Mojo: Split public/cpp/system/core.h into pieces

mojo/public/cpp/system/handle.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_PUBLIC_CPP_SYSTEM_HANDLE_H_
+#define MOJO_PUBLIC_CPP_SYSTEM_HANDLE_H_
+
+#include <assert.h>
+#include <limits>
+
+#include "mojo/public/c/system/functions.h"
+#include "mojo/public/c/system/types.h"
+#include "mojo/public/cpp/system/macros.h"
+
+namespace mojo {
+
+// OVERVIEW
+//
+// |Handle| and |...Handle|:
+//
+// |Handle| is a simple, copyable wrapper for the C type |MojoHandle| (which is
+// just an integer). Its purpose is to increase type-safety, not provide
+// lifetime management. For the same purpose, we have trivial *subclasses* of
+// |Handle|, e.g., |MessagePipeHandle| and |DataPipeProducerHandle|. |Handle|
+// and its subclasses impose *no* extra overhead over using |MojoHandle|s
+// directly.
+//
+// Note that though we provide constructors for |Handle|/|...Handle| from a
+// |MojoHandle|, we do not provide, e.g., a constructor for |MessagePipeHandle|
+// from a |Handle|. This is for type safety: If we did, you'd then be able to
+// construct a |MessagePipeHandle| from, e.g., a |DataPipeProducerHandle| (since
+// it's a |Handle|).
+//
+// |ScopedHandleBase| and |Scoped...Handle|:
+//
+// |ScopedHandleBase<HandleType>| is a templated scoped wrapper, for the handle
+// types above (in the same sense that a C++11 |unique_ptr<T>| is a scoped
+// wrapper for a |T*|). It provides lifetime management, closing its owned
+// handle on destruction. It also provides (emulated) move semantics, again
+// along the lines of C++11's |unique_ptr| (and exactly like Chromium's
+// |scoped_ptr|).
+//
+// |ScopedHandle| is just (a typedef of) a |ScopedHandleBase<Handle>|.
+// Similarly, |ScopedMessagePipeHandle| is just a
+// |ScopedHandleBase<MessagePipeHandle>|. Etc. Note that a
+// |ScopedMessagePipeHandle| is *not* a (subclass of) |ScopedHandle|.
+//
+// Wrapper functions:
+//
+// We provide simple wrappers for the |Mojo...()| functions (in
+// mojo/public/c/system/core.h -- see that file for details on individual
+// functions).
+//
+// The general guideline is functions that imply ownership transfer of a handle
+// should take (or produce) an appropriate |Scoped...Handle|, while those that
+// don't take a |...Handle|. For example, |CreateMessagePipe()| has two
+// |ScopedMessagePipe| "out" parameters, whereas |Wait()| and |WaitMany()| take
+// |Handle| parameters. Some, have both: e.g., |DuplicatedBuffer()| takes a
+// suitable (unscoped) handle (e.g., |SharedBufferHandle|) "in" parameter and
+// produces a suitable scoped handle (e.g., |ScopedSharedBufferHandle| a.k.a.
+// |ScopedHandleBase<SharedBufferHandle>|) as an "out" parameter.
+//
+// An exception are some of the |...Raw()| functions. E.g., |CloseRaw()| takes a
+// |Handle|, leaving the user to discard the handle.
+//
+// More significantly, |WriteMessageRaw()| exposes the full API complexity of
+// |MojoWriteMessage()| (but doesn't require any extra overhead). It takes a raw
+// array of |Handle|s as input, and takes ownership of them (i.e., invalidates
+// them) on *success* (but not on failure). There are a number of reasons for
+// this. First, C++03 |std::vector|s cannot contain the move-only
+// |Scoped...Handle|s. Second, |std::vector|s impose extra overhead
+// (necessitating heap-allocation of the buffer). Third, |std::vector|s wouldn't
+// provide the desired level of flexibility/safety: a vector of handles would
+// have to be all of the same type (probably |Handle|/|ScopedHandle|). Fourth,
+// it's expected to not be used directly, but instead be used by generated
+// bindings.
+//
+// Other |...Raw()| functions expose similar rough edges, e.g., dealing with raw
+// pointers (and lengths) instead of taking |std::vector|s or similar.
+
+// ScopedHandleBase ------------------------------------------------------------
+
+// Scoper for the actual handle types defined further below. It's move-only,
+// like the C++11 |unique_ptr|.
+template <class HandleType>
+class ScopedHandleBase {
+  MOJO_MOVE_ONLY_TYPE_FOR_CPP_03(ScopedHandleBase, RValue)
+
+ public:
+  ScopedHandleBase() {}
+  explicit ScopedHandleBase(HandleType handle) : handle_(handle) {}
+  ~ScopedHandleBase() { CloseIfNecessary(); }
+
+  template <class CompatibleHandleType>
+  explicit ScopedHandleBase(ScopedHandleBase<CompatibleHandleType> other)
+      : handle_(other.release()) {
+  }
+
+  // Move-only constructor and operator=.
