Prevent invalid credentials: no missing server URL or username (#62)

* Prevent invalid credentials: no missing server URL or username

Signed-off-by: Nassim 'Nass' Eddequiouaq <eddequiouaq.nassim@gmail.com>

* Add missing username/serverURL error checks in credstore client

Signed-off-by: Nassim 'Nass' Eddequiouaq <eddequiouaq.nassim@gmail.com>

* Clean up doc on invalid creds errors and client's error checks

Signed-off-by: Nassim 'Nass' Eddequiouaq <eddequiouaq.nassim@gmail.com>

* Add tests for missing ServerURL/Username

Signed-off-by: Nassim 'Nass' Eddequiouaq <eddequiouaq.nassim@gmail.com>

* Clean isValidCredsMessage prototype

Signed-off-by: Nassim 'Nass' Eddequiouaq <eddequiouaq.nassim@gmail.com>

* Add test for missing server URL and more detailed error msg

Signed-off-by: Nassim 'Nass' Eddequiouaq <eddequiouaq.nassim@gmail.com>

credentials/error.go

@@ -1,8 +1,15 @@
 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"
+const (
+	// ErrCredentialsNotFound standardizes the not found error, so every helper returns
+	// the same message and docker can handle it properly.
+	errCredentialsNotFoundMessage = "credentials not found in native keychain"
+
+	// ErrCredentialsMissingServerURL and ErrCredentialsMissingUsername standardize
+	// invalid credentials or credentials management operations
+	errCredentialsMissingServerURLMessage = "no credentials server URL"
+	errCredentialsMissingUsernameMessage = "no credentials username"
+)
 
 // errCredentialsNotFound represents an error
 // raised when credentials are not in the store.
@@ -35,3 +42,64 @@ func IsErrCredentialsNotFound(err error) bool {
 func IsErrCredentialsNotFoundMessage(err string) bool {
 	return err == errCredentialsNotFoundMessage
 }
+
+
+// errCredentialsMissingServerURL represents an error raised
+// when the credentials object has no server URL or when no
+// server URL is provided to a credentials operation requiring
+// one.
+type errCredentialsMissingServerURL struct{}
+
+func (errCredentialsMissingServerURL) Error() string {
+	return errCredentialsMissingServerURLMessage
+}
+
+// errCredentialsMissingUsername represents an error raised
+// when the credentials object has no username or when no
+// username is provided to a credentials operation requiring
+// one.
+type errCredentialsMissingUsername struct{}
+
+func (errCredentialsMissingUsername) Error() string {
+	return errCredentialsMissingUsernameMessage
+}
+
+
+// NewErrCredentialsMissingServerURL creates a new error for
+// errCredentialsMissingServerURL.
+func NewErrCredentialsMissingServerURL() error {
+	return errCredentialsMissingServerURL{}
+}
+
+// NewErrCredentialsMissingUsername creates a new error for
+// errCredentialsMissingUsername.
+func NewErrCredentialsMissingUsername() error {
+	return errCredentialsMissingUsername{}
+}
+
+
+// IsCredentialsMissingServerURL returns true if the error
+// was an errCredentialsMissingServerURL.
+func IsCredentialsMissingServerURL(err error) bool {
+	_, ok := err.(errCredentialsMissingServerURL)
+	return ok
+}
+
+// IsCredentialsMissingServerURLMessage checks for an
+// errCredentialsMissingServerURL in the error message.
+func IsCredentialsMissingServerURLMessage(err string) bool {
+	return err == errCredentialsMissingServerURLMessage
+}
+
+// IsCredentialsMissingUsername returns true if the error
+// was an errCredentialsMissingUsername.
+func IsCredentialsMissingUsername(err error) bool {
+	_, ok := err.(errCredentialsMissingUsername)
+	return ok
+}
+
+// IsCredentialsMissingUsernameMessage checks for an
+// errCredentialsMissingUsername in the error message.
+func IsCredentialsMissingUsernameMessage(err string) bool {
+	return err == errCredentialsMissingUsernameMessage
+}