svchost/disco: Disco.CredentialsSource

Previously we allowed access to the credentials store only indirectly
through the Disco object, and that's fine for callers that only need to
_read_ credentials, but more specialized callers like "terraform login"
and "terraform logout" need more information in order to be transparent
to the user about what they are going to do and where the credentials
are going to be stored.

svchost/disco/disco.go

@@ -75,6 +75,18 @@ func (d *Disco) SetCredentialsSource(src auth.CredentialsSource) {
 	d.credsSrc = src
 }
 
+// CredentialsSource returns the credentials source associated with the receiver,
+// or an empty credentials source if none is associated.
+func (d *Disco) CredentialsSource() auth.CredentialsSource {
+	if d.credsSrc == nil {
+		// We'll return an empty one just to save the caller from having to
+		// protect against the nil case, since this interface already allows
+		// for the possibility of there being no credentials at all.
+		return auth.StaticCredentialsSource(nil)
+	}
+	return d.credsSrc
+}
+
 // CredentialsForHost returns a non-nil HostCredentials if the embedded source has
 // credentials available for the host, and a nil HostCredentials if it does not.
 func (d *Disco) CredentialsForHost(hostname svchost.Hostname) (auth.HostCredentials, error) {