cabal build interprets Extra-source-files relative to CWD

[leonschoorl]

Describe the bug
When using a cabal.project file cabal build seems to interpret "Extra-source-files" relative to the current working directory.
This has changed in cabal-install-head-3.1+git20191103.2.129775a-6~18.04 (from hvr's ppa).
cabal-install-3.0 did not have this problem.

To Reproduce
Here is a minimal setup:
/tmp/test-cabal/cabal.project:

packages:
  ./somepackage

/tmp/test-cabal/somepackage/somepackage.cabal:

Cabal-version:        2.2
Name:                 somepackage
Version:              0.1

Extra-source-files:   README.md

Library
  Build-depends:      base
  Default-language:   Haskell2010

/tmp/test-cabal/somepackage/README.md:

Readme ...

Building somepackage from the toplevel dir fails because it can't find the README.md:

$ cd /tmp/test-cabal
$ cabal-head v2-build somepackage
Resolving dependencies...
Build profile: -w ghc-8.8.1 -O1
In order, the following will be built (use -v for more details):
 - somepackage-0.1 (lib) (configuration changed)
Configuring library for somepackage-0.1..
Preprocessing library for somepackage-0.1..
Building library for somepackage-0.1..
cabal-head: filepath wildcard 'README.md' does not match any files.

But if I go into somepackage (where it's located)

$ cd /tmp/test-cabal/somepackage
$ cabal-head v2-build somepackage
Build profile: -w ghc-8.8.1 -O1
In order, the following will be built (use -v for more details):
 - somepackage-0.1 (lib) (first run)
Preprocessing library for somepackage-0.1..
Building library for somepackage-0.1..
$

Expected behavior
I expect locations in a .cabal file would be interpreted relative to the location of that .cabal file.

Additional context
Interestingly cabal sdist works fine:

$ cd /tmp/cabal-test
$ cabal-head v2-sdist somepackage
Wrote tarball sdist to
/tmp/test-cabal/dist-newstyle/sdist/somepackage-0.1.tar.gz
$ tar taf dist-newstyle/sdist/somepackage-0.1.tar.gz
somepackage-0.1/
somepackage-0.1/README.md
somepackage-0.1/somepackage.cabal

[phadej]

my 5cents: Cabal (starting from cabal-install?) should move from

• using FilePath (in favour of something like Path Absolute & Path Unrooted distinction as in hackage-security)
• never change its own CWD, but rather be specific in which cwd the subprocesses are spawned.

[phadej]

This is reproducible with current cabal-install-head in HVR PPA 3.1+git20191103.2.129775a-6~18.04. (e.g. on cabal own codebase)

I'll bisect which commit causes this, if it's easy to fix, I'll try that, otherwise hopefully revert would help.

[phadej]

My git-bisect says this is caused by 3b54835 by @quasicomputational