MM-12193: illustrate simplified, but safe configuration handling

server/configuration.go

@@ -0,0 +1,69 @@
+package main
+
+import (
+	"github.com/pkg/errors"
+)
+
+// configuration captures the plugin's external configuration as exposed in the Mattermost server
+// configuration, as well as values computed from the configuration. Any public fields will be
+// deserialized from the Mattermost server configuration in OnConfigurationChange, while any
+// private fields will be ignored.
+//
+// As plugins are inherently concurrent (hooks being called asynchronously), and the plugin
+// configuration can change at any time, access to the configuration must be synchronized. The
+// strategy used in this plugin is to guard a pointer to the configuration, and clone the entire
+// struct whenever it changes. You may replace this with whatever strategy you choose.
+//
+// If you add non-reference types to your configuration struct, be sure to rewrite Clone as a deep
+// copy appropriate for your types.
+type configuration struct {
+}
+
+// Clone shallow copies the configuration. Your implementation may require a deep copy if
+// your configuration has reference types.
+func (c *configuration) Clone() *configuration {
+	if c == nil {
+		return &configuration{}
+	}
+
+	var clone = *c
+	return &clone
+}
+
+// getConfiguration retrieves the active configuration under lock, making it safe to use
+// concurrently. The active configuration may change underneath the client of this method, but
+// the struct returned by this API call is considered immutable.
+func (p *Plugin) getConfiguration() *configuration {
+	p.configurationLock.RLock()
+	defer p.configurationLock.RUnlock()
+
+	if p.configuration == nil {
+		return &configuration{}
+	}
+
+	return p.configuration
+}
+
+// setConfiguration replaces the active configuration under lock.
+//
+// Do not call setConfiguration while holding the configurationLock, as sync.Mutex is not
+// reentrant.
+func (p *Plugin) setConfiguration(configuration *configuration) {
+	p.configurationLock.Lock()
+	defer p.configurationLock.Unlock()
+	p.configuration = configuration
+}
+
+// OnConfigurationChange is invoked when configuration changes may have been made.
+func (p *Plugin) OnConfigurationChange() error {
+	var configuration = new(configuration)
+
+	// Load the public configuration fields from the Mattermost server configuration.
+	if err := p.API.LoadPluginConfiguration(configuration); err != nil {
+		return errors.Wrap(err, "failed to load plugin configuration")
+	}
+
+	p.setConfiguration(configuration)
+
+	return nil
+}

server/plugin.go

@@ -1,11 +1,20 @@
 package main
 
 import (
+	"sync"
+
 	"github.com/mattermost/mattermost-server/plugin"
 )
 
 type Plugin struct {
 	plugin.MattermostPlugin
+
+	// configurationLock synchronizes access to the configuration.
+	configurationLock sync.RWMutex
+
+	// configuration is the active plugin configuration. Consult getConfiguration and
+	// setConfiguration for usage.
+	configuration *configuration
 }
 
 // See https://developers.mattermost.com/extend/plugins/server/reference/