Add additional ldflags constants for Kubernetes versioning

Now it is possible to push these variables through ldflags:
- gitMajor
- gitMinor
- gitVersion (from latest annotated tag, output of git describe)
- gitCommit (renamed commitFromGit, intended to have the full sha1)
- gitTreeState (either "clean" or "dirty")

These are spawned into its separate source file, since they are meant to
be updated separately when a new version is released.

Also use the notation vX.Y+ for when git information is not present.
(This is consistent with the kernel build, e.g. Linux 3.15+ means its
version is >= 3.15 and < 3.16.)

v2: Added comments to the individual fields in pkg/version/base.go

Tested:
- Built it and checked the -version output:
  $ hack/build-go.sh
  $ output/go/bin/kubelet -version
  Kubernetes version 0.1+, build c328679ef8aa

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

hack/build-go.sh

@@ -45,4 +45,4 @@ done
 # our cluster deploy.  If we add more command line options to our standard build
 # we'll want to duplicate them there.  As we move to distributing pre- built
 # binaries we can eliminate this duplication.
-go install -ldflags "-X github.com/GoogleCloudPlatform/kubernetes/pkg/version.commitFromGit '${version}'" "${binaries[@]}"
+go install -ldflags "-X github.com/GoogleCloudPlatform/kubernetes/pkg/version.gitCommit '${version}'" "${binaries[@]}"

pkg/version/base.go

@@ -0,0 +1,41 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package version
+
+// Base version information.
+//
+// This is the fallback data used when version information from git is not
+// provided via go ldflags. It provides an approximation of the Kubernetes
+// version for ad-hoc builds (e.g. `go build`) that cannot get the version
+// information from git.
+//
+// The "+" in the version info indicates that fact, and it means the current
+// build is from a version greater or equal to that version.
+// (e.g. v0.7+ means version >= 0.7 and < 0.8)
+//
+// When releasing a new Kubernetes version, this file should be updated to
+// reflect the new version, and then a git annotated tag (using format vX.Y
+// where X == Major version and Y == Minor version) should be created to point
+// to the commit that updates pkg/version/base.go
+
+var (
+	gitMajor     string = "0"              // major version, always numeric
+	gitMinor     string = "1+"             // minor version, numeric possibly followed by "+"
+	gitVersion   string = "v0.1+"          // version from git, output of $(git describe)
+	gitCommit    string = ""               // sha1 from git, output of $(git rev-parse HEAD)
+	gitTreeState string = "not a git tree" // state of git tree, either "clean" or "dirty"
+)

pkg/version/version.go

@@ -20,26 +20,28 @@ import (
 	"fmt"
 )
 
-// commitFromGit is a constant representing the source version that
-// generated this build. It should be set during build via -ldflags.
-var commitFromGit string
-
 // Info contains versioning information.
 // TODO: Add []string of api versions supported? It's still unclear
 // how we'll want to distribute that information.
 type Info struct {
-	Major     string `json:"major" yaml:"major"`
+	Major        string `json:"major" yaml:"major"`
-	Minor     string `json:"minor" yaml:"minor"`
+	Minor        string `json:"minor" yaml:"minor"`
-	GitCommit string `json:"gitCommit" yaml:"gitCommit"`
+	GitVersion   string `json:"gitVersion" yaml:"gitVersion"`
+	GitCommit    string `json:"gitCommit" yaml:"gitCommit"`
+	GitTreeState string `json:"gitTreeState" yaml:"gitTreeState"`
 }
 
 // Get returns the overall codebase version. It's for detecting
 // what code a binary was built from.
 func Get() Info {
+	// These variables typically come from -ldflags settings and in
+	// their absence fallback to the settings in pkg/version/base.go
 	return Info{
-		Major:     "0",
+		Major:        gitMajor,
-		Minor:     "1",
+		Minor:        gitMinor,
-		GitCommit: commitFromGit,
+		GitVersion:   gitVersion,
+		GitCommit:    gitCommit,
+		GitTreeState: gitTreeState,
 	}
 }