Refactor plugin system

When the plugin loader itself had to know the types in the arguments
and return values of the plugin functions, it was very difficult to
avoid import loops, given that the types were often defined in the
same package that needed to invoke the plugins.

Under this model, the plugin loader has much less knowledge of the
plugins themselves, and within each package we define functions to
interact with the plugins.

eth/tracers/api.go

@@ -40,7 +40,6 @@ import (
 	"github.com/ethereum/go-ethereum/internal/ethapi"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/params"
-	"github.com/ethereum/go-ethereum/plugins"
 	"github.com/ethereum/go-ethereum/rlp"
 	"github.com/ethereum/go-ethereum/rpc"
 )
@@ -801,7 +800,7 @@ func (api *API) traceTx(ctx context.Context, message core.Message, txctx *txTrac
 			}
 		}
 		// Get the tracer from the plugin loader
-		if tr, ok := plugins.GetTracer(*config.Tracer); ok {
+		if tr, ok := getPluginTracer(*config.Tracer); ok {
 			tracer = tr(statedb)
 		} else {
 			// Constuct the JavaScript tracer to execute with
@@ -851,7 +850,7 @@ func (api *API) traceTx(ctx context.Context, message core.Message, txctx *txTrac
 			StructLogs:  ethapi.FormatLogs(tracer.StructLogs()),
 		}, nil
 
-	case plugins.TracerResult:
+	case TracerResult:
 		return tracer.GetResult()
 
 	case *Tracer:

eth/tracers/plugin_hooks.go

@@ -0,0 +1,28 @@
+package tracers
+
+import (
+  "github.com/ethereum/go-ethereum/plugins"
+  "github.com/ethereum/go-ethereum/core/vm"
+  "github.com/ethereum/go-ethereum/core/state"
+)
+
+
+type TracerResult interface {
+	vm.Tracer
+	GetResult() (interface{}, error)
+}
+
+func getPluginTracer(name string) (func(*state.StateDB)TracerResult, bool) {
+  tracers := plugins.Lookup("Tracers", func(item interface{}) bool {
+    _, ok := item.(map[string]func(*state.StateDB)TracerResult)
+    return ok
+  })
+  for _, tmap := range tracers {
+    if tracerMap, ok := tmap.(map[string]func(*state.StateDB)TracerResult); ok {
+      if tracer, ok := tracerMap[name]; ok {
+        return tracer, true
+      }
+    }
+  }
+  return nil, false
+}