utils: add comment and tests for adding name to a TLF

Suggested by mmaxim.

Issue: #16067

go/chat/utils/utils.go

@@ -1708,6 +1708,10 @@ func AddUserToTLFName(g *globals.Context, tlfName string, vis keybase1.TLFVisibi
 			return username
 		}
 
+		// KBFS creates TLFs with suffixes (e.g., folder names that
+		// conflict after an assertion has been resolved) and readers,
+		// so we need to handle those types of TLF names here so that
+		// edit history works correctly.
 		split1 := strings.SplitN(tlfName, " ", 2) // split off suffix
 		split2 := strings.Split(split1[0], "#")   // split off readers
 		// Add the name to the writers list (assume the current user

go/chat/utils/utils_test.go

@@ -8,7 +8,9 @@ import (
 	"testing"
 	"time"
 
+	"github.com/keybase/client/go/chat/globals"
 	"github.com/keybase/client/go/chat/types"
+	"github.com/keybase/client/go/externalstest"
 	"github.com/keybase/client/go/libkb"
 	"github.com/keybase/client/go/protocol/chat1"
 	"github.com/keybase/client/go/protocol/gregor1"
@@ -237,3 +239,39 @@ func TestDecorateMentions(t *testing.T) {
 		require.Equal(t, c.result, res)
 	}
 }
+
+type configUsernamer struct {
+	libkb.ConfigReader
+	username libkb.NormalizedUsername
+}
+
+func (c configUsernamer) GetUsername() libkb.NormalizedUsername {
+	return c.username
+}
+
+func TestAddUserToTlfName(t *testing.T) {
+	tc := externalstest.SetupTest(t, "chat-utils", 0)
+	g := globals.NewContext(tc.G, &globals.ChatContext{})
+	g.Env.SetConfig(
+		&configUsernamer{g.Env.GetConfig(), "charlie"}, g.Env.GetConfigWriter())
+
+	priv := keybase1.TLFVisibility_PRIVATE
+	mem := chat1.ConversationMembersType_IMPTEAMNATIVE
+	s := AddUserToTLFName(g, "alice,bob", priv, mem)
+	require.Equal(t, "alice,bob,charlie", s)
+	s = AddUserToTLFName(g, "charlie", priv, mem)
+	require.Equal(t, "charlie,charlie", s)
+	s = AddUserToTLFName(
+		g, "alice,bob (conflicted copy 2019-02-14 #1)", priv, mem)
+	require.Equal(t, "alice,bob,charlie (conflicted copy 2019-02-14 #1)", s)
+	s = AddUserToTLFName(
+		g, "alice#bob", priv, mem)
+	require.Equal(t, "alice,charlie#bob", s)
+	s = AddUserToTLFName(
+		g, "alice#bob (conflicted copy 2019-02-14 #1)", priv, mem)
+	require.Equal(t, "alice,charlie#bob (conflicted copy 2019-02-14 #1)", s)
+
+	pub := keybase1.TLFVisibility_PUBLIC
+	s = AddUserToTLFName(g, "alice,bob", pub, mem)
+	require.Equal(t, "alice,bob", s)
+}