Expose public/ API as submodule (#23345)

* model -> public/model

* plugin -> public/plugin

* public/model/utils -> public/utils

* platform/shared/mlog -> public/shared/mlog

* platform/shared/i18n -> public/shared/i18n

* platform/shared/markdown -> public/shared/markdown

* platform/services/timezones -> public/shared/timezones

* channels/einterfaces -> einterfaces

* expose public/ submodule

* go mod tidy

* .github: cache-dependency-path, setup-go-work

* modules-tidy for public/ too

* remove old gomodtidy

server/public/plugin/example_help_test.go

@@ -0,0 +1,120 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin_test
+
+import (
+	"strings"
+	"sync"
+
+	"github.com/pkg/errors"
+
+	"github.com/mattermost/mattermost-server/server/public/model"
+	"github.com/mattermost/mattermost-server/server/public/plugin"
+)
+
+// configuration represents the configuration for this plugin as exposed via the Mattermost
+// server configuration.
+type configuration struct {
+	TeamName    string
+	ChannelName string
+
+	// channelID is resolved when the public configuration fields above change
+	channelID string
+}
+
+// HelpPlugin implements the interface expected by the Mattermost server to communicate
+// between the server and plugin processes.
+type HelpPlugin 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
+}
+
+// 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 *HelpPlugin) 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 *HelpPlugin) setConfiguration(configuration *configuration) {
+	// Replace the active configuration under lock.
+	p.configurationLock.Lock()
+	defer p.configurationLock.Unlock()
+	p.configuration = configuration
+}
+
+// OnConfigurationChange updates the active configuration for this plugin under lock.
+func (p *HelpPlugin) 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")
+	}
+
+	team, err := p.API.GetTeamByName(configuration.TeamName)
+	if err != nil {
+		return errors.Wrapf(err, "failed to find team %s", configuration.TeamName)
+	}
+
+	channel, err := p.API.GetChannelByName(team.Id, configuration.ChannelName, false)
+	if err != nil {
+		return errors.Wrapf(err, "failed to find channel %s", configuration.ChannelName)
+	}
+
+	configuration.channelID = channel.Id
+
+	p.setConfiguration(configuration)
+
+	return nil
+}
+
+// MessageHasBeenPosted automatically replies to posts that plea for help.
+func (p *HelpPlugin) MessageHasBeenPosted(c *plugin.Context, post *model.Post) {
+	configuration := p.getConfiguration()
+
+	// Ignore posts not in the configured channel
+	if post.ChannelId != configuration.channelID {
+		return
+	}
+
+	// Ignore posts this plugin made.
+	if sentByPlugin, _ := post.GetProp("sent_by_plugin").(bool); sentByPlugin {
+		return
+	}
+
+	// Ignore posts without a plea for help.
+	if !strings.Contains(post.Message, "help") {
+		return
+	}
+
+	p.API.SendEphemeralPost(post.UserId, &model.Post{
+		ChannelId: configuration.channelID,
+		Message:   "You asked for help? Checkout https://support.mattermost.com/hc/en-us",
+		Props: map[string]any{
+			"sent_by_plugin": true,
+		},
+	})
+}
+
+func Example_helpPlugin() {
+	plugin.ClientMain(&HelpPlugin{})
+}