Make selfupdate maintain client version as well.

cipd/client/cipd/client.go

Index: cipd/client/cipd/client.go
diff --git a/cipd/client/cipd/client.go b/cipd/client/cipd/client.go
index 008ddaf1568a9e90ebc74836424264a7340dad84..f1658a2fc4df52739b3d6919a2d484f488d71a95 100644
--- a/cipd/client/cipd/client.go
+++ b/cipd/client/cipd/client.go
@@ -32,6 +32,7 @@ package cipd
 
 import (
 	"bufio"
+	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
@@ -52,6 +53,7 @@ import (
 	"github.com/luci/luci-go/cipd/client/cipd/common"
 	"github.com/luci/luci-go/cipd/client/cipd/internal"
 	"github.com/luci/luci-go/cipd/client/cipd/local"
+	"github.com/luci/luci-go/cipd/version"
 )
 
 // PackageACLChangeAction defines a flavor of PackageACLChange.
@@ -719,6 +721,32 @@ func init() {
 	clientPackage = fmt.Sprintf("%s/%s-%s", clientPackageBase, platform, arch)
 }
 
+func (client *clientImpl) ensureClientVersionInfo(ctx context.Context, fs local.FileSystem, pin common.Pin, exePath string) {
+	verFile := version.GetVersionFile(exePath)
+	vfStat, err := os.Stat(verFile)

[Vadim Sh., 2016/11/27 01:25:55]

I'd remove both of these checks and just read the file and compare its content
to what it should be, it is tiny. I don't like reliance on ModTime.

[iannucci, 2016/11/29 01:46:20]

Done.

+	if err == nil {
+		eStat, err := os.Stat(exePath)
+		if err == nil {
+			if vfStat.ModTime().After(eStat.ModTime()) {
+				// version file already up to date
+				return
+			}
+		}
+	}
+	// there was an error statting one or both files, or the version file is older
+	// than the exe. Proceed with EnsureFile.
+
+	err = fs.EnsureFile(ctx, verFile, func(of *os.File) error {

[Vadim Sh., 2016/11/27 01:25:55]

I forgot, does it create a full path? i.e. if ".versions" directory is missing?

[iannucci, 2016/11/29 01:46:20]

Yes :)

+		return json.NewEncoder(of).Encode(version.Info{
+			PackageName: pin.PackageName,
+			InstanceID:  pin.InstanceID,
+		})
+	})
+	if err != nil {
+		logging.Warningf(ctx, "Unable to update version info %q: %s", verFile, err)
+	}
+}
+
 func (client *clientImpl) MaybeUpdateClient(ctx context.Context, fs local.FileSystem, targetVersion, currentHash, destination string) error {
 	if err := common.ValidateFileHash(currentHash); err != nil {
 		return err
@@ -741,7 +769,8 @@ func (client *clientImpl) MaybeUpdateClient(ctx context.Context, fs local.FileSy
 		}
 	}
 	if exeHash == currentHash {
-		// already up-to-date
+		// already up-to-date. Make sure version file is up to date.
+		client.ensureClientVersionInfo(ctx, fs, pin, destination)
 		return nil
 	}
 
@@ -758,11 +787,18 @@ func (client *clientImpl) MaybeUpdateClient(ctx context.Context, fs local.FileSy
 	}
 	client.doBatchAwareOp(ctx, batchAwareOpSaveTagCache)
 	if info.clientBinary.SHA1 == currentHash {
-		// already up-to-date, but the cache didn't know that.
+		// already up-to-date, but the cache didn't know that. Make sure version
+		// file is update.
+		client.ensureClientVersionInfo(ctx, fs, pin, destination)
 		return nil
 	}
 
-	return client.installClient(ctx, fs, info.clientBinary.FetchURL, destination)
+	if err := client.installClient(ctx, fs, info.clientBinary.FetchURL, destination); err != nil {
+		return err
+	}
+
+	client.ensureClientVersionInfo(ctx, fs, pin, destination)
+	return nil
 }
 
 func (client *clientImpl) RegisterInstance(ctx context.Context, instance local.PackageInstance, timeout time.Duration) error {