Python binding for xmlRegisterInputCallback

It is possible to make xmlIO handle any protocol by means of
xmlRegisterInputCallback(). However, that function is currently only
available in C API. So, the natural solution seems to be implementing Python
bindings for the xmlRegisterInputCallback.

* python/generator.py: skip xmlPopInputCallbacks
* python/libxml.c python/libxml.py python/libxml_wrap.h: implement the
  wrappers
* python/tests/input_callback.py python/tests/Makefile.am: also add a test case

python/libxml.py

@@ -719,11 +719,35 @@ class xmlTextReaderCore:
             return arg
 
 #
-# The cleanup now goes though a wrappe in libxml.c
+# The cleanup now goes though a wrapper in libxml.c
 #
 def cleanupParser():
     libxml2mod.xmlPythonCleanupParser()
 
+#
+# The interface to xmlRegisterInputCallbacks.
+# Since this API does not allow to pass a data object along with
+# match/open callbacks, it is necessary to maintain a list of all
+# Python callbacks.
+#
+__input_callbacks = []
+def registerInputCallback(func):
+    def findOpenCallback(URI):
+        for cb in reversed(__input_callbacks):
+            o = cb(URI)
+            if o is not None:
+                return o
+    libxml2mod.xmlRegisterInputCallback(findOpenCallback)
+    __input_callbacks.append(func)
+
+def popInputCallbacks():
+    # First pop python-level callbacks, when no more available - start
+    # popping built-in ones.
+    if len(__input_callbacks) > 0:
+        __input_callbacks.pop()
+    if len(__input_callbacks) == 0:
+        libxml2mod.xmlUnregisterInputCallback()
+
 # WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING
 #
 # Everything before this line comes from libxml.py