Use show prefix instead of show toplevel for finding (#520)

this takes the idea from https://github.com/msys2/MINGW-packages/blob/bf4d6f85053b66db09977ecffac0c1b3db735a0f/mingw-w64-python-setuptools-scm/0001-git-Use-show-prefix-instead-of-show-toplevel-for-fin.patch 
and simplified the logic

additionally msys2 testing is added

Fixes https://github.com/pypa/setuptools_scm/issues/415

3 files changed, 39 insertions, 4 deletions

diff --git a/.github/workflows/python-tests.yml b/.github/workflows/python-tests.yml
index ec2390b..4963733 100644
--- a/.github/workflows/python-tests.yml
+++ b/.github/workflows/python-tests.yml
@@ -23,13 +23,15 @@ jobs:
         include:
         - os: ubuntu-latest
           python_version: '3.9-dev'
+        - os: windows-latest
+          python_version: 'msys2'
 
     name: ${{ matrix.os }} - Python ${{ matrix.python_version }}
     steps:
       - uses: actions/checkout@v1
       - name: Setup python
         uses: actions/setup-python@v2
-        if: matrix.python_version != '3.9-dev'
+        if: matrix.python_version != '3.9-dev' && matrix.python_version != 'msys2'
         with:
           python-version: ${{ matrix.python_version }}
           architecture: x64
@@ -39,7 +41,15 @@ jobs:
         with:
           python-version: ${{ matrix.python_version }}
           architecture: x64
+      - name: Setup MSYS2
+        uses: msys2/setup-msys2@v2
+        if: matrix.python_version == 'msys2'
+        with:
+          msystem: MINGW64
+          install: git mingw-w64-x86_64-python mingw-w64-x86_64-python-setuptools
+          update: true
       - run: pip install -U setuptools
+        if: matrix.python_version != 'msys2'
       - run: pip install -e .[toml] pytest
       - run: pytest
 
diff --git a/src/setuptools_scm/file_finder_git.py b/src/setuptools_scm/file_finder_git.py
index 1d3e69b..fbaf07c 100644
--- a/src/setuptools_scm/file_finder_git.py
+++ b/src/setuptools_scm/file_finder_git.py
@@ -11,13 +11,27 @@ log = logging.getLogger(__name__)
 
 def _git_toplevel(path):
     try:
+        cwd = os.path.abspath(path or ".")
         with open(os.devnull, "wb") as devnull:
             out = subprocess.check_output(
-                ["git", "rev-parse", "--show-toplevel"],
-                cwd=(path or "."),
+                ["git", "rev-parse", "--show-prefix"],
+                cwd=cwd,
                 universal_newlines=True,
                 stderr=devnull,
             )
+        out = out.strip()[:-1]  # remove the trailing pathsep
+        if not out:
+            out = cwd
+        else:
+            # Here, ``out`` is a relative path to root of git.
+            # ``cwd`` is absolute path to current working directory.
+            # the below method removes the length of ``out`` from
+            # ``cwd``, which gives the git toplevel
+            assert cwd.replace("\\", "/").endswith(out)
+            # In windows cwd contains ``\`` which should be replaced by ``/``
+            # for this assertion to work. Length of string isn't changed by replace
+            # ``\\`` is just and escape for `\`
+            out = cwd[: -len(out)]
         trace("find files toplevel", out)
         return os.path.normcase(os.path.realpath(out.strip()))
     except subprocess.CalledProcessError:
diff --git a/src/setuptools_scm/git.py b/src/setuptools_scm/git.py
index a193f93..4bd5d15 100644
--- a/src/setuptools_scm/git.py
+++ b/src/setuptools_scm/git.py
@@ -2,6 +2,7 @@ from .config import Configuration
 from .utils import do_ex, trace, require_command
 from .version import meta
 
+import os
 from os.path import isfile, join
 import warnings
 
@@ -26,9 +27,19 @@ class GitWorkdir(object):
 
     @classmethod
     def from_potential_worktree(cls, wd):
-        real_wd, _, ret = do_ex("git rev-parse --show-toplevel", wd)
+        wd = os.path.abspath(wd)
+        real_wd, _, ret = do_ex("git rev-parse --show-prefix", wd)
+        real_wd = real_wd[:-1]  # remove the trailing pathsep
         if ret:
             return
+        if not real_wd:
+            real_wd = wd
+        else:
+            assert wd.replace("\\", "/").endswith(real_wd)
+            # In windows wd contains ``\`` which should be replaced by ``/``
+            # for this assertion to work.  Length of string isn't changed by replace
+            # ``\\`` is just and escape for `\`
+            real_wd = wd[: -len(real_wd)]
         trace("real root", real_wd)
         if not samefile(real_wd, wd):
             return