+  ScopedHandleBase(RValue other) : handle_(other.object->release()) {}
+  ScopedHandleBase& operator=(RValue other) {
+    if (other.object != this) {
+      CloseIfNecessary();
+      handle_ = other.object->release();
+    }
+    return *this;
+  }
+
+  const HandleType& get() const { return handle_; }
+
+  template <typename PassedHandleType>
+  static ScopedHandleBase<HandleType> From(
+      ScopedHandleBase<PassedHandleType> other) {
+    MOJO_COMPILE_ASSERT(
+        sizeof(static_cast<PassedHandleType*>(static_cast<HandleType*>(0))),
+        HandleType_is_not_a_subtype_of_PassedHandleType);
+    return ScopedHandleBase<HandleType>(
+        static_cast<HandleType>(other.release().value()));
+  }
+
+  void swap(ScopedHandleBase& other) {
+    handle_.swap(other.handle_);
+  }
+
+  HandleType release() MOJO_WARN_UNUSED_RESULT {
+    HandleType rv;
+    rv.swap(handle_);
+    return rv;
+  }
+
+  void reset(HandleType handle = HandleType()) {
+    CloseIfNecessary();
+    handle_ = handle;
+  }
+
+  bool is_valid() const {
+    return handle_.is_valid();
+  }
+
+ private:
+  void CloseIfNecessary() {
+    if (!handle_.is_valid())
+      return;
+    MojoResult result MOJO_ALLOW_UNUSED = MojoClose(handle_.value());
+    assert(result == MOJO_RESULT_OK);
+  }
+
+  HandleType handle_;
+};
+
+template <typename HandleType>
+inline ScopedHandleBase<HandleType> MakeScopedHandle(HandleType handle) {
+  return ScopedHandleBase<HandleType>(handle);
+}
+
+// Handle ----------------------------------------------------------------------
+
+const MojoHandle kInvalidHandleValue = MOJO_HANDLE_INVALID;
+
+// Wrapper base class for |MojoHandle|.
+class Handle {
+ public:
+  Handle() : value_(kInvalidHandleValue) {}
+  explicit Handle(MojoHandle value) : value_(value) {}
+  ~Handle() {}
+
+  void swap(Handle& other) {
+    MojoHandle temp = value_;
+    value_ = other.value_;
+    other.value_ = temp;
+  }
+
+  bool is_valid() const {
+    return value_ != kInvalidHandleValue;
+  }
+
+  MojoHandle value() const { return value_; }
+  MojoHandle* mutable_value() { return &value_; }
+  void set_value(MojoHandle value) { value_ = value; }
+
+ private:
+  MojoHandle value_;
+
+  // Copying and assignment allowed.
+};
+
+// Should have zero overhead.
+MOJO_COMPILE_ASSERT(sizeof(Handle) == sizeof(MojoHandle),
+                    bad_size_for_cpp_Handle);
+
+// The scoper should also impose no more overhead.
+typedef ScopedHandleBase<Handle> ScopedHandle;
+MOJO_COMPILE_ASSERT(sizeof(ScopedHandle) == sizeof(Handle),
+                    bad_size_for_cpp_ScopedHandle);
+
+inline MojoResult Wait(const Handle& handle,
+                       MojoHandleSignals signals,
+                       MojoDeadline deadline) {
+  return MojoWait(handle.value(), signals, deadline);
+}
+
+// |HandleVectorType| and |FlagsVectorType| should be similar enough to
+// |std::vector<Handle>| and |std::vector<MojoHandleSignals>|, respectively:
+//  - They should have a (const) |size()| method that returns an unsigned type.
+//  - They must provide contiguous storage, with access via (const) reference to
+//    that storage provided by a (const) |operator[]()| (by reference).
+template <class HandleVectorType, class FlagsVectorType>
+inline MojoResult WaitMany(const HandleVectorType& handles,
+                           const FlagsVectorType& signals,
+                           MojoDeadline deadline) {
+  if (signals.size() != handles.size())
+    return MOJO_RESULT_INVALID_ARGUMENT;
+  if (handles.size() > std::numeric_limits<uint32_t>::max())
+    return MOJO_RESULT_OUT_OF_RANGE;
+
+  if (handles.size() == 0)
+    return MojoWaitMany(NULL, NULL, 0, deadline);
+
+  const Handle& first_handle = handles[0];
+  const MojoHandleSignals& first_signals = signals[0];
+  return MojoWaitMany(
+      reinterpret_cast<const MojoHandle*>(&first_handle),
+      reinterpret_cast<const MojoHandleSignals*>(&first_signals),
+      static_cast<uint32_t>(handles.size()),
+      deadline);
+}
+
+// |Close()| takes ownership of the handle, since it'll invalidate it.
+// Note: There's nothing to do, since the argument will be destroyed when it
+// goes out of scope.
+template <class HandleType>
+inline void Close(ScopedHandleBase<HandleType> /*handle*/) {}
+
+// Most users should typically use |Close()| (above) instead.
+inline MojoResult CloseRaw(Handle handle) {
+  return MojoClose(handle.value());
+}
+
+// Strict weak ordering, so that |Handle|s can be used as keys in |std::map|s,
+// etc.
+inline bool operator<(const Handle& a, const Handle& b) {
+  return a.value() < b.value();
+}
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_SYSTEM_HANDLE_H_