rootless: use default_rootless_network_cmd config

Make sure we use the config field to know if we should use pasta or
slirp4netns as default.

While at it fix broken code which sets the default at two different
places, also do not set in Validate() as this should not modify the
specgen IMO, so set it directly before that.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

test/e2e/containers_conf_test.go

@@ -657,4 +657,35 @@ var _ = Describe("Verify podman containers.conf usage", func() {
 		Expect(result).Should(Exit(0))
 		Expect(result.OutputToString()).To(ContainSubstring("Path to the OCI-compatible binary used to run containers. (default \"testruntime\")"))
 	})
+
+	It("podman default_rootless_network_cmd", func() {
+		SkipIfNotRootless("default_rootless_network_cmd is only used rootless")
+
+		for _, mode := range []string{"pasta", "slirp4netns", "invalid"} {
+			conffile := filepath.Join(podmanTest.TempDir, "container.conf")
+			content := "[network]\ndefault_rootless_network_cmd=\"" + mode + "\"\n"
+			err := os.WriteFile(conffile, []byte(content), 0755)
+			Expect(err).ToNot(HaveOccurred())
+
+			os.Setenv("CONTAINERS_CONF_OVERRIDE", conffile)
+			if IsRemote() {
+				podmanTest.RestartRemoteService()
+			}
+
+			podman := podmanTest.Podman([]string{"create", "--name", mode, ALPINE, "ip", "addr"})
+			podman.WaitWithDefaultTimeout()
+
+			if mode == "invalid" {
+				Expect(podman).Should(Exit(125))
+				Expect(podman.ErrorToString()).Should(ContainSubstring("invalid default_rootless_network_cmd option \"invalid\""))
+				continue
+			}
+			Expect(podman).Should(Exit(0))
+
+			inspect := podmanTest.Podman([]string{"inspect", "--format", "{{.HostConfig.NetworkMode}}", mode})
+			inspect.WaitWithDefaultTimeout()
+			Expect(inspect).Should(Exit(0))
+			Expect(inspect.OutputToString()).Should(Equal(mode))
+		}
+	})
 })