Prevent traceback with non-exec interpreter

Fixes #121

Add tests for virtualenv  with nose and mock
Add helper method to virtualenv for is_executable
Raise SystemExit rather than sys.exit directly

Tested on 2.4.4, 2.7.1 and 3.2

1 files changed, 8 insertions, 1 deletions

diff --git a/virtualenv.py b/virtualenv.py
index f49f3a3..5bd5534 100755
--- a/virtualenv.py
+++ b/virtualenv.py
@@ -1228,9 +1228,16 @@ def resolve_interpreter(exe):
                 break
     if not os.path.exists(exe):
         logger.fatal('The executable %s (from --python=%s) does not exist' % (exe, exe))
-        sys.exit(3)
+        raise SystemExit(3)
+    if not is_executable(exe):
+        logger.fatal('The executable %s (from --python=%s) is not executable' % (exe, exe))
+        raise SystemExit(3)
     return exe
 
+def is_executable(exe):
+    """Checks a file is executable"""
+    return os.access(exe, os.X_OK)
+
 ############################################################
 ## Relocating the environment: