Nicer error when failing to read table due to type change not support…

…ed by the preview of type widening

spark/src/main/resources/error/delta-error-classes.json

@@ -2832,6 +2832,14 @@
     ],
     "sqlState" : "0AKDC"
   },
+  "DELTA_UNSUPPORTED_TYPE_CHANGE_IN_PREVIEW": {
+    "message": [
+      "This table can't be read by this version of Delta because an unsupported type change was applied. Field <fieldPath> was changed from <fromType> to <toType>.",
+      "Please upgrade to Delta 4.0 or higher to read this table, or drop the Type Widening table feature using a client that supports reading this table:",
+      "  ALTER TABLE tableName DROP FEATURE <typeWideningFeatureName>"
+    ],
+    "sqlState": "0AKDC"
+  },
   "DELTA_UNSUPPORTED_TYPE_CHANGE_IN_SCHEMA" : {
     "message" : [
       "Unable to operate on this table because an unsupported type change was applied. Field <fieldName> was changed from <fromType> to <toType>."

spark/src/main/scala/org/apache/spark/sql/delta/DeltaErrors.scala

@@ -667,6 +667,20 @@ trait DeltaErrorsBase
     )
   }
 
+  def unsupportedTypeChangeInPreview(
+      fieldPath: Seq[String],
+      fromType: DataType,
+      toType: DataType,
+      feature: TypeWideningTableFeatureBase): Throwable =
+    new DeltaUnsupportedOperationException(
+      errorClass = "DELTA_UNSUPPORTED_TYPE_CHANGE_IN_PREVIEW",
+      messageParameters = Array(
+        SchemaUtils.prettyFieldName(fieldPath),
+        fromType.sql,
+        toType.sql,
+        feature.name
+    ))
+
   def unsupportedTypeChangeInSchema(
       fieldPath: Seq[String],
       fromType: DataType,

spark/src/main/scala/org/apache/spark/sql/delta/TypeWidening.scala

@@ -89,6 +89,14 @@ object TypeWidening {
     TypeWideningMetadata.getAllTypeChanges(metadata.schema).foreach {
       case (_, TypeChange(_, from: AtomicType, to: AtomicType, _))
         if isTypeChangeSupported(from, to) =>
+      case (fieldPath, TypeChange(_, from: AtomicType, to: AtomicType, _))
+        if stableFeatureCanReadTypeChange(from, to) =>
+        val featureName = if (protocol.isFeatureSupported(TypeWideningPreviewTableFeature)) {
+          TypeWideningPreviewTableFeature
+        } else {
+          TypeWideningTableFeature
+        }
+        throw DeltaErrors.unsupportedTypeChangeInPreview(fieldPath, from, to, featureName)
       case (fieldPath, invalidChange) =>
         throw DeltaErrors.unsupportedTypeChangeInSchema(
           fieldPath ++ invalidChange.fieldPath,
@@ -97,4 +105,25 @@ object TypeWidening {
         )
     }
   }
+
+  /**
+   * Whether the given type change is supported in the stable version of the feature. Used to
+   * provide a helpful error message during the preview phase if upgrading to Delta 4.0 would allow
+   * reading the table.
+   */
+  private def stableFeatureCanReadTypeChange(fromType: AtomicType, toType: AtomicType): Boolean =
+    (fromType, toType) match {
+      case (from, to) if from == to => true
+      case (from: IntegralType, to: IntegralType) => from.defaultSize <= to.defaultSize
+      case (FloatType, DoubleType) => true
+      case (DateType, TimestampNTZType) => true
+      case (ByteType | ShortType | IntegerType, DoubleType) => true
+      case (from: DecimalType, to: DecimalType) => to.isWiderThan(from)
+      // Byte, Short, Integer are all stored as INT32 in parquet. The parquet readers support
+      // converting INT32 to Decimal(10, 0) and wider.
+      case (ByteType | ShortType | IntegerType, d: DecimalType) => d.isWiderThan(IntegerType)
+      // The parquet readers support converting INT64 to Decimal(20, 0) and wider.
+      case (LongType, d: DecimalType) => d.isWiderThan(LongType)
+      case _ => false
+    }
 }

spark/src/test/scala/org/apache/spark/sql/delta/DeltaErrorsSuite.scala

@@ -1030,6 +1030,24 @@ trait DeltaErrorsSuiteBase
           |""".stripMargin
       ))
     }
+    {
+      checkError(
+        exception = intercept[DeltaUnsupportedOperationException] {
+          throw DeltaErrors.unsupportedTypeChangeInPreview(
+            fieldPath = Seq("origin", "country"),
+            fromType = IntegerType,
+            toType = LongType,
+            feature = TypeWideningPreviewTableFeature
+          )
+        },
+        "DELTA_UNSUPPORTED_TYPE_CHANGE_IN_PREVIEW",
+        parameters = Map(
+          "fieldPath" -> "origin.country",
+          "fromType" -> "INT",
+          "toType" -> "BIGINT",
+          "typeWideningFeatureName" -> "typeWidening-preview"
+        ))
+    }
     {
       val e = intercept[DeltaIllegalStateException] {
         throw DeltaErrors.unsupportedTypeChangeInSchema(Seq("s", "a"), IntegerType, StringType)

spark/src/test/scala/org/apache/spark/sql/delta/typewidening/TypeWideningTableFeatureSuite.scala

@@ -45,7 +45,10 @@ class TypeWideningTableFeatureSuite
     with TypeWideningDropFeatureTestMixin
     with TypeWideningTableFeatureTests
 
-trait TypeWideningTableFeatureTests extends RowTrackingTestUtils with TypeWideningTestCases {
+trait TypeWideningTableFeatureTests
+  extends RowTrackingTestUtils
+    with DeltaExcludedBySparkVersionTestMixinShims
+    with TypeWideningTestCases {
   self: QueryTest
     with TypeWideningTestMixin
     with TypeWideningDropFeatureTestMixin =>
@@ -454,6 +457,43 @@ trait TypeWideningTableFeatureTests extends RowTrackingTestUtils with TypeWideni
     )
   }
 
+  testSparkLatestOnly(
+    "helpful error when reading type changes not supported yet during preview") {
+    sql(s"CREATE TABLE delta.`$tempDir` (a int) USING DELTA")
+    val metadata = new MetadataBuilder()
+      .putMetadataArray("delta.typeChanges", Array(
+        new MetadataBuilder()
+          .putString("toType", "long")
+          .putString("fromType", "int")
+          .putLong("tableVersion", 1)
+          .build()
+      )).build()
+
+    // Delta 3.2/3.3 doesn't support changing type from int->long, we manually commit that type
+    // change to simulate what Delta 4.0 could do.
+    deltaLog.withNewTransaction { txn =>
+      txn.commit(
+        Seq(txn.snapshot.metadata.copy(
+          schemaString = new StructType()
+            .add("a", LongType, nullable = true, metadata).json
+        )),
+        ManualUpdate)
+    }
+
+    checkError(
+      exception = intercept[DeltaUnsupportedOperationException] {
+        readDeltaTable(tempPath).collect()
+      },
+      "DELTA_UNSUPPORTED_TYPE_CHANGE_IN_PREVIEW",
+      parameters = Map(
+        "fieldPath" -> "a",
+        "fromType" -> "INT",
+        "toType" -> "BIGINT",
+        "typeWideningFeatureName" -> "typeWidening-preview"
+      )
+    )
+  }
+
   test("type widening rewrite metrics") {
     sql(s"CREATE TABLE delta.`$tempDir` (a byte) USING DELTA")
     addSingleFile(Seq(1, 2, 3), ByteType)