chore: Refactor CometScanRule to improve scan selection and fallback logic

spark/src/main/scala/org/apache/comet/rules/CometScanRule.scala

@@ -42,7 +42,7 @@ import org.apache.spark.sql.types._
 
 import org.apache.comet.{CometConf, CometNativeException, DataTypeSupport}
 import org.apache.comet.CometConf._
-import org.apache.comet.CometSparkSessionExtensions.{hasExplainInfo, isCometLoaded, withInfo, withInfos}
+import org.apache.comet.CometSparkSessionExtensions.{isCometLoaded, withInfo, withInfos}
 import org.apache.comet.DataTypeSupport.isComplexType
 import org.apache.comet.iceberg.{CometIcebergNativeScanMetadata, IcebergReflection}
 import org.apache.comet.objectstore.NativeConfig
@@ -145,21 +145,9 @@ case class CometScanRule(session: SparkSession) extends Rule[SparkPlan] with Com
         if (!CometScanExec.isFileFormatSupported(r.fileFormat)) {
           return withInfo(scanExec, s"Unsupported file format ${r.fileFormat}")
         }
+        val hadoopConf = r.sparkSession.sessionState.newHadoopConfWithOptions(r.options)
 
-        var scanImpl = COMET_NATIVE_SCAN_IMPL.get()
-
-        val hadoopConf = scanExec.relation.sparkSession.sessionState
-          .newHadoopConfWithOptions(scanExec.relation.options)
-
-        // if scan is auto then pick the best available scan
-        if (scanImpl == SCAN_AUTO) {
-          scanImpl = selectScan(scanExec, r.partitionSchema, hadoopConf)
-        }
-
-        if (scanImpl == SCAN_NATIVE_DATAFUSION && !CometNativeScan.isSupported(scanExec)) {
-          return scanExec
-        }
-
+        // TODO is this restriction valid for all native scan types?
         val possibleDefaultValues = getExistenceDefaultValues(scanExec.requiredSchema)
         if (possibleDefaultValues.exists(d => {
             d != null && (d.isInstanceOf[ArrayBasedMapData] || d
@@ -171,31 +159,72 @@ case class CometScanRule(session: SparkSession) extends Rule[SparkPlan] with Com
           withInfo(
             scanExec,
             "Full native scan disabled because nested types for default values are not supported")
+          return scanExec
         }
 
-        if (encryptionEnabled(hadoopConf) && scanImpl != CometConf.SCAN_NATIVE_COMET) {
-          if (!isEncryptionConfigSupported(hadoopConf)) {
-            withInfo(scanExec, s"$scanImpl does not support encryption")
-          }
-        }
-
-        // check that schema is supported
-        checkSchema(scanExec, scanImpl, r)
-
-        if (hasExplainInfo(scanExec)) {
-          // could not accelerate, and plan is already tagged with fallback reasons
-          scanExec
-        } else {
-          // this is confusing, but we always insert a CometScanExec here, which may replaced
-          // with a CometNativeExec when CometExecRule runs, depending on the scanImpl value.
-          CometScanExec(scanExec, session, scanImpl)
+        COMET_NATIVE_SCAN_IMPL.get() match {
+          case SCAN_AUTO =>
+            // TODO add support for native_datafusion in the future
+            nativeIcebergCompatScan(session, scanExec, r, hadoopConf)
+              .orElse(nativeCometScan(session, scanExec, r, hadoopConf))
+              .getOrElse(scanExec)
+          case SCAN_NATIVE_DATAFUSION =>
+            nativeDataFusionScan(session, scanExec, r, hadoopConf).getOrElse(scanExec)
+          case SCAN_NATIVE_ICEBERG_COMPAT =>
+            nativeIcebergCompatScan(session, scanExec, r, hadoopConf).getOrElse(scanExec)
+          case SCAN_NATIVE_COMET =>
+            nativeCometScan(session, scanExec, r, hadoopConf).getOrElse(scanExec)
         }
 
       case _ =>
         withInfo(scanExec, s"Unsupported relation ${scanExec.relation}")
     }
   }
 
