Update //third_party/protobuf to version 3.

third_party/protobuf/protoc-artifacts/README.md

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+# Build scripts that publish pre-compiled protoc artifacts
+``protoc`` is the compiler for ``.proto`` files. It generates language bindings
+for the messages and/or RPC services from ``.proto`` files.
+
+Because ``protoc`` is a native executable, the scripts under this directory
+build and publish a ``protoc`` executable (a.k.a. artifact) to Maven
+repositories. The artifact can be used by build automation tools so that users
+would not need to compile and install ``protoc`` for their systems.
+
+## Versioning
+The version of the ``protoc`` artifact must be the same as the version of the
+Protobuf project.
+
+## Artifact name
+The name of a published ``protoc`` artifact is in the following format:
+``protoc-<version>-<os>-<arch>.exe``, e.g., ``protoc-3.0.0-alpha-3-windows-x86_64.exe``.
+
+## System requirement
+Install [Apache Maven](http://maven.apache.org/) if you don't have it.
+
+The scripts only work under Unix-like environments, e.g., Linux, MacOSX, and
+Cygwin or MinGW for Windows. Please see ``README.md`` of the Protobuf project
+for how to set up the build environment.
+
+## To install artifacts locally
+The following command will install the ``protoc`` artifact to your local Maven repository.
+```
+$ mvn install
+```
+
+## Cross-compilation
+The Maven script will try to detect the OS and the architecture from Java
+system properties. It's possible to build a protoc binary for an architecture
+that is different from what Java has detected, as long as you have the proper
+compilers installed.
+
+You can override the Maven properties ``os.detected.name`` and
+``os.detected.arch`` to force the script to generate binaries for a specific OS
+and/or architecture. Valid values are defined as the return values of
+``normalizeOs()`` and ``normalizeArch()`` of ``Detector`` from
+[os-maven-plugin](https://github.com/trustin/os-maven-plugin/blob/master/src/main/java/kr/motd/maven/os/Detector.java).
+Frequently used values are:
+- ``os.detected.name``: ``linux``, ``osx``, ``windows``.
+- ``os.detected.arch``: ``x86_32``, ``x86_64``
+
+For example, MingGW32 only ships with 32-bit compilers, but you can still build
+32-bit protoc under 64-bit Windows, with the following command:
+```
+$ mvn install -Dos.detected.arch=x86_32
+```
+
+## To push artifacts to Maven Central
+Before you can upload artifacts to Maven Central repository, make sure you have
+read [this page](http://central.sonatype.org/pages/apache-maven.html) on how to
+configure GPG and Sonatype account.
+
+You need to perform the deployment for every platform that you want to
+suppport. DO NOT close the staging repository until you have done the
+deployment for all platforms. Currently the following platforms are supported:
+- Linux (x86_32 and x86_64)
+- Windows (x86_32 and x86_64) with
+ - Cygwin with MinGW compilers (both x86_32 and x86_64)
+ - MSYS with MinGW32 (x86_32 only)
+- MacOSX (x86_32 and x86_64)
+
+Use the following command to deploy artifacts for the host platform to a
+staging repository.
+```
+$ mvn clean deploy -P release
+```
+It creates a new staging repository. Go to
+https://oss.sonatype.org/#stagingRepositories and find the repository, usually
+in the name like ``comgoogle-123``.
+
+You will want to run this command on a different platform. Remember, in
+subsequent deployments you will need to provide the repository name that you
+have found in the first deployment so that all artifacts go to the same
+repository:
+```
+$ mvn clean deploy -P release -Dstaging.repository=comgoogle-123
+```
+
+A 32-bit artifact can be deployed from a 64-bit host with
+``-Dos.detected.arch=x86_32``
+
+When you have done deployment for all platforms, go to
+https://oss.sonatype.org/#stagingRepositories, verify that the staging
+repository has all the binaries, close and release this repository.
+
+### Tips for deploying on Linux
+We build on Centos 6.6 to provide a good compatibility for not very new
+systems. We have provided a ``Dockerfile`` under this directory to build the
+environment. It has been tested with Docker 1.6.1.
+
+To build a image:
+```
+$ docker build -t protoc-artifacts .
+```
+
+To run the image:
+```
+$ docker run -it --rm=true protoc-artifacts
+```
+
+The Protobuf repository has been cloned into ``/protobuf``.
+
+### Tips for deploying on Windows
+Under Windows the following error may occur: ``gpg: cannot open tty `no tty':
+No such file or directory``. This can be fixed by configuring gpg through an
+active profile in ``.m2\settings.xml`` where also the Sonatype password is
+stored:
+```xml
+<settings>
+  <servers>
+    <server>
+      <id>ossrh</id>
+      <username>[username]</username>
+      <password>[password]</password>
+    </server>
+  </servers>
+  <profiles>
+    <profile>
+      <id>gpg</id>
+      <properties>
+        <gpg.executable>gpg</gpg.executable>
+        <gpg.passphrase>[password]</gpg.passphrase>
+      </properties>
+    </profile>
+  </profiles>
+  <activeProfiles>
+    <activeProfile>gpg</activeProfile>
+  </activeProfiles>
+</settings>
+```
+
+### Tested build environments
+We have succesfully built artifacts on the following environments:
+- Linux x86_32 and x86_64:
+ - Centos 6.6 (within Docker 1.6.1)
+ - Ubuntu 14.04.2 64-bit
+- Windows x86_32: MSYS with ``mingw32-gcc-g++ 4.8.1-4`` on Windows 7 64-bit
+- Windows x86_64: Cygwin64 with ``mingw64-x86_64-gcc-g++ 4.8.3-1`` on Windows 7 64-bit
+- Mac OS X x86_32 and x86_64: Mac OS X 10.9.5