backends: implemented functions for fetching backend versions via remote calls.

Ref:  #4276

vcsserver/git.py

@@ -521,6 +521,9 b' class GitRemote(object):'
     def discover_git_version(self):
         stdout, _ = self.run_git_command(
             {}, ['--version'], _bare=True, _safe=True)
+        prefix = 'git version'
+        if stdout.startswith(prefix):
+            stdout = stdout[len(prefix):]
         return stdout
 
     @reraise_safe_exceptions

vcsserver/hg.py

@@ -142,6 +142,11 b' class HgRemote(object):'
         }
 
     @reraise_safe_exceptions
+    def discover_hg_version(self):
+        from mercurial import util
+        return util.version()
+
+    @reraise_safe_exceptions
     def archive_repo(self, archive_path, mtime, file_info, kind):
         if kind == "tgz":
             archiver = archival.tarit(archive_path, mtime, "gz")

vcsserver/svn.py

@@ -32,6 +32,7 b' import svn.fs'
 import svn.repos
 
 from vcsserver import svn_diff
+from vcsserver import exceptions
 from vcsserver.base import RepoFactory
 
 
@@ -48,6 +49,30 b' svn_compatible_versions = set(['
 ])
 
 
+def reraise_safe_exceptions(func):
+    """Decorator for converting svn exceptions to something neutral."""
+    def wrapper(*args, **kwargs):
+        try:
+            return func(*args, **kwargs)
+        except Exception as e:
+            if not hasattr(e, '_vcs_kind'):
+                log.exception("Unhandled exception in hg remote call")
+                raise_from_original(exceptions.UnhandledException)
+            raise
+    return wrapper
+
+
+def raise_from_original(new_type):
+    """
+    Raise a new exception type with original args and traceback.
+    """
+    _, original, traceback = sys.exc_info()
+    try:
+        raise new_type(*original.args), None, traceback
+    finally:
+        del traceback
+
+
 class SubversionFactory(RepoFactory):
 
     def _create_repo(self, wire, create, compatible_version):
@@ -88,6 +113,15 b' class SvnRemote(object):'
         # for subversion
         self._hg_factory = hg_factory
 
+    @reraise_safe_exceptions
+    def discover_svn_version(self):
+        try:
+            import svn.core
+            svn_ver = svn.core.SVN_VERSION
+        except ImportError:
+            svn_ver = None
+        return svn_ver
+
     def check_url(self, url, config_items):
         # this can throw exception if not installed, but we detect this
         from hgsubversion import svnrepo