Schema loading from the wire

Cap'n Proto provides a schema loader, which can be used to dynamically
load schemas during runtime. To port this functionality to pycapnp,
a new class is provided `C_SchemaLoader`, which exposes the Cap'n
Proto C++ interface, and `SchemaLoader`, which is part of the pycapnp
library.

The specific use case for this is when a capnp message contains
a Node.Reader: The schema for a yet unseen message can be loaded
dynamically, allowing the future message to be properly processed.

If the message is a struct containing other structs, all the schemas for
every struct must be loaded to correctly parse the message. See
https://github.com/DaneSlattery/capnp_generic_poc for a
proof-of-concept.

Add docs and cleanup

Add more docs

Reduce changes

Fix flake8 formatting

Fix get datatype

capnp/helpers/deserialize.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include "capnp/dynamic.h"
+#include "capnp/schema.capnp.h"
+
+/// @brief Convert the dynamic struct to a Node::Reader
+::capnp::schema::Node::Reader toReader(capnp::DynamicStruct::Reader reader)
+{
+    // requires an intermediate step to AnyStruct before going directly to Node::Reader,
+    // since there exists no direct conversion from DynamicStruct::Reader to Node::Reader.
+    return reader.as<capnp::AnyStruct>().as<capnp::schema::Node>();
+}

capnp/helpers/helpers.pxd

@@ -1,7 +1,7 @@
 from capnp.includes.capnp_cpp cimport (
     Maybe, PyPromise, VoidPromise, RemotePromise,
     DynamicCapability, InterfaceSchema, EnumSchema, StructSchema, DynamicValue, Capability, 
-    RpcSystem, MessageBuilder, Own, PyRefCounter
+    RpcSystem, MessageBuilder, Own, PyRefCounter, Node, DynamicStruct
 )
 
 from capnp.includes.schema_cpp cimport ByteArray
@@ -30,3 +30,7 @@ cdef extern from "capnp/helpers/rpcHelper.h":
 
 cdef extern from "capnp/helpers/serialize.h":
     ByteArray messageToPackedBytes(MessageBuilder &, size_t wordCount)
+
+cdef extern from "capnp/helpers/deserialize.h":
+    Node.Reader toReader(DynamicStruct.Reader reader) except +reraise_kj_exception
+

capnp/includes/capnp_cpp.pxd

@@ -446,6 +446,12 @@ cdef extern from "capnp/dynamic.h" namespace " ::capnp":
             DynamicStruct.Pipeline asStruct"releaseAs< ::capnp::DynamicStruct>"()
             Type getType()
 
+cdef extern from "capnp/schema-loader.h" namespace " ::capnp":
+    cdef cppclass SchemaLoader:
+        SchemaLoader()
+        Schema load(Node.Reader reader) except +reraise_kj_exception
+        Schema get(uint64_t id_) except +reraise_kj_exception
+
 cdef extern from "capnp/schema-parser.h" namespace " ::capnp":
     cdef cppclass ParsedSchema(Schema) nogil:
         ParsedSchema getNested(char * name) except +reraise_kj_exception

capnp/lib/capnp.pxd

@@ -5,7 +5,7 @@ from capnp.includes cimport schema_cpp
 from capnp.includes.capnp_cpp cimport (
     Schema as C_Schema, StructSchema as C_StructSchema, InterfaceSchema as C_InterfaceSchema,
     EnumSchema as C_EnumSchema, ListSchema as C_ListSchema, DynamicStruct as C_DynamicStruct,
-    DynamicValue as C_DynamicValue, Type as C_Type, DynamicList as C_DynamicList,
+    DynamicValue as C_DynamicValue, Type as C_Type, DynamicList as C_DynamicList, SchemaLoader as C_SchemaLoader,
     SchemaParser as C_SchemaParser, ParsedSchema as C_ParsedSchema, VOID, ArrayPtr, StringPtr,
     String, StringTree, DynamicOrphan as C_DynamicOrphan, AnyPointer as C_DynamicObject,
     DynamicCapability as C_DynamicCapability, Request, Response, RemotePromise, Promise,
@@ -30,6 +30,9 @@ cdef class _StringArrayPtr:
     cdef size_t size
     cdef ArrayPtr[StringPtr] asArrayPtr(self) except +reraise_kj_exception
 
