capnproto

schema-loader.h (8544B)

---

 // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
 // Licensed under the MIT License:
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to deal
 // in the Software without restriction, including without limitation the rights
 // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 // copies of the Software, and to permit persons to whom the Software is
 // furnished to do so, subject to the following conditions:
 //
 // The above copyright notice and this permission notice shall be included in
 // all copies or substantial portions of the Software.
 //
 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
 #pragma once
 
 #include "schema.h"
 #include <kj/memory.h>
 #include <kj/mutex.h>
 
 CAPNP_BEGIN_HEADER
 
 namespace capnp {
 
 class SchemaLoader {
   // Class which can be used to construct Schema objects from schema::Nodes as defined in
   // schema.capnp.
   //
   // It is a bad idea to use this class on untrusted input with exceptions disabled -- you may
   // be exposing yourself to denial-of-service attacks, as attackers can easily construct schemas
   // that are subtly inconsistent in a way that causes exceptions to be thrown either by
   // SchemaLoader or by the dynamic API when the schemas are subsequently used.  If you enable and
   // properly catch exceptions, you should be OK -- assuming no bugs in the Cap'n Proto
   // implementation, of course.
 
 public:
   class LazyLoadCallback {
   public:
     virtual void load(const SchemaLoader& loader, uint64_t id) const = 0;
     // Request that the schema node with the given ID be loaded into the given SchemaLoader.  If
     // the callback is able to find a schema for this ID, it should invoke `loadOnce()` on
     // `loader` to load it.  If no such node exists, it should simply do nothing and return.
     //
     // The callback is allowed to load schema nodes other than the one requested, e.g. because it
     // expects they will be needed soon.
     //
     // If the `SchemaLoader` is used from multiple threads, the callback must be thread-safe.
     // In particular, it's possible for multiple threads to invoke `load()` with the same ID.
     // If the callback performs a large amount of work to look up IDs, it should be sure to
     // de-dup these requests.
   };
 
   SchemaLoader();
 
   SchemaLoader(const LazyLoadCallback& callback);
   // Construct a SchemaLoader which will invoke the given callback when a schema node is requested
   // that isn't already loaded.
 
   ~SchemaLoader() noexcept(false);
   KJ_DISALLOW_COPY(SchemaLoader);
 
   Schema get(uint64_t id, schema::Brand::Reader brand = schema::Brand::Reader(),
              Schema scope = Schema()) const;
   // Gets the schema for the given ID, throwing an exception if it isn't present.
   //
   // The returned schema may be invalidated if load() is called with a new schema for the same ID.
   // In general, you should not call load() while a schema from this loader is in-use.
   //
   // `brand` and `scope` are used to determine brand bindings where relevant. `brand` gives
   // parameter bindings for the target type's brand parameters that were specified at the reference
   // site. `scope` specifies the scope in which the type ID appeared -- if `brand` itself contains
   // parameter references or indicates that some parameters will be inherited, these will be
   // interpreted within / inherited from `scope`.
 
   kj::Maybe<Schema> tryGet(uint64_t id, schema::Brand::Reader bindings = schema::Brand::Reader(),
                            Schema scope = Schema()) const;
   // Like get() but doesn't throw.
 
   Schema getUnbound(uint64_t id) const;
   // Gets a special version of the schema in which all brand parameters are "unbound". This means
   // that if you look up a type via the Schema API, and it resolves to a brand parameter, the
   // returned Type's getBrandParameter() method will return info about that parameter. Otherwise,
   // normally, all brand parameters that aren't otherwise bound are assumed to simply be
   // "AnyPointer".
 
   Type getType(schema::Type::Reader type, Schema scope = Schema()) const;
   // Convenience method which interprets a schema::Type to produce a Type object. Implemented in
   // terms of get().
 
   Schema load(const schema::Node::Reader& 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.
   //
   // If the node has any dependencies which are not already loaded, they will be initialized as
   // stubs -- empty schemas of whichever kind is expected.
   //
   // If another schema for the given reader has already been seen, the loader will inspect both
   // schemas to determine which one is newer, and use that that one.  If the two versions are
   // found to be incompatible, an exception is thrown.  If the two versions differ but are
   // compatible and the loader cannot determine which is newer (e.g., the only changes are renames),
   // the existing schema will be preferred.  Note that in any case, the loader will end up keeping
   // around copies of both schemas, so you shouldn't repeatedly reload schemas into the same loader.
   //
   // The following properties of the schema node are validated:
   // - Struct size and preferred list encoding are valid and consistent.
   // - Struct members are fields or unions.
   // - Union members are fields.
   // - Field offsets are in-bounds.
   // - Ordinals and codeOrders are sequential starting from zero.
   // - Values are of the right union case to match their types.
   //
   // You should assume anything not listed above is NOT validated.  In particular, things that are
   // not validated now, but could be in the future, include but are not limited to:
   // - Names.
   // - Annotation values.  (This is hard because the annotation declaration is not always
   //   available.)
   // - Content of default/constant values of pointer type.  (Validating these would require knowing
   //   their schema, but even if the schemas are available at validation time, they could be
   //   updated by a subsequent load(), invalidating existing values.  Instead, these values are
   //   validated at the time they are used, as usual for Cap'n Proto objects.)
   //
   // Also note that unknown types are not considered invalid.  Instead, the dynamic API returns
   // a DynamicValue with type UNKNOWN for these.
 
   Schema loadOnce(const schema::Node::Reader& reader) const;
   // Like `load()` but does nothing if a schema with the same ID is already loaded.  In contrast,
   // `load()` would attempt to compare the schemas and take the newer one.  `loadOnce()` is safe
   // to call even while concurrently using schemas from this loader.  It should be considered an
   // error to call `loadOnce()` with two non-identical schemas that share the same ID, although
   // this error may or may not actually be detected by the implementation.
 
   template <typename T>
   void loadCompiledTypeAndDependencies();
   // Load the schema for the given compiled-in type and all of its dependencies.
   //
   // If you want to be able to cast a DynamicValue built from this SchemaLoader to the compiled-in
   // type using as<T>(), you must call this method before constructing the DynamicValue.  Otherwise,
   // as<T>() will throw an exception complaining about type mismatch.
 
   kj::Array<Schema> getAllLoaded() const;
   // Get a complete list of all loaded schema nodes.  It is particularly useful to call this after
   // loadCompiledTypeAndDependencies<T>() in order to get a flat list of all of T's transitive
   // dependencies.
 
 private:
   class Validator;
   class CompatibilityChecker;
   class Impl;
   class InitializerImpl;
   class BrandedInitializerImpl;
   kj::MutexGuarded<kj::Own<Impl>> impl;
 
   void loadNative(const _::RawSchema* nativeSchema);
 };
 
 template <typename T>
 inline void SchemaLoader::loadCompiledTypeAndDependencies() {
   loadNative(&_::rawSchema<T>());
 }
 
 }  // namespace capnp
 
 CAPNP_END_HEADER