal-sandbox/add-source-timestamps, even if the directory is already listed. Then cabal treats the source as modified during the next build. #1331 changed add-source to allow it to overwrite existing timestamps. However, I don't know if the issues discussed in that PR and the associated issue (#1197) are still relevant. When I modified cabal to prevent overwriting timestamps, I couldn't reproduce the problems discussed in #1197. Does anyone know if it is safe to stop overwriting existing timestamps?

[23Skidoo]

@grayjay I think that this solution should be OK. Please do a pull request, and I'll take a look. Note that you also need to support the following case:

$ cabal sandbox add-source /path/to/foo
$ cabal install --only-dependencies
$ cabal sandbox add-source /some/other/path/to/foo

[grayjay]

@23Skidoo I'm not sure how add-source is supposed to work in that case. Currently, it adds both directories to add-source-timestamps and the index. How should cabal determine which source to install later, if they both have the same package name and version?

As far as I can tell, cabal uses the source that was last added to the index. In your example, /some/other/path/to/foo is installed during the next build. But if I add /path/to/foo again, the package doesn't need to be added to the index, so it's not installed when I run cabal build. Handling adding a directory twice might require resetting its timestamp.

[phadej]

sandbox functionality is removed.