comparison osx/include/capnp/membrane.h @ 134:41e769c91eca

Add Capnp and KJ builds for OSX

+// Copyright (c) 2015 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.
+
+#ifndef CAPNP_MEMBRANE_H_
+#define CAPNP_MEMBRANE_H_
+// In capability theory, a "membrane" is a wrapper around a capability which (usually) forwards
+// calls but recursively wraps capabilities in those calls in the same membrane. The purpose of a
+// membrane is to enforce a barrier between two capabilities that cannot be bypassed by merely
+// introducing new objects.
+//
+// The most common use case for a membrane is revocation: Say Alice wants to give Bob a capability
+// to access Carol, but wants to be able to revoke this capability later. Alice can accomplish this
+// by wrapping Carol in a revokable wrapper which passes through calls until such a time as Alice
+// indicates it should be revoked, after which all calls through the wrapper will throw exceptions.
+// However, a naive wrapper approach has a problem: if Bob makes a call to Carol and sends a new
+// capability in that call, or if Carol returns a capability to Bob in the response to a call, then
+// the two are now able to communicate using this new capability, which Alice cannot revoke. In
+// order to avoid this problem, Alice must use not just a wrapper but a "membrane", which
+// recursively wraps all objects that pass through it in either direction. Thus, all connections
+// formed between Bob and Carol (originating from Alice's original introduction) can be revoked
+// together by revoking the membrane.
+//
+// Note that when a capability is passed into a membrane and then passed back out, the result is
+// the original capability, not a double-membraned capability. This means that in our revocation
+// example, if Bob uses his capability to Carol to obtain another capability from her, then send
+// it back to her, the capability Carol receives back will NOT be revoked when Bob's access to
+// Carol is revoked. Thus Bob can create long-term irrevocable connections. In most practical use
+// cases, this is what you want. APIs commonly rely on the fact that a capability obtained and then
+// passed back can be recognized as the original capability.
+//
+// Mark Miller on membranes: http://www.eros-os.org/pipermail/e-lang/2003-January/008434.html
+
+#include "capability.h"
+
+namespace capnp {
+
+class MembranePolicy {
+  // Applications may implement this interface to define a membrane policy, which allows some
+  // calls crossing the membrane to be blocked or redirected.
+
+public:
+  virtual kj::Maybe<Capability::Client> inboundCall(
+      uint64_t interfaceId, uint16_t methodId, Capability::Client target) = 0;
+  // Given an inbound call (a call originating "outside" the membrane destined for an object
+  // "inside" the membrane), decides what to do with it. The policy may:
+  //
+  // - Return null to indicate that the call should proceed to the destination. All capabilities
+  //   in the parameters or result will be properly wrapped in the same membrane.
+  // - Return a capability to have the call redirected to that capability. Note that the redirect
+  //   capability will be treated as outside the membrane, so the params and results will not be
+  //   auto-wrapped; however, the callee can easily wrap the returned capability in the membrane
+  //   itself before returning to achieve this effect.
+  // - Throw an exception to cause the call to fail with that exception.
+  //
+  // `target` is the underlying capability (*inside* the membrane) for which the call is destined.
+  // Generally, the only way you should use `target` is to wrap it in some capability which you
+  // return as a redirect. The redirect capability may modify the call in some way and send it to
+  // `target`. Be careful to use `copyIntoMembrane()` and `copyOutOfMembrane()` as appropriate when
+  // copying parameters or results across the membrane.
+  //
+  // Note that since `target` is inside the capability, if you were to directly return it (rather
+  // than return null), the effect would be that the membrane would be broken: the call would
+  // proceed directly and any new capabilities introduced through it would not be membraned. You
+  // generally should not do that.
+
+  virtual kj::Maybe<Capability::Client> outboundCall(
+      uint64_t interfaceId, uint16_t methodId, Capability::Client target) = 0;
+  // Like `inboundCall()`, but applies to calls originating *inside* the membrane and terminating
+  // outside.
+  //
+  // Note: It is strongly recommended that `outboundCall()` returns null in exactly the same cases
+  //   that `inboundCall()` return null. Conversely, for any case where `inboundCall()` would
+  //   redirect or throw, `outboundCall()` should also redirect or throw. Otherwise, you can run
+  //   into inconsistent behavion when a promise is returned across a membrane, and that promise
+  //   later resolves to a capability on the other side of the membrane: calls on the promise
+  //   will enter and then exit the membrane, but calls on the eventual resolution will not cross
+  //   the membrane at all, so it is important that these two cases behave the same.
