Channel default value by changing default logic

In a nutshell, the newly introduced plugin.PopulateDefaults function
populates all fields of a Plugin-implementing struct with those fields
from Info.ConfigAttributes where ConfigOption.Default is set. Thus, a
single function call before parsing the user-submitted configuration
within the Plugin.SetConfig method sets default values.

As a result of the discussion between the Go and the Web team,
summarized in #205, Web does not store key-value pairs to be omitted.

Prior, an already JSON-encoded version of the ConfigOption slice was
present in plugin.Info. Thus, this data wasn't easily available anymore.
As the new code now needs to access this field, it was changed and a
custom sql driver.Valuer was implemented for a slice type.

While working on this, all ConfigOptions were put in the same order as
the struct fields.

Requires <Icinga/icinga-notifications-web#230>.

Closes #205.

Author: oxzi

notifications/plugin/plugin.go

@@ -1,6 +1,7 @@
 package plugin
 
 import (
+	"database/sql/driver"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -69,13 +70,23 @@ type ConfigOption struct {
 	Max types.Int `json:"max,omitempty"`
 }
 
+// ConfigOptions describes all ConfigOption entries.
+//
+// This type became necessary to implement the database.sql.driver.Valuer to marshal it into JSON.
+type ConfigOptions []ConfigOption
+
+// Value implements database.sql's driver.Valuer to represent all ConfigOptions as a JSON array.
+func (c ConfigOptions) Value() (driver.Value, error) {
+	return json.Marshal(c)
+}
+
 // Info contains plugin information.
 type Info struct {
-	Type             string          `db:"type" json:"-"`
-	Name             string          `db:"name" json:"name"`
-	Version          string          `db:"version" json:"version"`
-	Author           string          `db:"author" json:"author"`
-	ConfigAttributes json.RawMessage `db:"config_attrs" json:"config_attrs"` // ConfigOption(s) as json-encoded list
+	Type             string        `db:"type" json:"-"`
+	Name             string        `db:"name" json:"name"`
+	Version          string        `db:"version" json:"version"`
+	Author           string        `db:"author" json:"author"`
+	ConfigAttributes ConfigOptions `db:"config_attrs" json:"config_attrs"`
 }
 
 // TableName implements the contracts.TableNamer interface.
@@ -131,6 +142,25 @@ type Plugin interface {
 	SendNotification(req *NotificationRequest) error
 }
 
+// PopulateDefaults sets the struct fields from Info.ConfigAttributes where ConfigOption.Default is set.
+//
+// It should be called from each channel plugin within its Plugin.SetConfig before doing any further configuration.
+func PopulateDefaults(typePtr Plugin) error {
+	defaults := make(map[string]any)
+	for _, confAttr := range typePtr.GetInfo().ConfigAttributes {
+		if confAttr.Default != nil {
+			defaults[confAttr.Name] = confAttr.Default
+		}
+	}
+
+	defaultConf, err := json.Marshal(defaults)
+	if err != nil {
+		return err
+	}
+
+	return json.Unmarshal(defaultConf, typePtr)
+}
+
 // RunPlugin reads the incoming stdin requests, processes and writes the responses to stdout
 func RunPlugin(plugin Plugin) {
 	encoder := json.NewEncoder(os.Stdout)