Push and pop error handler within Env context

Closes #876
Closes #902

rasterio/_drivers.pyx

@@ -9,9 +9,9 @@ import sys
 from rasterio.compat import string_types
 
 from rasterio._gdal cimport (
-    CPLSetConfigOption, CPLSetErrorHandler, GDALAllRegister, GDALGetDriver,
+    CPLSetConfigOption, GDALAllRegister, GDALGetDriver,
     GDALGetDriverCount, GDALGetDriverLongName, GDALGetDriverShortName,
-    OGRGetDriverCount, OGRRegisterAll)
+    OGRGetDriverCount, OGRRegisterAll, CPLPopErrorHandler, CPLPushErrorHandler)
 
 include "gdal.pxi"
 
@@ -131,7 +131,9 @@ cdef class GDALEnv(ConfigEnv):
     def start(self):
         GDALAllRegister()
         OGRRegisterAll()
-        CPLSetErrorHandler(<CPLErrorHandler>errorHandler)
+        CPLPushErrorHandler(<CPLErrorHandler>errorHandler)
+        log.debug("Error handler pushed")
+
         if driver_count() == 0:
             raise ValueError("Drivers not registered")
 
@@ -153,6 +155,8 @@ cdef class GDALEnv(ConfigEnv):
         # NB: do not restore the CPL error handler to its default
         # state here. If you do, log messages will be written to stderr
         # by GDAL instead of being sent to Python's logging module.
+        CPLPopErrorHandler()
+        log.debug("Error handler popped")
         log.debug("Env %r has been stopped", self)
 
     def drivers(self):

rasterio/_gdal.pxd

@@ -29,7 +29,8 @@ cdef extern from "cpl_error.h" nogil:
     int CPLGetLastErrorNo()
     const char* CPLGetLastErrorMsg()
     CPLErr CPLGetLastErrorType()
-    void CPLSetErrorHandler(CPLErrorHandler handler)
+    void CPLPushErrorHandler(CPLErrorHandler handler)
+    void CPLPopErrorHandler()
 
 
 cdef extern from "ogr_srs_api.h" nogil:

rasterio/env.py

@@ -1,5 +1,6 @@
 """Rasterio's GDAL/AWS environment"""
 
+from functools import wraps
 import logging
 
 from rasterio._drivers import (
@@ -197,5 +198,8 @@ def delenv():
 def ensure_env(f):
     """A decorator that ensures an env exists before a function
     calls any GDAL C functions."""
-    defenv()
-    return f
+    @wraps(f)
+    def wrapper(*args, **kwds):
+        with Env():
+            return f(*args, **kwds)
+    return wrapper

tests/test_env.py

@@ -72,7 +72,7 @@ def test_ensure_env_decorator(gdalenv):
     def f(x):
         return x
     wrapper = ensure_env(f)
-    assert wrapper == f
+    assert wrapper.func_name == f.func_name
 
 
 def test_no_aws_gdal_config(gdalenv):