stack_snapshot get invalidated too often

[thufschmitt]

Describe the bug

Dependencies built with stack_packages or haskell_cabal_package gets
invalidated at least each time the PATH changes due to the use of
use_default_shell_env.

To Reproduce

From the examples directory:

# Build a random cabal package
nix-shell --pure --run "bazel build @stackage//:random"
# Rebuild it again and see that it's cached
nix-shell --pure --run "bazel build @stackage//:random"
# Add `hello` to the `buildInputs` list in `shell.nix`
# Rebuild the random package and see that it isn't cached anymore
nix-shell --pure --run "bazel build @stackage//:random"

Expected behavior

A change in the environment shouldn't trigger a cache miss.

Environment

• OS name + version: nixos unstable
• Bazel version: 0.28.0 (from the example's shell.nix)
• Version of the rules: latest master (79e2c53)

[mboes]

Did you mean stack_snapshot?

[thufschmitt]

Did you mean stack_snapshot?

Oh yes, sorry, I'll update the title

[mboes]

each time the PATH changes due to the use of use_default_shell_env.

This is a side-effect of not having a shell toolchain. Cabal assumes quite a bit from the environment (in terms of available commands).

[jkachmar]

Should #1097 and #1117 (which have since been merged) automatically address the problem, and should this issue be closed accordingly?

Or is there a need for some sort of regression test to make sure stack_snapshot dependencies are always cached?

[aherrmann]

This should be fixed now. I've verified that the following does not cause a rebuild, while it did before those PRs were merged.

$ nix-shell --pure --run 'bazel build @stackage//:void'
$ nix-shell --pure --run 'PATH=foo:$PATH bazel build @stackage//:void'