[#316] ir: Add before-start and after-stop function to Server

Keep list of functions which are called first when the server starts (method
Server.Start). If any of the starters returns an error, the server will not
start. Such starters will mainly be used for resources that need to be
initialized after a successful server construction, but before its main work
(e.g. local files).

Keep list of functions which are called when the server stops (Server.Stop
method). Such closers will mainly be used for resources that need to be
released after server shutdown (e.g. initialized by starters).

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/innerring/innerring.go

@@ -3,6 +3,7 @@ package innerring
 import (
 	"context"
 	"crypto/ecdsa"
+	"io"
 
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
@@ -62,6 +63,23 @@ type (
 		predefinedValidators []keys.PublicKey
 
 		workers []func(context.Context)
+
+		// Set of local resources that must be
+		// initialized at the very beginning of
+		// Server's work, (e.g. opening files).
+		//
+		// If any starter returns an error, Server's
+		// starting fails immediately.
+		starters []func() error
+
+		// Set of local resources that must be
+		// released at Server's work completion
+		// (e.g closing files).
+		//
+		// Closer's wrong outcome shouldn't be critical.
+		//
+		// Errors are logged.
+		closers []func() error
 	}
 
 	contracts struct {
@@ -93,6 +111,12 @@ const (
 
 // Start runs all event providers.
 func (s *Server) Start(ctx context.Context, intError chan<- error) error {
+	for _, starter := range s.starters {
+		if err := starter(); err != nil {
+			return err
+		}
+	}
+
 	err := s.initConfigFromBlockchain()
 	if err != nil {
 		return err
@@ -152,6 +176,26 @@ func (s *Server) startWorkers(ctx context.Context) {
 func (s *Server) Stop() {
 	go s.morphListener.Stop()
 	go s.mainnetListener.Stop()
+
+	for _, c := range s.closers {
+		if err := c(); err != nil {
+			s.log.Warn("closer error",
+				zap.String("error", err.Error()),
+			)
+		}
+	}
+}
+
+func (s *Server) registerIOCloser(c io.Closer) {
+	s.registerCloser(c.Close)
+}
+
+func (s *Server) registerCloser(f func() error) {
+	s.closers = append(s.closers, f)
+}
+
+func (s *Server) registerStarter(f func() error) {
+	s.starters = append(s.starters, f)
 }
 
 // New creates instance of inner ring sever structure.