fromAbsoluteFilePath: not an absolute path when running cabal update on Windows in Github Actions with built-in ghcup

[newhoggy]

Describe the bug
I'm getting the following error when running cabal update in very specific circumstances.

fromAbsoluteFilePath: not an absolute path
CallStack (from HasCallStack):
  error, called at src\\Hackage\\Security\\Util\\Path.hs:297:31 in hackage-secur_-0.6.0.1-aa3b50000d0dcf2987b9cef364fe5c110fa92705:Hackage.Security.Util.Path

To Reproduce
Steps to reproduce the behavior:

For this repository: https://github.com/input-output-hk/cardano-node

Checkout always-use-ghcup. Create a new PR with the latest commit. (You may need to amend the commit to force a build).

I've not seen the error outside of this environment.

Expected behavior
The cabal update command should work.

System information

• Operating system: Windows on Github Actions
• cabal-3.6.2.0, ghc-8.10.7 versions

Additional context
Related PRs and tickets:

• Print filepath in fromAbsoluteFilePath error hackage-security#261
• https://gitlab.haskell.org/haskell/ghcup-hs/-/issues/308

[hasufell]

Seems HasCallStack is in fact a rather useless concept if it only prints the end of the callstack.

[hasufell]

I tracked down the error by using a patched binary: https://github.com/hasufell/cardano-node/runs/4991673164?check_suite_focus=true

Selected mirror http://objects-us-east-1.dream.io/hackage-mirror/
Downloading root
"C:\Windows\SYSTEM32\curl.exe" "http://objects-us-east-1.dream.io/hackage-mirror/root.json" "--output" "C:\Users\RUNNER~1\AppData\Local\Temp\tra6AE3" "--location" "--write-out" "%{http_code}" "--user-agent" "cabal-install/3.7 (windows; x86_64)" "--silent" "--show-error" "--dump-header" "C:\Users\RUNNER~1\AppData\Local\Temp\cur6AF4.txt"
fromAbsoluteFilePath: not an absolute path: C:Users\runneradmin\.cabal\packages\hackage.haskell.org\roo6AD3.json
CallStack (from HasCallStack):
  error, called at src\\Hackage\\Security\\Util\\Path.hs:297:31 in hackage-security-0.6.2.0-inplace:Hackage.Security.Util.Path
Error: Process completed with exit code 1.

So it fails because of C:Users\runneradmin\.cabal\packages\hackage.haskell.org\roo6AD3.json. First of all, this is a valid (relative) windows path. On windows C:foo means "file foo relative to the current directory on the drive C". I don't think that's what was meant here though, so somewhere along the callstack, something messed up the windows path.

[hasufell]

I was able to reproduce the error by adding this to the global cabal.config:

remote-repo-cache: C:/cabal/packages

Instead of C:\cabal\packages (which btw. should be semantically equivalent).

So I'm assuming that this line is causing issues: https://github.com/input-output-hk/cardano-node/pull/3551/files#diff-c09cc8dc973415b1221308559b96942a1c4b872e690bf360fc22d9b42822d519R32

echo "CABAL_DIR=$HOME/.cabal" >> $GITHUB_ENV

I printed the value and it shows /c/Users/runneradmin/.cabal, which is not a native absolute windows path ...it's a cygwin thing. Look here:

Prelude System.FilePath.Windows> isRelative "/c/Users/runneradmin/.cabal"
True

Under windows /c/foo is the relative path c\foo on the current drive.

How all this ends up as C:Users\runneradmin\.cabal is still not entirely clear to me.

---

So, what should cabal do here? Interpret cygwin style paths? How does it know what is meant?

But for sure the error needs to be improved.

[jneira]

@newhoggy what do you think about the comments above? maybe fixing the path in the gha config and a better error would be enough?

[Mikolaj]

@newhoggy: any news? and comments?

[newhoggy]

Definitely a better error message will help.

[Mikolaj]

Right, let's improve the error message; volunteers welcome!

Independently, let's try to code-dive and understand how the path arises and what cabal could do better here.