Improve error message when using closed ECDriver instances

Previously, we would wrap an error coming out of our pyeclib_c
extension to get something like

   pyeclib.ec_iface.ECInvalidParameter: pyeclib_c_encode ERROR:
   Invalid arguments. Please inspect syslog for liberasurecode
   error report.

Now, raise something more specific/useful:

   ECBackendInstanceNotAvailable: erasure coding backend is closed

Change-Id: Iec889153f2b8c034be6a2e91ffe6bc82ed079e61
Signed-off-by: Tim Burke <tim.burke@gmail.com>

pyeclib/core.py

@@ -21,6 +21,7 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 # THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+from .ec_iface import ECBackendInstanceNotAvailable
 from .ec_iface import ECDriverError
 from .ec_iface import ECInsufficientFragments
 from .ec_iface import PyECLib_FRAGHDRCHKSUM_Types
@@ -49,7 +50,7 @@ class ECPyECLibDriver(object):
         if self.chksum_type is PyECLib_FRAGHDRCHKSUM_Types.inline_crc32:
             self.inline_chksum = 1
 
-        self.handle = pyeclib_c.init(
+        self._handle = pyeclib_c.init(
             self.k,
             self.m,
             ec_type.value,
@@ -64,9 +65,16 @@ class ECPyECLibDriver(object):
             self.chksum_type)
 
     def close(self):
-        if self.handle:
+        if self._handle is not None:
-            pyeclib_c.destroy(self.handle)
+            pyeclib_c.destroy(self._handle)
-        self.handle = None
+        self._handle = None
+
+    @property
+    def handle(self):
+        if self._handle is None:
+            raise ECBackendInstanceNotAvailable(
+                'erasure coding handle is closed')
+        return self._handle
 
     def encode(self, data_bytes):
         return pyeclib_c.encode(self.handle, data_bytes)

test/test_pyeclib_api.py

@@ -33,6 +33,7 @@ import unittest
 from itertools import combinations
 
 import pyeclib.ec_iface
+from pyeclib.ec_iface import ECBackendInstanceNotAvailable
 from pyeclib.ec_iface import ECBackendNotSupported
 from pyeclib.ec_iface import ECDriver
 from pyeclib.ec_iface import ECDriverError
@@ -294,6 +295,27 @@ class TestPyECLibDriver(unittest.TestCase):
                                    chksum_type=csum))
         return pyeclib_drivers
 
+    def test_use_after_close(self):
+        pyeclib_drivers = self.get_pyeclib_testspec()
+        for pyeclib_driver in pyeclib_drivers:
+            frags = pyeclib_driver.encode(b"testdata")
+            self.assertEqual(
+                pyeclib_driver.reconstruct(frags[1:], [0])[0], frags[0])
+
+            pyeclib_driver.close()
+            with self.assertRaises(ECBackendInstanceNotAvailable) as ctx:
+                pyeclib_driver.encode(b"testdata")
+            self.assertEqual(str(ctx.exception),
+                             'erasure coding handle is closed')
+            with self.assertRaises(ECBackendInstanceNotAvailable) as ctx:
+                pyeclib_driver.decode(frags)
+            self.assertEqual(str(ctx.exception),
+                             'erasure coding handle is closed')
+            with self.assertRaises(ECBackendInstanceNotAvailable) as ctx:
+                pyeclib_driver.reconstruct(frags[1:], [0])
+            self.assertEqual(str(ctx.exception),
+                             'erasure coding handle is closed')
+
     def test_small_encode(self):
         pyeclib_drivers = self.get_pyeclib_testspec()
         encode_strs = [b"a", b"hello", b"hellohyhi", b"yo"]