Fix problem copying files when container is in host pid namespace

When attempting to copy files into and out of running containers
within the host pidnamespace, the code was attempting to join the
host pidns again, and getting an error. This was causing the podman
cp command to fail. Since we are already in the host pid namespace,
we should not be attempting to join.  This PR adds a check to see if
the container is in NOT host pid namespace, and only then attempts to
join.

Fixes: https://github.com/containers/podman/issues/9985

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

test/e2e/cp_test.go

@@ -91,6 +91,50 @@ var _ = Describe("Podman cp", func() {
 		Expect(roundtripContent).To(Equal(originalContent))
 	})
 
+	// Copy a file to the container, then back to the host in --pid=host
+	It("podman cp --pid=host file", func() {
+		SkipIfRootlessCgroupsV1("Not supported for rootless + CGroupsV1")
+		srcFile, err := ioutil.TempFile("", "")
+		Expect(err).To(BeNil())
+		defer srcFile.Close()
+		defer os.Remove(srcFile.Name())
+
+		originalContent := []byte("podman cp file test")
+		err = ioutil.WriteFile(srcFile.Name(), originalContent, 0644)
+		Expect(err).To(BeNil())
+
+		// Create a container. NOTE that container mustn't be running for copying.
+		session := podmanTest.Podman([]string{"create", "--pid=host", ALPINE, "top"})
+		session.WaitWithDefaultTimeout()
+		Expect(session.ExitCode()).To(Equal(0))
+		name := session.OutputToString()
+
+		session = podmanTest.Podman([]string{"start", name})
+		session.WaitWithDefaultTimeout()
+		Expect(session.ExitCode()).To(Equal(0))
+
+		// The file will now be created (and written to).
+		session = podmanTest.Podman([]string{"cp", srcFile.Name(), name + ":foo"})
+		session.WaitWithDefaultTimeout()
+		Expect(session.ExitCode()).To(Equal(0))
+
+		// Copy FROM the container.
+
+		destFile, err := ioutil.TempFile("", "")
+		Expect(err).To(BeNil())
+		defer destFile.Close()
+		defer os.Remove(destFile.Name())
+
+		session = podmanTest.Podman([]string{"cp", name + ":foo", destFile.Name()})
+		session.WaitWithDefaultTimeout()
+		Expect(session.ExitCode()).To(Equal(0))
+
+		// Now make sure the content matches.
+		roundtripContent, err := ioutil.ReadFile(destFile.Name())
+		Expect(err).To(BeNil())
+		Expect(roundtripContent).To(Equal(originalContent))
+	})
+
 	// Create a symlink in the container, use it as a copy destination and
 	// make sure that the link and the resolved path are accessible and
 	// give the right content.