libct: fix shared pidns detection

When someone is using libcontainer to start and kill containers from a
long lived process (i.e. the same process creates and removes the
container), initProcess.wait method is used, which has a kludge to work
around killing containers that do not have their own PID namespace.

The code that checks for own PID namespace is not entirely correct.
To be exact, it does not set sharePidns flag when the host/caller PID
namespace is implicitly used. As a result, the above mentioned kludge
does not work.

Fix the issue, add a test case (which fails without the fix).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/configs/namespaces_syscall.go

@@ -31,3 +31,15 @@ func (n *Namespaces) CloneFlags() uintptr {
 	}
 	return uintptr(flag)
 }
+
+// IsPrivate tells whether the namespace of type t is configured as private
+// (i.e. it exists and is not shared).
+func (n Namespaces) IsPrivate(t NamespaceType) bool {
+	for _, v := range n {
+		if v.Type == t {
+			return v.Path == ""
+		}
+	}
+	// Not found, so implicitly sharing a parent namespace.
+	return false
+}

libcontainer/container_linux.go

@@ -549,7 +549,6 @@ func (c *Container) newInitProcess(p *Process, cmd *exec.Cmd, messageSockPair, l
 			nsMaps[ns.Type] = ns.Path
 		}
 	}
-	_, sharePidns := nsMaps[configs.NEWPID]
 	data, err := c.bootstrapData(c.config.Namespaces.CloneFlags(), nsMaps, initStandard)
 	if err != nil {
 		return nil, err
@@ -594,7 +593,7 @@ func (c *Container) newInitProcess(p *Process, cmd *exec.Cmd, messageSockPair, l
 		container:       c,
 		process:         p,
 		bootstrapData:   data,
-		sharePidns:      sharePidns,
+		sharePidns:      !c.config.Namespaces.IsPrivate(configs.NEWPID),
 	}
 	c.initProcess = init
 	return init, nil

libcontainer/integration/exec_test.go

@@ -1355,16 +1355,26 @@ func TestPIDHost(t *testing.T) {
 	}
 }
 
-func TestPIDHostInitProcessWait(t *testing.T) {
+func TestHostPidnsInitKill(t *testing.T) {
+	config := newTemplateConfig(t, nil)
+	// Implicitly use host pid ns.
+	config.Namespaces.Remove(configs.NEWPID)
+	testPidnsInitKill(t, config)
+}
+
+func TestSharedPidnsInitKill(t *testing.T) {
+	config := newTemplateConfig(t, nil)
+	// Explicitly use host pid ns.
+	config.Namespaces.Add(configs.NEWPID, "/proc/1/ns/pid")
+	testPidnsInitKill(t, config)
+}
+
+func testPidnsInitKill(t *testing.T, config *configs.Config) {
 	if testing.Short() {
 		return
 	}
 
-	pidns := "/proc/1/ns/pid"
-
 	// Run a container with two long-running processes.
-	config := newTemplateConfig(t, nil)
-	config.Namespaces.Add(configs.NEWPID, pidns)
 	container, err := newContainer(t, config)
 	ok(t, err)
 	defer func() {