Make SharedMemoryHandle a class on Mac.

base/memory/shared_memory_handle.h

Index: base/memory/shared_memory_handle.h
diff --git a/base/memory/shared_memory_handle.h b/base/memory/shared_memory_handle.h
new file mode 100644
index 0000000000000000000000000000000000000000..b9336afa688dea6e99532f974aef29e30f83f030
--- /dev/null
+++ b/base/memory/shared_memory_handle.h
@@ -0,0 +1,90 @@
+// Copyright 2015 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 BASE_MEMORY_SHARED_MEMORY_HANDLE_H_
+#define BASE_MEMORY_SHARED_MEMORY_HANDLE_H_
+
+#include "build/build_config.h"
+
+#if defined(OS_WIN)
+#include <windows.h>
+#elif defined(OS_MACOSX) && !defined(OS_IOS)
+#include <sys/types.h>
+#include "base/base_export.h"
+#include "base/file_descriptor_posix.h"
+#include "base/macros.h"
+#elif defined(OS_POSIX)
+#include <sys/types.h>
+#include "base/file_descriptor_posix.h"
+#endif
+
+namespace base {
+
+class Pickle;
+
+// SharedMemoryHandle is a platform specific type which represents
+// the underlying OS handle to a shared memory segment.
+#if defined(OS_WIN)
+typedef HANDLE SharedMemoryHandle;
+#elif defined(OS_MACOSX) && !defined(OS_IOS)
+
+class BASE_EXPORT SharedMemoryHandle {
+ public:
+  enum Type {
+    // Indicates that the SharedMemoryHandle is backed by a POSIX fd.
+    POSIX,
+    // Indicates that the SharedMemoryHandle is backed by the Mach primitive
+    // "memory object".
+    MACH,
+  };
+
+  // The default constructor returns an invalid SharedMemoryHandle.
+  SharedMemoryHandle();
+
+  // Constructs a SharedMemoryHandle backed by |file_descriptor|.

[Tom Sepez, 2015/06/17 18:51:55]

nit: comment about who owns the FD.  Must the caller close it themselves?  How
long must it remain valid? etc.

[erikchen, 2015/06/18 17:58:33]

I've added a long comment indicating the ownership semantics. The ownership
semantics are copied from base::FileDescriptor (by necessity). Unfortunately,
those aren't well defined because base::FileDescriptor is a struct and code that
uses it frequently passes it around by assignment/copy rather than by reference.

+  explicit SharedMemoryHandle(const FileDescriptor& file_descriptor);
+
+  // Constructs a SharedMemoryHandle backed by the components of a
+  // FileDescriptor.
+  SharedMemoryHandle(int fd, bool auto_close);
+
+  // Standard copy constructor. The new instance shares the underlying OS
+  // primitives.
+  SharedMemoryHandle(const SharedMemoryHandle& handle);
+
+  // Standard assignment operator. The updated instance shares the underlying
+  // OS primitives.
+  SharedMemoryHandle& operator=(const SharedMemoryHandle& handle);
+
+  // Duplicates the underlying OS resources.
+  SharedMemoryHandle Duplicate() const;
+
+  // Comparison operators.
+  bool operator==(const SharedMemoryHandle& handle) const;
+  bool operator!=(const SharedMemoryHandle& handle) const;
+
+  // Getter and setter for a property.
+  Type GetType() const;
+  void SetType(Type type);

[Tom Sepez, 2015/06/17 18:51:55]

Can the type change after the SMH is created?  I'd assume this is set to one or
the other by ctors?

[erikchen, 2015/06/18 17:58:33]

Yes, since code that uses base::SharedMemoryHandles passes it by assignment.
That being said, there's no reason for this functionality to be exposed. I've
removed the method.

+
+  // Sets the POSIX fd backing the SharedMemoryHandle. Requires that the
+  // SharedMemoryHandle be backed by a POSIX fd.
+  void SetFileHandle(int fd, bool auto_close);
+
+  // Returns the address of the POSIX FileDescriptor backing the
+  // SharedMemoryHandle.
+  FileDescriptor* GetFileDescriptor();
+  const FileDescriptor* GetFileDescriptor() const;
+
+ private:
+  Type type_;

[Tom Sepez, 2015/06/17 18:51:55]

const Type type_; per above.

[erikchen, 2015/06/18 17:58:33]

That won't work, since SharedMemoryHandles are passed by assignment and copy.
(For example, your suggestion for not exposing the internals of
SharedMemoryHandle in ipc_message_utils is good, but itself relies on being able
to assign (and change the type_) of SharedMemoryHandle).

+  FileDescriptor file_descriptor_;
+};
+#elif defined(OS_POSIX)

[Tom Sepez, 2015/06/17 18:51:55]

nit: I might re-arrange this so that win/posix cases come first since they are
small, then this one last.  That way, when I hit this line, I don't have to
remember. "Oh yeah, there's still another possibility". No matter.

[erikchen, 2015/06/18 17:58:33]

Good suggestion, done.

+typedef FileDescriptor SharedMemoryHandle;
+#endif
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_SHARED_MEMORY_HANDLE_H_