hooks: make the Python interpreter for Git hooks configurable as 'git_hook_interpreter' (Issue #333)

Commit 5e501b6ee639 introduced the use of 'sys.executable' as interpreter
for git hooks instead of 'python2' with the following argument:

    "Windows doesn't necessarily have "python2" available in $PATH, but we
    still want to make sure we don't end up invoking a python3. Using the
    absolute path seems more safe."

But, sys.executable does not necessarily point to Python. When Kallithea is
started under uWSGI, sys.executable points to the uwsgi executable. As a
result, the interpreter encoded in the git hooks on the server repositories
would be:

    #!/path/to/uwsgi

And pushing to such repo would result in following client errors:

    $ git push
    Password for 'http://user@localhost:5050':
    Enumerating objects: 3, done.
    Counting objects: 100% (3/3), done.
    Writing objects: 100% (3/3), 241 bytes | 241.00 KiB/s, done.
    Total 3 (delta 0), reused 0 (delta 0)
    remote: unable to load configuration from hooks/pre-receive
    To http://localhost:5050/gitrepo-new
     ! [remote rejected] master -> master (pre-receive hook declined)
    error: failed to push some refs to 'http://user@localhost:5050/gitrepo-new'

Fix this problem by introducing a configuration setting 'git_hook_interpreter'
that allow administrators to specify which Python interpreter to use.

A subsequent commit will cause its value to be filled in automatically when
generating a new ini file, but an administrator can always override it.

development.ini

@@ -117,6 +117,15 b' use_htsts = false'
 ## number of commits stats will parse on each iteration
 commit_parse_limit = 25
 
+## Path to Python executable to be used for git hooks.
+## This value will be written inside the git hook scripts as the text
+## after '#!' (shebang). When empty or not defined, the value of
+## 'sys.executable' at the time of installation of the git hooks is
+## used, which is correct in many cases but for example not when using uwsgi.
+## If you change this setting, you should reinstall the Git hooks via
+## Admin > Settings > Remap and Rescan.
+# git_hook_interpreter = /srv/kallithea/venv/bin/python2
+
 ## path to git executable
 git_path = git
 

kallithea/lib/paster_commands/template.ini.mako

@@ -211,6 +211,18 b' use_htsts = false'
 <%text>## number of commits stats will parse on each iteration</%text>
 commit_parse_limit = 25
 
+<%text>## Path to Python executable to be used for git hooks.</%text>
+<%text>## This value will be written inside the git hook scripts as the text</%text>
+<%text>## after '#!' (shebang). When empty or not defined, the value of</%text>
+<%text>## 'sys.executable' at the time of installation of the git hooks is</%text>
+<%text>## used, which is correct in many cases but for example not when using uwsgi.</%text>
+<%text>## If you change this setting, you should reinstall the Git hooks via</%text>
+<%text>## Admin > Settings > Remap and Rescan.</%text>
+# git_hook_interpreter = /srv/kallithea/venv/bin/python2
+%if git_hook_interpreter:
+git_hook_interpreter = ${git_hook_interpreter}
+%endif
+
 <%text>## path to git executable</%text>
 git_path = git
 

kallithea/model/scm.py

@@ -727,8 +727,11 b' class ScmModel(object):'
         Git hook scripts so they invoke Kallithea code with the right Python
         interpreter and in the right environment.
         """
+        # Note: sys.executable might not point at a usable Python interpreter. For
+        # example, when using uwsgi, it will point at the uwsgi program itself.
         # FIXME This may not work on Windows and may need a shell wrapper script.
-        return (sys.executable
+        return (kallithea.CONFIG.get('git_hook_interpreter')
+                or sys.executable
                 or '/usr/bin/env python2')
 
     def install_git_hooks(self, repo, force_create=False):