Changes to PATH quietly ignored

[hamishmack]

Describe the bug
cabal v2-build caches the PATH environment variable and does not always pass on changes (perhaps unless they impact program selection). Other environment variable changes are passed on. This is confusing for tools like ghcjs-hsc2hs that wrap other executables and issue errors when they are not in the PATH. Updating the PATH and rerunning cabal v2-build does not work without a rm -rf dist-newstyle because cabal v2-build continues to use the old PATH.

To Reproduce
Steps to reproduce the behavior:

Here is an example of the new PATH value being ignored (while NOTPATH is not). Adding a bug to the source makes sure cabal prints the environment instead of just skipping the step that does that.

~/haskell$ cabal unpack hello
Unpacking to hello-1.0.0.2/
~/haskell$ cd hello-1.0.0.2
~/haskell/hello-1.0.0.2$ echo bug >> src/hello.hs 
~/haskell/hello-1.0.0.2$ cabal v2-build --verbose=3 2>&1 | grep HELLO
~/haskell/hello-1.0.0.2$ PATH=HELLO:$PATH cabal new-build --verbose=3 2>&1 | grep HELLO
~/haskell/hello-1.0.0.2$ NOTPATH=HELLO cabal new-build --verbose=3 2>&1 | grep HELLO
Environment: [[dump of the environment including HELLO]]

Expected behavior
I would have expected the Environment: to include HELLO in both cases, but it only does when the NOTPATH variable is set. Changes in PATH should be passed on to child processes even if they do not impact cabal's program selection.

System information

~/haskell/hello-1.0.0.2$ cabal --version
cabal-install version 3.0.0.0
compiled using version 3.0.0.0 of the Cabal library 
~/haskell/hello-1.0.0.2$ uname -a
Darwin MacBook-Pro-4.local 18.7.0 Darwin Kernel Version 18.7.0: Tue Aug 20 16:57:14 PDT 2019; root:xnu-4903.271.2~2/RELEASE_X86_64 x86_64
~/haskell/hello-1.0.0.2$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.6.5

[phadej]

I agree that the behavior is not good.

Currently the changes in PATH don't cause automatic reconfiguration. I'm not sure whether they should. After all there are other changes which cabal won't be possibly able to notice. So where to draw the line is somewhat arbitrary.

Related to #2662.

This makes me reconsider my comment in #3886 (comment), maybe there should some flat to v2-build after all, which would force reconfiguration of each package in the project (then the PATH will be picked as well).

[adamgundry]

I recently tripped over this as well. My instinct is that changing PATH should force a reconfigure, because I'd expect cabal to select programs from the current PATH rather than some previous cached version. Even if there are circumstances in which manually forced reconfiguration is necessary, we might as well make it as rare as possible, no?

In the absence of that, how easy would it be to emit a warning that the PATH has changed since configuration time and that the user might want to manually reconfigure? (Presumably by passing the flag contemplated in #3886.)