agent: Return an error, not just an interface

Make `newAgentConfig()` return an explicit error rather than handling
the error scenario by simply returning the `error` object in the
`interface{}` return type. The old behaviour was confusing and
inconsistent with the other functions creating a new config type (shim,
proxy, etc).

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

virtcontainers/agent.go

@@ -91,19 +91,19 @@ func newAgent(agentType AgentType) agent {
 }
 
 // newAgentConfig returns an agent config from a generic SandboxConfig interface.
-func newAgentConfig(agentType AgentType, agentConfig interface{}) interface{} {
+func newAgentConfig(agentType AgentType, agentConfig interface{}) (interface{}, error) {
 	switch agentType {
 	case NoopAgentType:
-		return nil
+		return nil, nil
 	case KataContainersAgent:
 		var kataAgentConfig KataAgentConfig
 		err := mapstructure.Decode(agentConfig, &kataAgentConfig)
 		if err != nil {
-			return err
+			return nil, err
 		}
-		return kataAgentConfig
+		return kataAgentConfig, nil
 	default:
-		return nil
+		return nil, nil
 	}
 }
 

virtcontainers/agent_test.go

@@ -86,7 +86,12 @@ func TestNewAgentFromUnknownAgentType(t *testing.T) {
 }
 
 func testNewAgentConfig(t *testing.T, config SandboxConfig, expected interface{}) {
-	agentConfig := newAgentConfig(config.AgentType, config.AgentConfig)
+	agentConfig, err := newAgentConfig(config.AgentType, config.AgentConfig)
+	if err != nil {
+		t.Fatal(err)
+
+	}
+
 	if reflect.DeepEqual(agentConfig, expected) == false {
 		t.Fatal()
 	}

virtcontainers/sandbox.go

@@ -583,7 +583,11 @@ func newSandbox(ctx context.Context, sandboxConfig SandboxConfig, factory Factor
 		return nil, err
 	}
 
-	agentConfig := newAgentConfig(sandboxConfig.AgentType, sandboxConfig.AgentConfig)
+	agentConfig, err := newAgentConfig(sandboxConfig.AgentType, sandboxConfig.AgentConfig)
+	if err != nil {
+		return nil, err
+	}
+
 	if err = s.agent.init(ctx, s, agentConfig); err != nil {
 		return nil, err
 	}