Moves list_resources to the library and call it from resource manager

PyVISA was leaking memory because a resource was not properly closed.
Until this commit, `list_resources` was implemented in the ResourceManager
and called `find_resources` and `find_next` in the library.

This design is flawed because different backends might implement this
differently. Moreover, it required all backends to implement
`find_resources` and `find_next` instead of more direct code.

See #135

pyvisa/ctwrapper/highlevel.py

@@ -185,4 +185,28 @@ def _return_handler(self, ret_value, func, arguments):
 
         return ret_value
 
+    def list_resources(self, query='?*::INSTR'):
+        """Returns a tuple of all connected devices matching query.
+
+        :param query: regular expression used to match devices.
+        """
+
+        lib = self.lib
+
+        resources = []
+
+        try:
+            find_list, return_counter, instrument_description, err = lib.find_resources(self.session, query)
+        except errors.VisaIOError as e:
+            if e.error_code == constants.StatusCode.error_resource_not_found:
+                return tuple()
+            raise e
+
+        resources.append(instrument_description)
+        for i in range(return_counter - 1):
+            resources.append(lib.find_next(find_list)[0])
+
+        lib.close(find_list)
+
+        return tuple(resource for resource in resources)
 

pyvisa/highlevel.py

@@ -665,6 +665,13 @@ def install_handler(self, session, event_type, handler, user_handle):
         """
         raise NotImplementedError
 
+    def list_resources(self, query='?*::INSTR'):
+        """Returns a tuple of all connected devices matching query.
+
+        :param query: regular expression used to match devices.
+        """
+        raise NotImplementedError
+
     def lock(self, session, lock_type, timeout, requested_key=None):
         """Establishes an access mode to the specified resources.
 
@@ -1546,22 +1553,7 @@ def list_resources(self, query='?*::INSTR'):
         :param query: regular expression used to match devices.
         """
 
-        lib = self.visalib
-
-        resources = []
-
-        try:
-            find_list, return_counter, instrument_description, err = lib.find_resources(self.session, query)
-        except errors.VisaIOError as e:
-            if e.error_code == constants.StatusCode.error_resource_not_found:
-                return tuple()
-            raise e
-
-        resources.append(instrument_description)
-        for i in range(return_counter - 1):
-            resources.append(lib.find_next(find_list)[0])
-
-        return tuple(resource for resource in resources)
+        return self.visalib.list_resources(query)
 
     def list_resources_info(self, query='?*::INSTR'):
         """Returns a dictionary mapping resource names to resource extended