Fix /etc/hostname backward compatibility issue for in-place upgrade.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/server/container_create.go

@@ -483,11 +483,19 @@ func (c *criService) generateContainerMounts(sandboxID string, config *runtime.C
 	var mounts []*runtime.Mount
 	securityContext := config.GetLinux().GetSecurityContext()
 	if !isInCRIMounts(etcHostname, config.GetMounts()) {
-		mounts = append(mounts, &runtime.Mount{
-			ContainerPath: etcHostname,
-			HostPath:      c.getSandboxHostname(sandboxID),
-			Readonly:      securityContext.GetReadonlyRootfs(),
-		})
+		// /etc/hostname is added since 1.1.6, 1.2.4 and 1.3.
+		// For in-place upgrade, the old sandbox doesn't have the hostname file,
+		// do not mount this in that case.
+		// TODO(random-liu): Remove the check and always mount this when
+		// containerd 1.1 and 1.2 are deprecated.
+		hostpath := c.getSandboxHostname(sandboxID)
+		if _, err := c.os.Stat(hostpath); err == nil {
+			mounts = append(mounts, &runtime.Mount{
+				ContainerPath: etcHostname,
+				HostPath:      hostpath,
+				Readonly:      securityContext.GetReadonlyRootfs(),
+			})
+		}
 	}
 
 	if !isInCRIMounts(etcHosts, config.GetMounts()) {