fn: agent call overrider (#1080)

Similar to LB Agent call overrider, this PR adds Agent overrider
for Agents to modify/analyze a Call/Extensions during GetCall().

api/agent/agent.go

@@ -120,6 +120,8 @@ type agent struct {
 	shutonce            sync.Once
 	callEndCount        int64
 	disableAsyncDequeue bool
+
+	callOverrider CallOverrider
 }
 
 type AgentOption func(*agent) error
@@ -194,6 +196,17 @@ func WithoutAsyncDequeue() AgentOption {
 	}
 }
 
+// Agents can use this to register a CallOverrider to modify a Call and extensions
+func WithCallOverrider(fn CallOverrider) AgentOption {
+	return func(a *agent) error {
+		if a.callOverrider != nil {
+			return errors.New("lb-agent call overriders already exists")
+		}
+		a.callOverrider = fn
+		return nil
+	}
+}
+
 // Create a default docker driver from agent config
 func NewDockerDriver(cfg *AgentConfig) *docker.DockerDriver {
 	return docker.NewDocker(drivers.Config{

api/agent/call.go

@@ -41,6 +41,9 @@ type Call interface {
 	End(ctx context.Context, err error) error
 }
 
+// Interceptor in GetCall
+type CallOverrider func(*models.Call, map[string]string) (map[string]string, error)
+
 // TODO build w/o closures... lazy
 type CallOpt func(c *call) error
 
@@ -261,6 +264,16 @@ func (a *agent) GetCall(opts ...CallOpt) (Call, error) {
 		return nil, errors.New("no model or request provided for call")
 	}
 
+	// If overrider is present, let's allow it to modify models.Call
+	// and call extensions
+	if a.callOverrider != nil {
+		ext, err := a.callOverrider(c.Call, c.extensions)
+		if err != nil {
+			return nil, err
+		}
+		c.extensions = ext
+	}
+
 	mem := c.Memory + uint64(c.TmpFsSize)
 	if !a.resources.IsResourcePossible(mem, uint64(c.CPUs), c.Type == models.TypeAsync) {
 		// if we're not going to be able to run this call on this machine, bail here.

api/agent/lb_agent.go

@@ -17,8 +17,6 @@ import (
 	"github.com/fnproject/fn/fnext"
 )
 
-type CallOverrider func(*models.Call, map[string]string) (map[string]string, error)
-
 type lbAgent struct {
 	cfg           AgentConfig
 	da            DataAccess
@@ -40,7 +38,7 @@ func WithLBAgentConfig(cfg *AgentConfig) LBAgentOption {
 }
 
 // LB agents can use this to register a CallOverrider to modify a Call and extensions
-func WithCallOverrider(fn CallOverrider) LBAgentOption {
+func WithLBCallOverrider(fn CallOverrider) LBAgentOption {
 	return func(a *lbAgent) error {
 		if a.callOverrider != nil {
 			return errors.New("lb-agent call overriders already exists")