Indexer ValidateAndWaitOnly startup mode for canton participant HA

ledger/participant-integration-api/src/main/scala/platform/indexer/IndexerStartupMode.scala

@@ -15,4 +15,6 @@ object IndexerStartupMode {
 
   case object MigrateOnly extends IndexerStartupMode
 
+  case object ValidateAndWaitOnly extends IndexerStartupMode
+
 }

ledger/participant-integration-api/src/main/scala/platform/indexer/JdbcIndexer.scala

@@ -87,6 +87,12 @@ object JdbcIndexer {
         resourceContext: ResourceContext
     ): Future[ResourceOwner[Indexer]] = initialized(resetSchema = true)
 
+    def validateAndWaitOnly()(implicit
+        resourceContext: ResourceContext
+    ): Future[Unit] =
+      flywayMigrations
+        .validateAndWaitOnly(config.enableAppendOnlySchema)
+
     private[this] def initializedMutatingSchema(
         resetSchema: Boolean
     )(implicit resourceContext: ResourceContext): Future[ResourceOwner[Indexer]] =

ledger/participant-integration-api/src/main/scala/platform/indexer/StandaloneIndexerServer.scala

@@ -69,6 +69,13 @@ final class StandaloneIndexerServer(
             logger.debug("Waiting for the indexer to initialize the database.")
             healthReporter
           }
+      case IndexerStartupMode.ValidateAndWaitOnly =>
+        Resource
+          .fromFuture(indexerFactory.validateAndWaitOnly())
+          .map[ReportsHealth] { _ =>
+            logger.debug("Waiting for the indexer to validate the schema migrations.")
+            () => HealthStatus.healthy
+          }
     }
   }
 

ledger/participant-integration-api/src/main/scala/platform/store/FlywayMigrations.scala

@@ -13,6 +13,7 @@ import org.flywaydb.core.Flyway
 import org.flywaydb.core.api.MigrationVersion
 import org.flywaydb.core.api.configuration.FluentConfiguration
 
+import scala.annotation.tailrec
 import scala.concurrent.Future
 import scala.concurrent.duration.DurationInt
 
@@ -68,6 +69,49 @@ private[platform] class FlywayMigrations(jdbcUrl: String)(implicit loggingContex
       }
     }
 
+  def validateAndWaitOnly(
+      // TODO append-only: remove after removing support for the current (mutating) schema
+      enableAppendOnlySchema: Boolean = false
+  )(implicit resourceContext: ResourceContext): Future[Unit] =
+    dataSource.use { ds =>
+      val flyway = configurationBase(dbType, enableAppendOnlySchema)
+        .dataSource(ds)
+        .ignoreFutureMigrations(false)
+        .load()
+      logger.info("Running Flyway validation...")
+
+      @tailrec
+      def flywayMigrationDone(
+          retries: Int,
+          pendingMigrationsSoFar: Option[Int],
+      ): Unit = {
+        val pendingMigrations = flyway.info().pending().length
+        if (pendingMigrations == 0) {
+          ()
+        } else if (retries <= 0) {
+          throw ExhaustedRetries(pendingMigrations)
+        } else if (pendingMigrationsSoFar.exists(pendingMigrations >= _)) {
+          throw StoppedProgressing(pendingMigrations)
+        } else {
+          logger.debug(
+            s"Concurrent migration has reduced the pending migrations set to ${pendingMigrations}, waiting until pending set is empty.."
+          )
+          Thread.sleep(1000)
+          flywayMigrationDone(retries - 1, Some(pendingMigrations))
+        }
+      }
+
+      try {
+        flywayMigrationDone(10, None)
+        logger.info("Flyway schema validation finished successfully.")
+        Future.unit
+      } catch {
+        case ex: RuntimeException =>
+          logger.error(s"Failed to validate and wait only: ${ex.getMessage}", ex)
+          Future.failed(ex)
+      }
+    }
+
   private def dataSource: ResourceOwner[HikariDataSource] =
     HikariConnection.owner(
       serverRole = ServerRole.IndexMigrations,
@@ -116,4 +160,9 @@ private[platform] object FlywayMigrations {
     Flyway
       .configure()
       .locations(locations(enableAppendOnlySchema, dbType): _*)
+
+  case class ExhaustedRetries(pendingMigrations: Int)
+      extends RuntimeException(s"Ran out of retries with ${pendingMigrations} migrations remaining")
+  case class StoppedProgressing(pendingMigrations: Int)
+      extends RuntimeException(s"Stopped progressing with ${pendingMigrations} migrations")
 }