[MM-47896] Channel names can collide with GM names (#21639)

* Block channel creation if name matches GM naming pattern

* Fix regex and error text

* Add test for DM names

* Update error string

* Trigger build

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

i18n/en.json

@@ -8121,7 +8121,7 @@
   },
   {
     "id": "model.channel.is_valid.name.app_error",
-    "translation": "Invalid channel name. User ids are not permitted in channel name for non-direct message channels."
+    "translation": "Channel names can't be in a hexadecimal format. Please enter a different channel name."
   },
   {
     "id": "model.channel.is_valid.purpose.app_error",

model/channel.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"io"
 	"net/http"
+	"regexp"
 	"sort"
 	"strings"
 	"unicode/utf8"
@@ -185,6 +186,8 @@ type ChannelMemberCountByGroup struct {
 
 type ChannelOption func(channel *Channel)
 
+var gmNameRegex = regexp.MustCompile("^[a-f0-9]{40}$")
+
 func WithID(ID string) ChannelOption {
 	return func(channel *Channel) {
 		channel.Id = ID
@@ -281,9 +284,11 @@ func (o *Channel) IsValid() *AppError {
 		return NewAppError("Channel.IsValid", "model.channel.is_valid.creator_id.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	userIds := strings.Split(o.Name, "__")
+	if o.Type != ChannelTypeDirect && o.Type != ChannelTypeGroup {
-	if o.Type != ChannelTypeDirect && len(userIds) == 2 && IsValidId(userIds[0]) && IsValidId(userIds[1]) {
+		userIds := strings.Split(o.Name, "__")
-		return NewAppError("Channel.IsValid", "model.channel.is_valid.name.app_error", nil, "", http.StatusBadRequest)
+		if ok := gmNameRegex.MatchString(o.Name); ok || (o.Type != ChannelTypeDirect && len(userIds) == 2 && IsValidId(userIds[0]) && IsValidId(userIds[1])) {
+			return NewAppError("Channel.IsValid", "model.channel.is_valid.name.app_error", nil, "", http.StatusBadRequest)
+		}
 	}
 
 	return nil

model/channel_test.go

@@ -79,6 +79,12 @@ func TestChannelIsValid(t *testing.T) {
 
 	o.Purpose = strings.Repeat("0123456789", 25)
 	require.Nil(t, o.IsValid())
+
+	o.Name = "beu8cc6b3jnxfe9r4na9baooma__36atajbs87dqmpym6o8eiy9saa"
+	require.NotNil(t, o.IsValid())
+
+	o.Name = "71b03afcbb2d503d49f87f057549c43db4e19f92"
+	require.NotNil(t, o.IsValid())
 }
 
 func TestChannelPreSave(t *testing.T) {