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+Deterministic OSX Dmg Notes.
+
+Working OSX DMG's are created in Linux by combining a recent clang,
+the Apple's binutils (ld, ar, etc), and DMG authoring tools.
+
+Apple uses clang extensively for development and has upstreamed the necessary
+functionality so that a vanilla clang can take advantage. It supports the use
+of -F, -target, -mmacosx-version-min, and --sysroot, which are all necessary
+when building for OSX. A pre-compiled version of 3.2 is used because it was not
+available in the Precise repositories at the time this work was started. In the
+future, it can be switched to use system packages instead.
+
+Apple's version of binutils (called cctools) contains lots of functionality
+missing in the FSF's binutils. In addition to extra linker options for
+frameworks and sysroots, several other tools are needed as well such as
+install_name_tool, lipo, and nmedit. These do not build under linux, so they
+have been patched to do so. The work here was used as a starting point:
+https://github.com/mingwandroid/toolchain4
+
+In order to build a working toolchain, the following source packages are needed
+from Apple: cctools, dyld, and ld64.
+
+Beware. This part is ugly. Very very very ugly. In the future, this should be
+broken out into a new repository and cleaned up. Additionally, the binaries
+only work when built as x86 and not x86_64. This is an especially nasty
+limitation because it must be linked with the toolchain's libLTO.so, meaning
+that the entire toolchain must be x86. Gitian x86_64 should not be used until
+this has been fixed, because it would mean that several native dependencies
+(openssl, libuuid, etc) would need to be built as x86 first.
+
+These tools inject timestamps by default, which produce non-deterministic
+binaries. The ZERO_AR_DATE environment variable is used to disable that.
+
+This version of cctools has been patched to use the current version of clang's
+headers and and its libLTO.so rather than those from llvmgcc, as it was
+originally done in toolchain4.
+
+To complicate things further, all builds must target an Apple SDK. These SDKs
+are free to download, but not redistributable.
+To obtain it, register for a developer account, then download xcode4630916281a.dmg:
+https://developer.apple.com/downloads/download.action?path=Developer_Tools/xcode_4.6.3/xcode4630916281a.dmg
+This file is several gigabytes in size, but only a single directory inside is
+needed: Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk
+
+Unfortunately, the usual linux tools (7zip, hpmount, loopback mount) are incapable of opening this file.
+To create a tarball suitable for gitian input, mount the dmg in OSX, then create it with:
+  $ tar -C /Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/ -czf MacOSX10.7.sdk.tar.gz MacOSX10.7.sdk
+
+
+The gitian descriptors build 2 sets of files: Linux tools, then Apple binaries
+which are created using these tools. The build process has been designed to
+avoid including the SDK's files in Gitian's outputs. All interim tarballs are
+fully deterministic and may be freely redistributed.
+
+genisoimage is used to create the initial DMG. It is not deterministic as-is,
+so it has been patched. A system genisoimage will work fine, but it will not
+be deterministic because the file-order will change between invocations.
+The patch can be seen here:
+https://raw.githubusercontent.com/theuni/osx-cross-depends/master/patches/cdrtools/genisoimage.diff
+No effort was made to fix this cleanly, so it likely leaks memory badly. But
+it's only used for a single invocation, so that's no real concern.
+
+genisoimage cannot compress DMGs, so afterwards, the 'dmg' tool from the
+libdmg-hfsplus project is used to compress it. There are several bugs in this
+tool and its maintainer has seemingly abandoned the project. It has been forked
+and is available (with fixes) here: https://github.com/theuni/libdmg-hfsplus .
+
+The 'dmg' tool has the ability to create DMG's from scratch as well, but this
+functionality is broken. Only the compression feature is currently used.
+Ideally, the creation could be fixed and genisoimage would no longer be necessary.
+
+Background images and other features can be added to DMG files by inserting a
+.DS_Store before creation. The easiest way to create this file is to build a
+DMG without one, move it to a device running OSX, customize the layout, then
+grab the .DS_Store file for later use. That is the approach taken here.