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>

credentials/credentials.go

@@ -10,6 +10,13 @@ import (
 	"strings"
 )
 
+// Credentials holds the information shared between docker and the credentials store.
+type Credentials struct {
+	ServerURL string
+	Username  string
+	Secret    string
+}
+
 // Serve initializes the credentials helper and parses the action argument.
 // This function is designed to be called from a command line interface.
 // It uses os.Args[1] as the key for the action.

credentials/error.go

@@ -0,0 +1,37 @@
+package credentials
+
+// ErrCredentialsNotFound standarizes the not found error, so every helper returns
+// the same message and docker can handle it properly.
+const errCredentialsNotFoundMessage = "credentials not found in native keychain"
+
+// errCredentialsNotFound represents an error
+// raised when credentials are not in the store.
+type errCredentialsNotFound struct{}
+
+// Error returns the standard error message
+// for when the credentials are not in the store.
+func (errCredentialsNotFound) Error() string {
+	return errCredentialsNotFoundMessage
+}
+
+// NewErrCredentialsNotFound creates a new error
+// for when the credentials are not in the store.
+func NewErrCredentialsNotFound() error {
+	return errCredentialsNotFound{}
+}
+
+// IsErrCredentialsNotFound returns true if the error
+// was caused by not having a set of credentials in a store.
+func IsErrCredentialsNotFound(err error) bool {
+	_, ok := err.(errCredentialsNotFound)
+	return ok
+}
+
+// IsErrCredentialsNotFoundMessage returns true if the error
+// was caused by not having a set of credentials in a store.
+//
+// This function helps to check messages returned by an
+// external program via its standard output.
+func IsErrCredentialsNotFoundMessage(err string) bool {
+	return err == errCredentialsNotFoundMessage
+}

credentials/helper.go

@@ -1,14 +1,5 @@
 package credentials
 
-import "errors"
-
-// Credentials holds the information shared between docker and the credentials store.
-type Credentials struct {
-	ServerURL string
-	Username  string
-	Secret    string
-}
-
 // Helper is the interface a credentials store helper must implement.
 type Helper interface {
 	// Add appends credentials to the store.
@@ -19,7 +10,3 @@ type Helper interface {
 	// It returns username and secret as strings.
 	Get(serverURL string) (string, string, error)
 }
-
-// ErrCredentialsNotFound standarizes the not found error, so every helper returns
-// the same message and docker can handle it properly.
-var ErrCredentialsNotFound = errors.New("credentials not found in native keychain")