+cdef class SchemaLoader:
+    cdef C_SchemaLoader * thisptr
+
 cdef class SchemaParser:
     cdef C_SchemaParser * thisptr
     cdef public dict modules_by_id

capnp/lib/capnp.pyx

@@ -363,6 +363,11 @@ cdef class _NodeReader:
         def __get__(self):
             return self.thisptr.isEnum()
     
+    property node:
+        """A property that returns the NodeReader as a DynamicStructReader."""
+        def __get__(self):
+            return _DynamicStructReader()._init(self.thisptr, self)
+
 
 cdef class _NestedNodeReader:
     cdef C_Node.NestedNode.Reader thisptr
@@ -3231,6 +3236,34 @@ cdef class _StringArrayPtr:
         return ArrayPtr[StringPtr](self.thisptr, self.size)
 
 
+cdef class SchemaLoader:
+    """ Class which can be used to construct Schema objects from schema::Nodes as defined in
+    schema.capnp.
+    
+    This class wraps capnproto/c++/src/capnp/schema-loader.h directly."""
+    def __cinit__(self):
+        self.thisptr = new C_SchemaLoader()
+    
+    def __dealloc__(self):
+        del self.thisptr
+
+    def load(self, _NodeReader reader):
+        """Loads the given schema node.  Validates the node and throws an exception if invalid.  This
+        makes a copy of the schema, so the object passed in can be destroyed after this returns.
+        
+        """
+        return _Schema()._init(self.thisptr.load(reader.thisptr))
+
+    def load_dynamic(self, _DynamicStructReader reader):
+        """Loads the given schema node with self.load, but converts from a _DynamicStructReader
+        first."""
+        return _Schema()._init(self.thisptr.load(helpers.toReader(reader.thisptr)))
+    
+    def get(self, id_):
+        """Gets the schema for the given ID, throwing an exception if it isn't present."""
+        return _Schema()._init(self.thisptr.get(<uint64_t>id_))
+
+
 cdef class SchemaParser:
     """A class for loading Cap'n Proto schema files.
 

docs/capnp.rst

@@ -88,6 +88,10 @@ Miscellaneous
   :undoc-members:
   :inherited-members:
 
+.. autoclass:: SchemaLoader
+  :members:
+  :undoc-members:
+  :inherited-members:
 
 Functions
 ---------

test/foo.capnp

@@ -5,3 +5,16 @@ struct Foo {
   name @1 :Text;
 }
 
+
+struct Baz{
+    text @0 :Text;
+    qux @1 :Qux;
+}
+
+struct Qux{
+    id @0 :UInt64;
+}
+
+interface Wrapper  {
+    wrapped @0 (object :AnyPointer);
+}

test/test_load.py

@@ -119,3 +119,30 @@ def test_bundled_import_hook():
     # stream.capnp should be bundled, or provided by the system capnproto
     capnp.add_import_hook()
     import stream_capnp  # noqa: F401
+
+
+def test_load_capnp(foo):
+    # test dynamically loading
+    loader = capnp.SchemaLoader()
+    loader.load(foo.Baz.schema.get_proto())
+    loader.load_dynamic(foo.Qux.schema.get_proto().node)
+
+    schema = loader.get(foo.Baz.schema.get_proto().node.id).as_struct()
+    assert "text" in schema.fieldnames
+    assert "qux" in schema.fieldnames
+    assert schema.fields["qux"].proto.slot.type.which == "struct"
+
+    class Wrapper(foo.Wrapper.Server):
+        def wrapped(self, object, **kwargs):
+            assert isinstance(object, capnp.lib.capnp._DynamicObjectReader)
+            baz_ = object.as_struct(schema)
+            assert baz_.text == "test"
+            assert baz_.qux.id == 2
+
+    # test calling into the wrapper with a Baz message.
+    baz_ = foo.Baz.new_message()
+    baz_.text = "test"
+    baz_.qux.id = 2
+
+    wrapper = foo.Wrapper._new_client(Wrapper())
+    wrapper.wrapped(baz_).wait()