Have daemon/logger.Context carry cgroup info

Make logger.Context into an object that also carries the cgroup path
of the container for which it's logging, and initialize the field when
we start a container.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Author: nalind

container/container.go

@@ -315,7 +315,7 @@ func (container *Container) StartLogger(cfg containertypes.LogConfig) (logger.Lo
 	if err != nil {
 		return nil, fmt.Errorf("Failed to get logging factory: %v", err)
 	}
-	ctx := logger.Context{
+	cctx := logger.CommonContext{
 		Config:              cfg.Config,
 		ContainerID:         container.ID,
 		ContainerName:       container.Name,
@@ -328,6 +328,7 @@ func (container *Container) StartLogger(cfg containertypes.LogConfig) (logger.Lo
 		ContainerLabels:     container.Config.Labels,
 		DaemonName:          "docker",
 	}
+	ctx := configurePlatformLogger(cctx, container)
 
 	// Set logging file for "json-logger"
 	if cfg.Type == jsonfilelog.Name {

container/logger_linux.go

@@ -0,0 +1,14 @@
+package container
+
+import (
+	"github.com/docker/docker/daemon/logger"
+)
+
+// configurePlatformLogger takes a logger.CommonContext and adds any
+// OS-specific information that is exclusive to a logger.Context.
+func configurePlatformLogger(ctx logger.CommonContext, container *Container) logger.Context {
+	return logger.Context{
+		CommonContext:   ctx,
+		ContainerCGroup: *container.Spec.Linux.CgroupsPath,
+	}
+}

container/logger_notlinux.go

@@ -0,0 +1,9 @@
+// +build !linux
+
+package container
+
+// configurePlatformLogger takes a logger.CommonContext and adds any
+// OS-specific information that is exclusive to a logger.Context.
+func configurePlatformLogger(ctx logger.CommonContext, container *Container) logger.Context {
+	return logger.Context(ctx)
+}

daemon/logger/context.go

@@ -7,8 +7,9 @@ import (
 	"time"
 )
 
-// Context provides enough information for a logging driver to do its function.
-type Context struct {
+// CommonContext provides almost enough information for a logging driver to do
+// its function, but not anything that's OS-specific.
+type CommonContext struct {
 	Config              map[string]string
 	ContainerID         string
 	ContainerName       string

daemon/logger/context_linux.go

@@ -0,0 +1,14 @@
+package logger
+
+// Context provides enough information for a logging driver to do its function.
+type Context struct {
+	// These fields are shared across all definitions.
+	CommonContext
+	// Fields below this point are platform-specific.
+	ContainerCGroup string
+}
+
+// CGroup returns the name of the container's cgroup.
+func (ctx *Context) CGroup() (string, error) {
+	return ctx.ContainerCGroup, nil
+}

daemon/logger/context_notlinux.go

@@ -0,0 +1,6 @@
+// +build !linux
+
+package logger
+
+// Context provides enough information for a logging driver to do its function.
+type Context CommonContext