+
+  virtual kj::Own<MembranePolicy> addRef() = 0;
+  // Return a new owned pointer to the same policy.
+  //
+  // Typically an implementation of MembranePolicy should also inherit kj::Refcounted and implement
+  // `addRef()` as `return kj::addRef(*this);`.
+  //
+  // Note that the membraning system considers two membranes created with the same MembranePolicy
+  // object actually to be the *same* membrane. This is relevant when an object passes into the
+  // membrane and then back out (or out and then back in): instead of double-wrapping the object,
+  // the wrapping will be removed.
+};
+
+Capability::Client membrane(Capability::Client inner, kj::Own<MembranePolicy> policy);
+// Wrap `inner` in a membrane specified by `policy`. `inner` is considered "inside" the membrane,
+// while the returned capability should only be called from outside the membrane.
+
+Capability::Client reverseMembrane(Capability::Client outer, kj::Own<MembranePolicy> policy);
+// Like `membrane` but treat the input capability as "outside" the membrane, and return a
+// capability appropriate for use inside.
+//
+// Applications typically won't use this directly; the membraning code automatically sets up
+// reverse membranes where needed.
+
+template <typename ClientType>
+ClientType membrane(ClientType inner, kj::Own<MembranePolicy> policy);
+template <typename ClientType>
+ClientType reverseMembrane(ClientType inner, kj::Own<MembranePolicy> policy);
+// Convenience templates which return the same interface type as the input.
+
+template <typename ServerType>
+typename ServerType::Serves::Client membrane(
+    kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy);
+template <typename ServerType>
+typename ServerType::Serves::Client reverseMembrane(
+    kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy);
+// Convenience templates which input a capability server type and return the appropriate client
+// type.
+
+template <typename Reader>
+Orphan<typename kj::Decay<Reader>::Reads> copyIntoMembrane(
+    Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy);
+// Copy a Cap'n Proto object (e.g. struct or list), adding the given membrane to any capabilities
+// found within it. `from` is interpreted as "outside" the membrane while `to` is "inside".
+
+template <typename Reader>
+Orphan<typename kj::Decay<Reader>::Reads> copyOutOfMembrane(
+    Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy);
+// Like copyIntoMembrane() except that `from` is "inside" the membrane and `to` is "outside".
+
+// =======================================================================================
+// inline implementation details
+
+template <typename ClientType>
+ClientType membrane(ClientType inner, kj::Own<MembranePolicy> policy) {
+  return membrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
+      .castAs<typename ClientType::Calls>();
+}
+template <typename ClientType>
+ClientType reverseMembrane(ClientType inner, kj::Own<MembranePolicy> policy) {
+  return reverseMembrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
+      .castAs<typename ClientType::Calls>();
+}
+
+template <typename ServerType>
+typename ServerType::Serves::Client membrane(
+    kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy) {
+  return membrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
+      .castAs<typename ServerType::Serves::Client>();
+}
+template <typename ServerType>
+typename ServerType::Serves::Client reverseMembrane(
+    kj::Own<ServerType> inner, kj::Own<MembranePolicy> policy) {
+  return reverseMembrane(Capability::Client(kj::mv(inner)), kj::mv(policy))
+      .castAs<typename ServerType::Serves::Client>();
+}
+
+namespace _ {  // private
+
+OrphanBuilder copyOutOfMembrane(PointerReader from, Orphanage to,
+                                kj::Own<MembranePolicy> policy, bool reverse);
+OrphanBuilder copyOutOfMembrane(StructReader from, Orphanage to,
+                                kj::Own<MembranePolicy> policy, bool reverse);
+OrphanBuilder copyOutOfMembrane(ListReader from, Orphanage to,
+                                kj::Own<MembranePolicy> policy, bool reverse);
+
+}  // namespace _ (private)
+
+template <typename Reader>
+Orphan<typename kj::Decay<Reader>::Reads> copyIntoMembrane(
+    Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy) {
+  return _::copyOutOfMembrane(
+      _::PointerHelpers<typename kj::Decay<Reader>::Reads>::getInternalReader(from),
+      to, kj::mv(policy), true);
+}
+
+template <typename Reader>
+Orphan<typename kj::Decay<Reader>::Reads> copyOutOfMembrane(
+    Reader&& from, Orphanage to, kj::Own<MembranePolicy> policy) {
+  return _::copyOutOfMembrane(
+      _::PointerHelpers<typename kj::Decay<Reader>::Reads>::getInternalReader(from),
+      to, kj::mv(policy), false);
+}
+
+} // namespace capnp
+
+#endif // CAPNP_MEMBRANE_H_