Add transports/StubTransport

This is useful primarily so that we can use build tags to remove transports
from transports/alltransports, while giving users more hints about what
to do instead of a bland “unknown transport”.

Tests can also use this instead of their own mocks.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

transports/stub.go

@@ -0,0 +1,36 @@
+package transports
+
+import (
+	"fmt"
+
+	"github.com/containers/image/types"
+)
+
+// stubTransport is an implementation of types.ImageTransport which has a name, but rejects any references with “the transport $name: is not supported in this build”.
+type stubTransport string
+
+// NewStubTransport returns an implementation of types.ImageTransport which has a name, but rejects any references with “the transport $name: is not supported in this build”.
+func NewStubTransport(name string) types.ImageTransport {
+	return stubTransport(name)
+}
+
+// Name returns the name of the transport, which must be unique among other transports.
+func (s stubTransport) Name() string {
+	return string(s)
+}
+
+// ParseReference converts a string, which should not start with the ImageTransport.Name prefix, into an ImageReference.
+func (s stubTransport) ParseReference(reference string) (types.ImageReference, error) {
+	return nil, fmt.Errorf(`The transport "%s:" is not supported in this build`, string(s))
+}
+
+// ValidatePolicyConfigurationScope checks that scope is a valid name for a signature.PolicyTransportScopes keys
+// (i.e. a valid PolicyConfigurationIdentity() or PolicyConfigurationNamespaces() return value).
+// It is acceptable to allow an invalid value which will never be matched, it can "only" cause user confusion.
+// scope passed to this function will not be "", that value is always allowed.
+func (s stubTransport) ValidatePolicyConfigurationScope(scope string) error {
+	// Allowing any reference in here allows tools with some transports stubbed-out to still
+	// use signature verification policies which refer to these stubbed-out transports.
+	// See also the treatment of unknown transports in policyTransportScopesWithTransport.UnmarshalJSON .
+	return nil
+}

transports/stub_test.go

@@ -0,0 +1,18 @@
+package transports
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStubTransport(t *testing.T) {
+	const name = "whatever"
+
+	s := NewStubTransport(name)
+	assert.Equal(t, name, s.Name())
+	_, err := s.ParseReference("this is rejected regardless of content")
+	assert.Error(t, err)
+	err = s.ValidatePolicyConfigurationScope("this is accepted regardless of content")
+	assert.NoError(t, err)
+}