services/horizon: Apply and check migrations on ingesting instances o…

…nly (#3586)

This commit reverts 7c92b23 and changes the DB migrations behaviour to only
trigger on ingesting instances.

When checking if there are any migrations required Horizon calls
`migrate.PlanMigration` which sends `CREATE TABLE gorp_migrations`. This write
query is forbidden when using read only database (like replica).

The new flag in 7c92b23 added unnecessary complexity. Ingesting instances are
guaranteed to have write access to a DB.

Thanks @jacekn for the idea!

services/horizon/internal/flags.go

@@ -445,17 +445,17 @@ func ApplyFlags(config *Config, flags support.ConfigOptions) {
 	flags.Require()
 	flags.SetValues()
 
-	if config.ApplyMigrations {
-		applyMigrations(*config)
-	}
-
-	// Migrations should be checked as early as possible
-	checkMigrations(*config)
-
 	// Validate options that should be provided together
 	validateBothOrNeither("tls-cert", "tls-key")
 
 	if config.Ingest {
+		// Migrations should be checked as early as possible. Apply and check
+		// only on ingesting instances which are required to have write-access
+		// to the DB.
+		if config.ApplyMigrations {
+			applyMigrations(*config)
+		}
+		checkMigrations(*config)
 
 		// config.HistoryArchiveURLs contains a single empty value when empty so using
 		// viper.GetString is easier.