Make Task type immutable

This change makes it impossible to mutate payload within Handler or
RetryDelayFunc.

payload.go

@@ -12,8 +12,9 @@ import (
 )
 
 // Payload is an arbitrary data needed for task execution.
-// The values have to be JSON serializable.
-type Payload map[string]interface{}
+type Payload struct {
+	data map[string]interface{}
+}
 
 type errKeyNotFound struct {
 	key string
@@ -25,14 +26,14 @@ func (e *errKeyNotFound) Error() string {
 
 // Has reports whether key exists.
 func (p Payload) Has(key string) bool {
-	_, ok := p[key]
+	_, ok := p.data[key]
 	return ok
 }
 
 // GetString returns a string value if a string type is associated with
 // the key, otherwise reports an error.
 func (p Payload) GetString(key string) (string, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return "", &errKeyNotFound{key}
 	}
@@ -42,7 +43,7 @@ func (p Payload) GetString(key string) (string, error) {
 // GetInt returns an int value if a numeric type is associated with
 // the key, otherwise reports an error.
 func (p Payload) GetInt(key string) (int, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return 0, &errKeyNotFound{key}
 	}
@@ -52,7 +53,7 @@ func (p Payload) GetInt(key string) (int, error) {
 // GetFloat64 returns a float64 value if a numeric type is associated with
 // the key, otherwise reports an error.
 func (p Payload) GetFloat64(key string) (float64, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return 0, &errKeyNotFound{key}
 	}
@@ -62,7 +63,7 @@ func (p Payload) GetFloat64(key string) (float64, error) {
 // GetBool returns a boolean value if a boolean type is associated with
 // the key, otherwise reports an error.
 func (p Payload) GetBool(key string) (bool, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return false, &errKeyNotFound{key}
 	}
@@ -72,7 +73,7 @@ func (p Payload) GetBool(key string) (bool, error) {
 // GetStringSlice returns a slice of strings if a string slice type is associated with
 // the key, otherwise reports an error.
 func (p Payload) GetStringSlice(key string) ([]string, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return nil, &errKeyNotFound{key}
 	}
@@ -82,7 +83,7 @@ func (p Payload) GetStringSlice(key string) ([]string, error) {
 // GetIntSlice returns a slice of ints if a int slice type is associated with
 // the key, otherwise reports an error.
 func (p Payload) GetIntSlice(key string) ([]int, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return nil, &errKeyNotFound{key}
 	}
@@ -93,7 +94,7 @@ func (p Payload) GetIntSlice(key string) ([]int, error) {
 // if a correct map type is associated with the key,
 // otherwise reports an error.
 func (p Payload) GetStringMap(key string) (map[string]interface{}, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return nil, &errKeyNotFound{key}
 	}
@@ -104,7 +105,7 @@ func (p Payload) GetStringMap(key string) (map[string]interface{}, error) {
 // if a correct map type is associated with the key,
 // otherwise reports an error.
 func (p Payload) GetStringMapString(key string) (map[string]string, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return nil, &errKeyNotFound{key}
 	}
@@ -115,7 +116,7 @@ func (p Payload) GetStringMapString(key string) (map[string]string, error) {
 // if a correct map type is associated with the key,
 // otherwise reports an error.
 func (p Payload) GetStringMapStringSlice(key string) (map[string][]string, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return nil, &errKeyNotFound{key}
 	}
@@ -126,7 +127,7 @@ func (p Payload) GetStringMapStringSlice(key string) (map[string][]string, error
 // if a correct map type is associated with the key,
 // otherwise reports an error.
 func (p Payload) GetStringMapInt(key string) (map[string]int, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return nil, &errKeyNotFound{key}
 	}
@@ -137,7 +138,7 @@ func (p Payload) GetStringMapInt(key string) (map[string]int, error) {
 // if a correct map type is associated with the key,
 // otherwise reports an error.
 func (p Payload) GetStringMapBool(key string) (map[string]bool, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return nil, &errKeyNotFound{key}
 	}
@@ -147,7 +148,7 @@ func (p Payload) GetStringMapBool(key string) (map[string]bool, error) {
 // GetTime returns a time value if a correct map type is associated with the key,
 // otherwise reports an error.
 func (p Payload) GetTime(key string) (time.Time, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return time.Time{}, &errKeyNotFound{key}
 	}
@@ -157,7 +158,7 @@ func (p Payload) GetTime(key string) (time.Time, error) {
 // GetDuration returns a duration value if a correct map type is associated with the key,
 // otherwise reports an error.
 func (p Payload) GetDuration(key string) (time.Duration, error) {
-	v, ok := p[key]
+	v, ok := p.data[key]
 	if !ok {
 		return 0, &errKeyNotFound{key}
 	}