ns: fix reading net namespace in multi-threaded processes

/proc/self/ns/net gives the main thread's namespace, not necessarily
the namespace of the thread that's running the testcases.  This causes
sporadic failures of the tests.

For example, with a testcase reading inodes after switching netns:

/proc/27686/task/27689/ns/net 4026532565
/proc/self/ns/net 4026531969
/proc/27686/task/27689/ns/net 4026532565

See also:
008d17ae00

Running Suite: pkg/ns Suite
===========================
Random Seed: 1459953577
Will run 6 of 6 specs

• Failure [0.028 seconds]
Linux namespace operations
/cni/gopath/src/github.com/appc/cni/pkg/ns/ns_test.go:167
  WithNetNS
  /cni/gopath/src/github.com/appc/cni/pkg/ns/ns_test.go:166
    executes the callback within the target network namespace [It]
    /cni/gopath/src/github.com/appc/cni/pkg/ns/ns_test.go:97

    Expected
        <uint64>: 4026531969
    to equal
        <uint64>: 4026532565

    /cni/gopath/src/github.com/appc/cni/pkg/ns/ns_test.go:96
------------------------------
•••••

Summarizing 1 Failure:

[Fail] Linux namespace operations WithNetNS [It] executes the callback within the target network namespace
/cni/gopath/src/github.com/appc/cni/pkg/ns/ns_test.go:96

Ran 6 of 6 Specs in 0.564 seconds
FAIL! -- 5 Passed | 1 Failed | 0 Pending | 0 Skipped --- FAIL: TestNs (0.56s)
FAIL

ns/ns_test.go

@@ -28,13 +28,9 @@ import (
 	. "github.com/onsi/gomega"
 )
 
-const CurrentNetNS = "/proc/self/ns/net"
-
 var _ = Describe("Linux namespace operations", func() {
 	Describe("WithNetNS", func() {
 		var (
-			originalNetNS *os.File
-
 			targetNetNSName string
 			targetNetNSPath string
 			targetNetNS     *os.File
@@ -42,8 +38,6 @@ var _ = Describe("Linux namespace operations", func() {
 
 		BeforeEach(func() {
 			var err error
-			originalNetNS, err = os.Open(CurrentNetNS)
-			Expect(err).NotTo(HaveOccurred())
 
 			targetNetNSName = fmt.Sprintf("test-netns-%d", rand.Int())
 
@@ -60,8 +54,6 @@ var _ = Describe("Linux namespace operations", func() {
 
 			err := exec.Command("ip", "netns", "del", targetNetNSName).Run()
 			Expect(err).NotTo(HaveOccurred())
-
-			Expect(originalNetNS.Close()).To(Succeed())
 		})
 
 		It("executes the callback within the target network namespace", func() {
@@ -71,7 +63,7 @@ var _ = Describe("Linux namespace operations", func() {
 			var actualInode uint64
 			var innerErr error
 			err = ns.WithNetNS(targetNetNS, false, func(*os.File) error {
-				actualInode, innerErr = testhelpers.GetInode(CurrentNetNS)
+				actualInode, innerErr = testhelpers.GetInodeCurNetNS()
 				return nil
 			})
 			Expect(err).NotTo(HaveOccurred())
@@ -81,7 +73,7 @@ var _ = Describe("Linux namespace operations", func() {
 		})
 
 		It("provides the original namespace as the argument to the callback", func() {
-			hostNSInode, err := testhelpers.GetInode(CurrentNetNS)
+			hostNSInode, err := testhelpers.GetInodeCurNetNS()
 			Expect(err).NotTo(HaveOccurred())
 
 			var inputNSInode uint64
@@ -97,7 +89,7 @@ var _ = Describe("Linux namespace operations", func() {
 		})
 
 		It("restores the calling thread to the original network namespace", func() {
-			preTestInode, err := testhelpers.GetInode(CurrentNetNS)
+			preTestInode, err := testhelpers.GetInodeCurNetNS()
 			Expect(err).NotTo(HaveOccurred())
 
 			err = ns.WithNetNS(targetNetNS, false, func(*os.File) error {
@@ -105,7 +97,7 @@ var _ = Describe("Linux namespace operations", func() {
 			})
 			Expect(err).NotTo(HaveOccurred())
 
-			postTestInode, err := testhelpers.GetInode(CurrentNetNS)
+			postTestInode, err := testhelpers.GetInodeCurNetNS()
 			Expect(err).NotTo(HaveOccurred())
 
 			Expect(postTestInode).To(Equal(preTestInode))
@@ -113,14 +105,14 @@ var _ = Describe("Linux namespace operations", func() {
 
 		Context("when the callback returns an error", func() {
 			It("restores the calling thread to the original namespace before returning", func() {
-				preTestInode, err := testhelpers.GetInode(CurrentNetNS)
+				preTestInode, err := testhelpers.GetInodeCurNetNS()
 				Expect(err).NotTo(HaveOccurred())
 
 				_ = ns.WithNetNS(targetNetNS, false, func(*os.File) error {
 					return errors.New("potato")
 				})
 
-				postTestInode, err := testhelpers.GetInode(CurrentNetNS)
+				postTestInode, err := testhelpers.GetInodeCurNetNS()
 				Expect(err).NotTo(HaveOccurred())
 
 				Expect(postTestInode).To(Equal(preTestInode))
@@ -136,7 +128,7 @@ var _ = Describe("Linux namespace operations", func() {
 
 		Describe("validating inode mapping to namespaces", func() {
 			It("checks that different namespaces have different inodes", func() {
-				hostNSInode, err := testhelpers.GetInode(CurrentNetNS)
+				hostNSInode, err := testhelpers.GetInodeCurNetNS()
 				Expect(err).NotTo(HaveOccurred())
 
 				testNsInode, err := testhelpers.GetInode(targetNetNSPath)

testhelpers/testhelpers.go

@@ -27,6 +27,16 @@ import (
 	. "github.com/onsi/gomega"
 )
 
+func getCurrentThreadNetNSPath() string {
+	pid := unix.Getpid()
+	tid := unix.Gettid()
+	return fmt.Sprintf("/proc/%d/task/%d/ns/net", pid, tid)
+}
+
+func GetInodeCurNetNS() (uint64, error) {
+	return GetInode(getCurrentThreadNetNSPath())
+}
+
 func GetInode(path string) (uint64, error) {
 	file, err := os.Open(path)
 	if err != nil {
@@ -68,9 +78,7 @@ func MakeNetworkNS(containerID string) string {
 		defer GinkgoRecover()
 
 		// capture current thread's original netns
-		pid := unix.Getpid()
+		currentThreadNetNSPath := getCurrentThreadNetNSPath()
-		tid := unix.Gettid()
-		currentThreadNetNSPath := fmt.Sprintf("/proc/%d/task/%d/ns/net", pid, tid)
 		originalNetNS, err := unix.Open(currentThreadNetNSPath, unix.O_RDONLY, 0)
 		Expect(err).NotTo(HaveOccurred())
 		defer unix.Close(originalNetNS)