If autolinking would make posts too long, don't

Substitutions that increase message length have the potential to grow a
post past the ability of the database to store it. This manifests to
users as their posts inexplicably failing. To avoid this fail state, we
check to see if the post-substitution message is too long and, if so,
don't apply the changes.

Unfortunately, the actual maximum post size is not currently accessible
to plugins. In the meantime, we use the smallest value for which the
server guarantees support, which is exposed through
model.POST_MESSAGE_MAX_RUNES_V1 and is in practice 4000 runes.

server/autolinkplugin/plugin.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 	"strings"
 	"sync"
+	"unicode/utf8"
 
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/plugin"
@@ -208,6 +209,11 @@ func (p *Plugin) ProcessPost(c *plugin.Context, post *model.Post) (*model.Post,
 			return nil, ""
 		}
 
+		if runes := utf8.RuneCountInString(postText); runes > model.POST_MESSAGE_MAX_RUNES_V1 {
+			p.API.LogWarn("Rewritten message would be too long, skipping", "postID", post.Id, "rewrittenLength", runes)
+			return nil, ""
+		}
+
 		post.Message = postText
 	}
 

server/autolinkplugin/plugin_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/mattermost/mattermost-server/v5/model"
@@ -305,6 +306,9 @@ func TestSpecialCases(t *testing.T) {
 		Pattern:  "(example)",
 		Template: "test",
 		Scope:    []string{"other-team/town-square"},
+	}, autolink.Autolink{
+		Pattern:  "(toolong)",
+		Template: strings.Repeat("1", model.POST_MESSAGE_MAX_RUNES_V1+1),
 	})
 	validConfig := Config{
 		EnableAdminCommand: false,
@@ -331,6 +335,8 @@ func TestSpecialCases(t *testing.T) {
 	api.On("GetChannel", mock.AnythingOfType("string")).Return(&testChannel, nil)
 	api.On("GetTeam", mock.AnythingOfType("string")).Return(&testTeam, nil)
 
+	api.On("LogWarn", mock.AnythingOfType("string"), mock.AnythingOfType("string"), mock.AnythingOfType("string"), mock.AnythingOfType("string"), mock.AnythingOfType("int"))
+
 	testUser := model.User{
 		IsBot: false,
 	}
@@ -454,6 +460,10 @@ func TestSpecialCases(t *testing.T) {
 			"check out MM-12345 too",
 			"check out [MM-12345](https://mattermost.atlassian.net/browse/MM-12345) too",
 		},
+		{
+			"toolong",
+			"toolong",
+		},
 	}
 
 	for _, tt := range tests {