Add client functions to allow integrations within other CLIs.

This is a simplification of how the docker engine implements
this feature, but it will be ported there once this is merged.

Signed-off-by: David Calavera <david.calavera@gmail.com>

client/client_test.go

@@ -0,0 +1,192 @@
+package client
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/docker/docker-credential-helpers/credentials"
+)
+
+const (
+	validServerAddress   = "https://index.docker.io/v1"
+	validServerAddress2  = "https://example.com:5002"
+	invalidServerAddress = "https://foobar.example.com"
+	missingCredsAddress  = "https://missing.docker.io/v1"
+)
+
+var errProgramExited = fmt.Errorf("exited 1")
+
+// mockProgram simulates interactions between the docker client and a remote
+// credentials helper.
+// Unit tests inject this mocked command into the remote to control execution.
+type mockProgram struct {
+	arg   string
+	input io.Reader
+}
+
+// Output returns responses from the remote credentials helper.
+// It mocks those responses based in the input in the mock.
+func (m *mockProgram) Output() ([]byte, error) {
+	in, err := ioutil.ReadAll(m.input)
+	if err != nil {
+		return nil, err
+	}
+	inS := string(in)
+
+	switch m.arg {
+	case "erase":
+		switch inS {
+		case validServerAddress:
+			return nil, nil
+		default:
+			return []byte("program failed"), errProgramExited
+		}
+	case "get":
+		switch inS {
+		case validServerAddress:
+			return []byte(`{"Username": "foo", "Secret": "bar"}`), nil
+		case validServerAddress2:
+			return []byte(`{"Username": "<token>", "Secret": "abcd1234"}`), nil
+		case missingCredsAddress:
+			return []byte(credentials.NewErrCredentialsNotFound().Error()), errProgramExited
+		case invalidServerAddress:
+			return []byte("program failed"), errProgramExited
+		}
+	case "store":
+		var c credentials.Credentials
+		err := json.NewDecoder(strings.NewReader(inS)).Decode(&c)
+		if err != nil {
+			return []byte("error storing credentials"), errProgramExited
+		}
+		switch c.ServerURL {
+		case validServerAddress:
+			return nil, nil
+		case validServerAddress2:
+			return nil, nil
+		default:
+			return []byte("error storing credentials"), errProgramExited
+		}
+	}
+
+	return []byte(fmt.Sprintf("unknown argument %q with %q", m.arg, inS)), errProgramExited
+}
+
+// Input sets the input to send to a remote credentials helper.
+func (m *mockProgram) Input(in io.Reader) {
+	m.input = in
+}
+
+func mockProgramFn(args ...string) Program {
+	return &mockProgram{
+		arg: args[0],
+	}
+}
+
+func ExampleStore() {
+	p := NewShellProgramFunc("docker-credential-secretservice")
+
+	c := &credentials.Credentials{
+		ServerURL: "https://example.com",
+		Username:  "calavera",
+		Secret:    "my super secret token",
+	}
+
+	if err := Store(p, c); err != nil {
+		fmt.Println(err)
+	}
+}
+
+func TestStore(t *testing.T) {
+	valid := []credentials.Credentials{
+		{validServerAddress, "foo", "bar"},
+		{validServerAddress2, "<token>", "abcd1234"},
+	}
+
+	for _, v := range valid {
+		if err := Store(mockProgramFn, &v); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	invalid := []credentials.Credentials{
+		{invalidServerAddress, "foo", "bar"},
+	}
+
+	for _, v := range invalid {
+		if err := Store(mockProgramFn, &v); err == nil {
+			t.Fatalf("Expected error for server %s, got nil", v.ServerURL)
+		}
+	}
+}
+
+func ExampleGet() {
+	p := NewShellProgramFunc("docker-credential-secretservice")
+
+	creds, err := Get(p, "https://example.com")
+	if err != nil {
+		fmt.Println(err)
+	}
+
+	fmt.Printf("Got credentials for user `%s` in `%s`\n", creds.Username, creds.ServerURL)
+}
+
+func TestGet(t *testing.T) {
+	valid := []credentials.Credentials{
+		{validServerAddress, "foo", "bar"},
+		{validServerAddress2, "<token>", "abcd1234"},
+	}
+
+	for _, v := range valid {
+		c, err := Get(mockProgramFn, v.ServerURL)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if c.Username != v.Username {
+			t.Fatalf("expected username `%s`, got %s", v.Username, c.Username)
+		}
+		if c.Secret != v.Secret {
+			t.Fatalf("expected secret `%s`, got %s", v.Secret, c.Secret)
+		}
+	}
+
+	invalid := []struct {
+		serverURL string
+		err       string
+	}{
+		{missingCredsAddress, credentials.NewErrCredentialsNotFound().Error()},
+		{invalidServerAddress, "error getting credentials - err: exited 1, out: `program failed`"},
+	}
+
+	for _, v := range invalid {
+		_, err := Get(mockProgramFn, v.serverURL)
+		if err == nil {
+			t.Fatalf("Expected error for server %s, got nil", v.serverURL)
+		}
+		if err.Error() != v.err {
+			t.Fatalf("Expected error `%s`, got `%v`", v.err, err)
+		}
+	}
+}
+
+func ExampleErase() {
+	p := NewShellProgramFunc("docker-credential-secretservice")
+
+	if err := Erase(p, "https://example.com"); err != nil {
+		fmt.Println(err)
+	}
+}
+
+func TestErase(t *testing.T) {
+	if err := Erase(mockProgramFn, validServerAddress); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := Erase(mockProgramFn, invalidServerAddress); err == nil {
+		t.Fatalf("Expected error for server %s, got nil", invalidServerAddress)
+	}
+}