Increase Channel Purpose length to 250, and add channel field length handling code to the slack importer (#4458)

* Increase Channel Purpose length to 250.

This commit increases the maxmimum length of the channel purpose field
to 250, including the database migration necessary.

It also adds a method to the Slack importer to check the lengths of
channel properties before importing, and truncate them if necessary.

Fixes #4168

* Fix database migration.

store/sql_channel_store.go

@@ -38,7 +38,7 @@ func NewSqlChannelStore(sqlStore *SqlStore) ChannelStore {
 		table.ColMap("Name").SetMaxSize(64)
 		table.SetUniqueTogether("Name", "TeamId")
 		table.ColMap("Header").SetMaxSize(1024)
-		table.ColMap("Purpose").SetMaxSize(128)
+		table.ColMap("Purpose").SetMaxSize(250)
 		table.ColMap("CreatorId").SetMaxSize(26)
 
 		tablem := db.AddTableWithName(model.ChannelMember{}, "ChannelMembers").SetKeys(false, "ChannelId", "UserId")

store/sql_upgrade.go

@@ -201,6 +201,11 @@ func UpgradeDatabaseToVersion35(sqlStore *SqlStore) {
 	// The rest of the migration from Filenames -> FileIds is done lazily in api.GetFileInfosForPost
 	sqlStore.CreateColumnIfNotExists("Posts", "FileIds", "varchar(150)", "varchar(150)", "[]")
 
+	// Increase maximum length of the Channel table Purpose column.
+	if sqlStore.GetMaxLengthOfColumnIfExists("Channels", "Purpose") != "250" {
+		sqlStore.AlterColumnTypeIfExists("Channels", "Purpose", "varchar(250)", "varchar(250)")
+	}
+
 	// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 	// UNCOMMENT WHEN WE DO RELEASE
 	// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!