setup.py: Fix gbp problem when installed with pip

when installed with "pip install gbp" it will be installed as version "0.0"
because the released version doesn't have a debian/changelog file, so it will
be installed with a warning:
balabit-bbos-tools 1.0.0 has requirement gbp==0.9.8, but you'll have gbp 0.0 which is incompatible.

and will cause an Exception when using gbp from a setuptools entrypoint script:

  File "/home/walkman/stew/projects/platform/source/balabit-os-tools/.venv/lib/python3.6/site-packages/pkg_resources/__init__.py", line 574, in _build_master
    ws.require(__requires__)
  File "/home/walkman/stew/projects/platform/source/balabit-os-tools/.venv/lib/python3.6/site-packages/pkg_resources/__init__.py", line 892, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/home/walkman/stew/projects/platform/source/balabit-os-tools/.venv/lib/python3.6/site-packages/pkg_resources/__init__.py", line 783, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (gbp 0.0 (/home/walkman/stew/projects/platform/source/balabit-os-tools/.venv/lib/python3.6/site-packages), Requirement.parse('gbp==0.9.8'), {'requiresgbp'})

This is because pkg_resources checks every dependency version and if it doesn't
match with the egg-info, it raises this Exception.

Instead, we try to parse the debian/changelog first, then load the version, or
if the debian/changelog file doesn't exists (this is the case at pip install)
we read the gbp/version.py and parse the version from it.

There is a "round trip check", which means the parsed version is written, then
it will be read back immediately to see if there is any problem.

2 files changed, 45 insertions, 24 deletions

diff --git a/setup.py b/setup.py
index f505b39d..150dfdde 100755
--- a/setup.py
+++ b/setup.py
@@ -17,30 +17,9 @@
 #    <http://www.gnu.org/licenses/>
 # END OF COPYRIGHT #
 
-import subprocess
-from setuptools import setup, find_packages
 import os
-
-
-def fetch_version():
-    """Get version from debian changelog and write it to gbp/version.py"""
-    version = "0.0"
-
-    try:
-        popen = subprocess.Popen('dpkg-parsechangelog', stdout=subprocess.PIPE)
-        out, ret = popen.communicate()
-        for line in out.decode('utf-8').split('\n'):
-            if line.startswith('Version:'):
-                version = line.split(' ')[1].strip()
-                break
-    except OSError:
-        pass  # Failing is fine, we just can't print the version then
-
-    with open('gbp/version.py', 'w') as f:
-        f.write('"The current gbp version number"\n')
-        f.write('gbp_version = "%s"\n' % version)
-
-    return version
+from setuptools import setup, find_packages
+from version_parser import parse_and_fetch_version
 
 
 def readme():
@@ -56,7 +35,7 @@ def setup_requires():
 
 
 setup(name="gbp",
-      version=fetch_version(),
+      version=parse_and_fetch_version(),
       author=u'Guido Günther',
       author_email='agx@sigxcpu.org',
       url='https://honk.sigxcpu.org/piki/projects/git-buildpackage/',
diff --git a/version_parser.py b/version_parser.py
new file mode 100644
index 00000000..793d77db
--- /dev/null
+++ b/version_parser.py
@@ -0,0 +1,42 @@
+import os
+import subprocess
+
+
+VERSION_PY_PATH = 'gbp/version.py'
+
+
+def _parse_changelog():
+    """Get version from debian changelog and write it to gbp/version.py"""
+    popen = subprocess.Popen('dpkg-parsechangelog', stdout=subprocess.PIPE)
+    out, ret = popen.communicate()
+    for line in out.decode('utf-8').split('\n'):
+        if line.startswith('Version:'):
+            version = line.split(' ')[1].strip()
+            return version
+
+    raise ValueError('Could not parse version from debian/changelog')
+
+
+def _save_version_py(version):
+    with open(VERSION_PY_PATH, 'w') as f:
+        f.write('"The current gbp version number"\n')
+        f.write('gbp_version = "%s"\n' % version)
+
+
+def _load_version():
+    with open(VERSION_PY_PATH, 'r') as f:
+        version_py = f.read()
+    version_py_globals = {}
+    exec(version_py, version_py_globals)
+    return version_py_globals['gbp_version']
+
+
+def parse_and_fetch_version():
+    if os.path.exists('debian/changelog'):
+        version = _parse_changelog()
+        _save_version_py(version)
+        # we could return with the version here, but instead we check that
+        # the file has been properly written and it can be loaded back
+
+    version = _load_version()
+    return version