feat: implement the scheduler plugin

This also performs a one-time migration of the DOKKU_SCHEDULER values the scheduler plugin properties.

Closes #4739

plugins/scheduler/triggers.go

@@ -0,0 +1,71 @@
+package scheduler
+
+import (
+	"fmt"
+
+	"github.com/dokku/dokku/plugins/common"
+	"github.com/dokku/dokku/plugins/config"
+)
+
+// TriggerSchedulerDetect outputs a manually selected scheduler for the app
+func TriggerSchedulerDetect(appName string) error {
+	if scheduler := common.PropertyGet("scheduler", appName, "selected"); scheduler != "" {
+		fmt.Println(scheduler)
+		return nil
+	}
+
+	if scheduler := common.PropertyGet("scheduler", "--global", "selected"); scheduler != "" {
+		fmt.Println(scheduler)
+		return nil
+	}
+
+	fmt.Println("docker-local")
+	return nil
+}
+
+// TriggerInstall runs the install step for the scheduler plugin
+func TriggerInstall() error {
+	if err := common.PropertySetup("scheduler"); err != nil {
+		return fmt.Errorf("Unable to install the scheduler plugin: %s", err.Error())
+	}
+
+	apps, err := common.DokkuApps()
+	if err != nil {
+		return nil
+	}
+
+	globalScheduler := config.GetWithDefault("--scheduler", "DOKKU_SCHEDULER", "")
+	if globalScheduler != "" {
+		common.LogVerboseQuiet(fmt.Sprintf("Setting scheduler property 'selected' to %v", globalScheduler))
+		if err := common.PropertyWrite("scheduler", "--global", "selected", globalScheduler); err != nil {
+			return err
+		}
+
+		if err := config.UnsetMany("--global", []string{"DOKKU_SCHEDULER"}, false); err != nil {
+			common.LogWarn(err.Error())
+		}
+	}
+
+	for _, appName := range apps {
+		scheduler := config.GetWithDefault(appName, "DOKKU_SCHEDULER", "")
+		if scheduler == "" {
+			continue
+		}
+
+		common.LogVerboseQuiet(fmt.Sprintf("Setting scheduler property 'selected' to %v", scheduler))
+		if err := common.PropertyWrite("scheduler", appName, "selected", scheduler); err != nil {
+			return err
+		}
+
+		if err := config.UnsetMany(appName, []string{"DOKKU_SCHEDULER"}, false); err != nil {
+			common.LogWarn(err.Error())
+		}
+	}
+
+	return nil
+}
+
+// TriggerPostDelete destroys the scheduler property for a given app container
+func TriggerPostDelete(appName string) error {
+	return common.PropertyDestroy("scheduler", appName)
+}