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.go

@@ -0,0 +1,70 @@
+package client
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"github.com/docker/docker-credential-helpers/credentials"
+)
+
+// Store uses an external program to save credentials.
+func Store(program ProgramFunc, credentials *credentials.Credentials) error {
+	cmd := program("store")
+
+	buffer := new(bytes.Buffer)
+	if err := json.NewEncoder(buffer).Encode(credentials); err != nil {
+		return err
+	}
+	cmd.Input(buffer)
+
+	out, err := cmd.Output()
+	if err != nil {
+		t := strings.TrimSpace(string(out))
+		return fmt.Errorf("error storing credentials - err: %v, out: `%s`", err, t)
+	}
+
+	return nil
+}
+
+// Get executes an external program to get the credentials from a native store.
+func Get(program ProgramFunc, serverURL string) (*credentials.Credentials, error) {
+	cmd := program("get")
+	cmd.Input(strings.NewReader(serverURL))
+
+	out, err := cmd.Output()
+	if err != nil {
+		t := strings.TrimSpace(string(out))
+
+		if credentials.IsErrCredentialsNotFoundMessage(t) {
+			return nil, credentials.NewErrCredentialsNotFound()
+		}
+
+		return nil, fmt.Errorf("error getting credentials - err: %v, out: `%s`", err, t)
+	}
+
+	resp := &credentials.Credentials{
+		ServerURL: serverURL,
+	}
+
+	if err := json.NewDecoder(bytes.NewReader(out)).Decode(resp); err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
+
+// Erase executes a program to remove the server credentails from the native store.
+func Erase(program ProgramFunc, serverURL string) error {
+	cmd := program("erase")
+	cmd.Input(strings.NewReader(serverURL))
+
+	out, err := cmd.Output()
+	if err != nil {
+		t := strings.TrimSpace(string(out))
+		return fmt.Errorf("error erasing credentials - err: %v, out: `%s`", err, t)
+	}
+
+	return nil
+}