Implement "global" state for modules, OnLoad and OnUnload callbacks

Tested for memory leaks and performance. Obviously the added locking and
global state is not awesome, but the alternative is a little uglier IMO:
we'd have to make some sort of "liaison" value which stores the state,
then pass it around to every module, and so LoadModule becomes a lot
less accessible, and each module would need to maintain a reference to
it... nope, just ugly. I think this is the cleaner solution: just make
sure only one Start() happens at a time, and keep global things global.

Very simple log middleware is an example.

Might need to reorder the operations in Start() and handle errors
differently, etc. Otherwise, I'm mostly happy with this solution...

caddy.go

@@ -10,13 +10,34 @@ import (
 	"time"
 )
 
-var currentCfg *Config
-var currentCfgMu sync.Mutex
-
 // Start runs Caddy with the given config.
 func Start(cfg Config) error {
-	cfg.runners = make(map[string]Runner)
+	// allow only one call to Start at a time,
+	// since various calls to LoadModule()
+	// access shared map moduleInstances
+	startMu.Lock()
+	defer startMu.Unlock()
 
+	// prepare the config for use
+	cfg.runners = make(map[string]Runner)
+	cfg.moduleStates = make(map[string]interface{})
+
+	// reset the shared moduleInstances map; but
+	// keep a temporary reference to the current
+	// one so we can transfer over any necessary
+	// state to the new modules; or in case this
+	// function returns an error, we need to put
+	// the "old" one back where we found it
+	var err error
+	oldModuleInstances := moduleInstances
+	defer func() {
+		if err != nil {
+			moduleInstances = oldModuleInstances
+		}
+	}()
+	moduleInstances = make(map[string][]interface{})
+
+	// load (decode) each runner module
 	for modName, rawMsg := range cfg.Modules {
 		val, err := LoadModule(modName, rawMsg)
 		if err != nil {
@@ -34,19 +55,60 @@ func Start(cfg Config) error {
 		}
 	}
 
-	// shut down down the old ones
+	// shut down down the old runners
 	currentCfgMu.Lock()
 	if currentCfg != nil {
-		for _, r := range currentCfg.runners {
+		for name, r := range currentCfg.runners {
 			err := r.Cancel()
 			if err != nil {
-				log.Println(err)
+				log.Printf("[ERROR] cancel %s: %v", name, err)
 			}
 		}
 	}
+	oldCfg := currentCfg
 	currentCfg = &cfg
 	currentCfgMu.Unlock()
 
+	// invoke unload callbacks on old configuration
+	for modName := range oldModuleInstances {
+		mod, err := GetModule(modName)
+		if err != nil {
+			return err
+		}
+		if mod.OnUnload != nil {
+			var unloadingState interface{}
+			if oldCfg != nil {
+				unloadingState = oldCfg.moduleStates[modName]
+			}
+			err := mod.OnUnload(unloadingState)
+			if err != nil {
+				log.Printf("[ERROR] module OnUnload: %s: %v", modName, err)
+				continue
+			}
+		}
+	}
+
+	// invoke load callbacks on new configuration
+	for modName, instances := range moduleInstances {
+		mod, err := GetModule(modName)
+		if err != nil {
+			return err
+		}
+		if mod.OnLoad != nil {
+			var priorState interface{}
+			if oldCfg != nil {
+				priorState = oldCfg.moduleStates[modName]
+			}
+			modState, err := mod.OnLoad(instances, priorState)
+			if err != nil {
+				return fmt.Errorf("module OnLoad: %s: %v", modName, err)
+			}
+			if modState != nil {
+				cfg.moduleStates[modName] = modState
+			}
+		}
+	}
+
 	// shut down listeners that are no longer being used
 	listenersMu.Lock()
 	for key, info := range listeners {
@@ -74,7 +136,15 @@ type Runner interface {
 type Config struct {
 	TestVal string                     `json:"testval"`
 	Modules map[string]json.RawMessage `json:"modules"`
+
+	// runners stores the decoded Modules values,
+	// keyed by module name.
 	runners map[string]Runner
+
+	// moduleStates stores the optional "global" state
+	// values of every module used by this configuration,
+	// keyed by module name.
+	moduleStates map[string]interface{}
 }
 
 // Duration is a JSON-string-unmarshable duration type.
@@ -95,3 +165,23 @@ func (d *Duration) UnmarshalJSON(b []byte) error {
 func (d Duration) MarshalJSON() ([]byte, error) {
 	return []byte(fmt.Sprintf(`"%s"`, time.Duration(d).String())), nil
 }
+
+// currentCfg is the currently-loaded configuration.
+var (
+	currentCfg   *Config
+	currentCfgMu sync.Mutex
+)
+
+// moduleInstances stores the individual instantiated
+// values of modules, keyed by module name. The list
+// of instances of each module get passed into the
+// respective module's OnLoad callback, so they can
+// set up any global state and/or make sure their
+// configuration, when viewed as a whole, is valid.
+// Since this list is shared, only one Start() routine
+// must be allowed to happen at any given time.
+var moduleInstances = make(map[string][]interface{})
+
+// startMu ensures that only one Start() happens at a time.
+// This is important since
+var startMu sync.Mutex

modules.go

@@ -13,6 +13,8 @@ import (
 type Module struct {
 	Name     string
 	New      func() (interface{}, error)
+	OnLoad   func(instances []interface{}, priorState interface{}) (newState interface{}, err error)
+	OnUnload func(state interface{}) error
 }
 
 func (m Module) String() string { return m.Name }
@@ -145,6 +147,8 @@ func LoadModule(name string, rawMsg json.RawMessage) (interface{}, error) {
 		}
 	}
 
+	moduleInstances[mod.Name] = append(moduleInstances[mod.Name], val)
+
 	return val, nil
 }
 

modules/caddyhttp/caddylog/log.go

@@ -12,13 +12,31 @@ import (
 func init() {
 	caddy2.RegisterModule(caddy2.Module{
 		Name: "http.middleware.log",
-		New:  func() (interface{}, error) { return &Log{}, nil },
+		New:  func() (interface{}, error) { return new(Log), nil },
+		OnLoad: func(instances []interface{}, priorState interface{}) (interface{}, error) {
+			var counter int
+			if priorState != nil {
+				counter = priorState.(int)
+			}
+			counter++
+			for _, inst := range instances {
+				logInst := inst.(*Log)
+				logInst.counter = counter
+			}
+			log.Println("State is now:", counter)
+			return counter, nil
+		},
+		OnUnload: func(state interface{}) error {
+			log.Println("Closing log files, state:", state)
+			return nil
+		},
 	})
 }
 
 // Log implements a simple logging middleware.
 type Log struct {
 	Filename string
+	counter  int
 }
 
 func (l *Log) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
@@ -28,7 +46,7 @@ func (l *Log) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.H
 		return err
 	}
 
-	log.Println("latency:", time.Now().Sub(start))
+	log.Println("latency:", time.Now().Sub(start), l.counter)
 
 	return nil
 }