Adds versioning for out-of-process storage driver

The registry currently only accepts storage driver versions with the
same major version and an equal or lower minor version as its own
current storage driver api version, but this may be changed in the
future if we decide to implement specific version cross-compatibility.

storagedriver/ipc/client.go

@@ -11,6 +11,7 @@ import (
 	"path"
 	"syscall"
 
+	"github.com/docker/docker-registry/storagedriver"
 	"github.com/docker/libchan"
 	"github.com/docker/libchan/spdy"
 )
@@ -22,6 +23,7 @@ type StorageDriverClient struct {
 	socket     *os.File
 	transport  *spdy.Transport
 	sender     libchan.Sender
+	version    storagedriver.Version
 }
 
 // NewDriverClient constructs a new out-of-process storage driver using the driver name and
@@ -65,42 +67,62 @@ func (driver *StorageDriverClient) Start() error {
 	}
 
 	childSocket := os.NewFile(uintptr(fileDescriptors[0]), "childSocket")
-	parentSocket := os.NewFile(uintptr(fileDescriptors[1]), "parentSocket")
+	driver.socket = os.NewFile(uintptr(fileDescriptors[1]), "parentSocket")
 
 	driver.subprocess.Stdout = os.Stdout
 	driver.subprocess.Stderr = os.Stderr
 	driver.subprocess.ExtraFiles = []*os.File{childSocket}
 
 	if err = driver.subprocess.Start(); err != nil {
-		parentSocket.Close()
+		driver.Stop()
 		return err
 	}
 
 	if err = childSocket.Close(); err != nil {
-		parentSocket.Close()
+		driver.Stop()
 		return err
 	}
 
-	connection, err := net.FileConn(parentSocket)
+	connection, err := net.FileConn(driver.socket)
 	if err != nil {
-		parentSocket.Close()
+		driver.Stop()
 		return err
 	}
-	transport, err := spdy.NewClientTransport(connection)
+	driver.transport, err = spdy.NewClientTransport(connection)
 	if err != nil {
-		parentSocket.Close()
+		driver.Stop()
 		return err
 	}
-	sender, err := transport.NewSendChannel()
+	driver.sender, err = driver.transport.NewSendChannel()
 	if err != nil {
-		transport.Close()
-		parentSocket.Close()
+		driver.Stop()
 		return err
 	}
 
-	driver.socket = parentSocket
-	driver.transport = transport
-	driver.sender = sender
+	// Check the driver's version to determine compatibility
+	receiver, remoteSender := libchan.Pipe()
+	err = driver.sender.Send(&Request{Type: "Version", ResponseChannel: remoteSender})
+	if err != nil {
+		driver.Stop()
+		return err
+	}
+
+	var response VersionResponse
+	err = receiver.Receive(&response)
+	if err != nil {
+		driver.Stop()
+		return err
+	}
+
+	if response.Error != nil {
+		return response.Error
+	}
+
+	driver.version = response.Version
+
+	if driver.version.Major() != storagedriver.CurrentVersion.Major() || driver.version.Minor() > storagedriver.CurrentVersion.Minor() {
+		return IncompatibleVersionError{driver.version}
+	}
 
 	return nil
 }
@@ -108,10 +130,20 @@ func (driver *StorageDriverClient) Start() error {
 // Stop stops the child process storage driver
 // storagedriver.StorageDriver methods called after Stop will fail
 func (driver *StorageDriverClient) Stop() error {
-	closeSenderErr := driver.sender.Close()
-	closeTransportErr := driver.transport.Close()
-	closeSocketErr := driver.socket.Close()
-	killErr := driver.subprocess.Process.Kill()
+	var closeSenderErr, closeTransportErr, closeSocketErr, killErr error
+
+	if driver.sender != nil {
+		closeSenderErr = driver.sender.Close()
+	}
+	if driver.transport != nil {
+		closeTransportErr = driver.transport.Close()
+	}
+	if driver.socket != nil {
+		closeSocketErr = driver.socket.Close()
+	}
+	if driver.subprocess != nil {
+		killErr = driver.subprocess.Process.Kill()
+	}
 
 	if closeSenderErr != nil {
 		return closeSenderErr