+  private def nativeDataFusionScan(
+      session: SparkSession,
+      scanExec: FileSourceScanExec,
+      r: HadoopFsRelation,
+      hadoopConf: Configuration): Option[SparkPlan] = {
+    if (!CometNativeScan.isSupported(scanExec)) {
+      return None
+    }
+    if (encryptionEnabled(hadoopConf) && !isEncryptionConfigSupported(hadoopConf)) {
+      withInfo(scanExec, s"$SCAN_NATIVE_DATAFUSION does not support encryption")
+      return None
+    }
+    if (!isSchemaSupported(scanExec, SCAN_NATIVE_DATAFUSION, r)) {
+      return None
+    }
+    Some(CometScanExec(scanExec, session, SCAN_NATIVE_DATAFUSION))
+  }
+
+  private def nativeIcebergCompatScan(
+      session: SparkSession,
+      scanExec: FileSourceScanExec,
+      r: HadoopFsRelation,
+      hadoopConf: Configuration): Option[SparkPlan] = {
+    if (encryptionEnabled(hadoopConf) && !isEncryptionConfigSupported(hadoopConf)) {
+      withInfo(scanExec, s"$SCAN_NATIVE_ICEBERG_COMPAT does not support encryption")
+      return None
+    }
+    if (!isSchemaSupported(scanExec, SCAN_NATIVE_ICEBERG_COMPAT, r)) {
+      return None
+    }
+    Some(CometScanExec(scanExec, session, SCAN_NATIVE_ICEBERG_COMPAT))
+  }
+
+  private def nativeCometScan(
+      session: SparkSession,
+      scanExec: FileSourceScanExec,
+      r: HadoopFsRelation,
+      hadoopConf: Configuration): Option[SparkPlan] = {
+    if (!isSchemaSupported(scanExec, SCAN_NATIVE_COMET, r)) {
+      return None
+    }
+    Some(CometScanExec(scanExec, session, SCAN_NATIVE_COMET))
+  }
+
   private def transformV2Scan(scanExec: BatchScanExec): SparkPlan = {
 
     scanExec.scan match {
@@ -612,20 +641,32 @@ case class CometScanRule(session: SparkSession) extends Rule[SparkPlan] with Com
   private def isDynamicPruningFilter(e: Expression): Boolean =
     e.exists(_.isInstanceOf[PlanExpression[_]])
 
-  def checkSchema(scanExec: FileSourceScanExec, scanImpl: String, r: HadoopFsRelation): Unit = {
+  private def isSchemaSupported(
+      scanExec: FileSourceScanExec,
+      scanImpl: String,
+      r: HadoopFsRelation): Boolean = {
     val fallbackReasons = new ListBuffer[String]()
     val typeChecker = CometScanTypeChecker(scanImpl)
     val schemaSupported =
       typeChecker.isSchemaSupported(scanExec.requiredSchema, fallbackReasons)
     if (!schemaSupported) {
-      withInfo(scanExec, s"Unsupported schema ${scanExec.requiredSchema} for $scanImpl")
+      withInfo(
+        scanExec,
+        s"Unsupported schema ${scanExec.requiredSchema} " +
+          s"for $scanImpl: ${fallbackReasons.mkString(", ")}")
+      return false
     }
     val partitionSchemaSupported =
       typeChecker.isSchemaSupported(r.partitionSchema, fallbackReasons)
     if (!partitionSchemaSupported) {
-      fallbackReasons += s"Unsupported partitioning schema ${r.partitionSchema} for $scanImpl"
+      withInfo(
+        scanExec,
+        s"Unsupported partitioning schema ${scanExec.requiredSchema} " +
+          s"for $scanImpl: ${fallbackReasons
+              .mkString(", ")}")
+      return false
     }
-    withInfos(scanExec, fallbackReasons.toSet)
+    true
   }
 }