Add tests

kernel/kernel-api/src/main/java/io/delta/kernel/internal/DeltaErrors.java

@@ -25,6 +25,7 @@
 import java.io.IOException;
 import java.sql.Timestamp;
 import java.util.List;
+import java.util.Locale;
 import java.util.Optional;
 import java.util.Set;
 import java.util.function.Supplier;
@@ -146,21 +147,27 @@ public static KernelException mismatchedProtocolVersionFeatureSet(
       int tableFeatureVersion,
       int minRequiredVersion,
       Set<String> tableFeatures) {
+    final String featureTypeStr = featureType.toString().toLowerCase(Locale.ROOT);
     final String message =
         String.format(
-            "Found %s Features %s in protocol version %s but these are only supported in "
-                + "protocol version %s and above.",
-            featureType, tableFeatureVersion, minRequiredVersion, tableFeatures);
+            "Found %s features %s in %s protocol version %s but these are only supported in "
+                + "%s protocol version %s and above.",
+            featureTypeStr,
+            tableFeatures,
+            featureTypeStr,
+            tableFeatureVersion,
+            featureTypeStr,
+            minRequiredVersion);
     return new KernelException(message);
   }
 
   public static KernelException tableFeatureReadRequiresWrite(
       int tableReaderVersion, int tableWriterVersion) {
     final String message =
         String.format(
-            "Table Reader Features are supported (current reader protocol version is %s) yet "
-                + "table Writer Features are not (current writer protocol version is %s). Writer "
-                + "protocol version must be at least %s to proceed",
+            "Table reader features are supported (current reader protocol version is %s) yet "
+                + "table writer features are not (current writer protocol version is %s). Writer "
+                + "protocol version must be at least %s to proceed.",
             tableReaderVersion,
             tableWriterVersion,
             TableFeatures.TABLE_FEATURES_MIN_WRITER_VERSION);

kernel/kernel-api/src/main/java/io/delta/kernel/internal/TableFeatures.java

@@ -33,11 +33,6 @@ public class TableFeatures {
   public enum TableFeatureType {
     READER,
     WRITER;
-
-    @Override
-    public String toString() {
-      return name().charAt(0) + name().substring(1).toLowerCase();
-    }
   }
 
   /** Min reader version that supports reader features. */

kernel/kernel-api/src/test/scala/io/delta/kernel/internal/TableFeaturesSuite.scala

@@ -30,6 +30,31 @@ import scala.collection.JavaConverters._
  * Suite that tests Kernel throws error when it receives a unsupported protocol and metadata
  */
 class TableFeaturesSuite extends AnyFunSuite {
+  test("throws when table does not support reader features but readerFeatures is non-empty") {
+    val exMsg = intercept[KernelException] {
+      new Protocol(1, 7, Set("columnMapping").asJava, Collections.emptySet())
+    }.getMessage
+    assert(exMsg == "Found reader features [columnMapping] in reader protocol version 1 but " +
+      "these are only supported in reader protocol version 3 and above.")
+  }
+
+  test("throws when table does not support writer features but writerFeatures is non-empty") {
+    val exMsg = intercept[KernelException] {
+      new Protocol(1, 1, Collections.emptySet(), Set("appendOnly").asJava)
+    }.getMessage
+    assert(exMsg == "Found writer features [appendOnly] in writer protocol version 1 but these " +
+      "are only supported in writer protocol version 7 and above.")
+  }
+
+  test("throws when table supports reader features but does not support writer features") {
+    val exMsg = intercept[KernelException] {
+      new Protocol(3, 1, Collections.emptySet(), Collections.emptySet())
+    }.getMessage
+    assert(exMsg == "Table reader features are supported (current reader protocol version is 3) " +
+      "yet table writer features are not (current writer protocol version is 1). Writer protocol " +
+      "version must be at least 7 to proceed.")
+  }
+
   test("validate write supported: protocol 1") {
     checkSupported(createTestProtocol(minWriterVersion = 1))
   }
@@ -70,37 +95,41 @@ class TableFeaturesSuite extends AnyFunSuite {
 
   Seq("appendOnly", "inCommitTimestamp", "columnMapping", "typeWidening-preview", "typeWidening")
     .foreach { supportedWriterFeature =>
-    test(s"validateWriteSupported: protocol 7 with $supportedWriterFeature") {
-      checkSupported(createTestProtocol(minWriterVersion = 7, supportedWriterFeature))
+      test(s"validateWriteSupported: protocol 7 with $supportedWriterFeature") {
+        checkSupported(createTestProtocol(minWriterVersion = 7, supportedWriterFeature))
+      }
     }
-  }
 
   Seq("invariants", "checkConstraints", "generatedColumns", "allowColumnDefaults", "changeDataFeed",
-      "identityColumns", "deletionVectors", "rowTracking", "timestampNtz",
-      "domainMetadata", "v2Checkpoint", "icebergCompatV1", "icebergCompatV2", "clustering",
-      "vacuumProtocolCheck").foreach { unsupportedWriterFeature =>
+    "identityColumns", "deletionVectors", "rowTracking", "timestampNtz",
+    "domainMetadata", "v2Checkpoint", "icebergCompatV1", "icebergCompatV2", "clustering",
+    "vacuumProtocolCheck").foreach { unsupportedWriterFeature =>
     test(s"validateWriteSupported: protocol 7 with $unsupportedWriterFeature") {
       checkUnsupported(createTestProtocol(minWriterVersion = 7, unsupportedWriterFeature))
     }
   }
 
-  def checkSupported(
-    protocol: Protocol,
-    metadata: Metadata = null,
-    schema: StructType = createTestSchema()): Unit = {
+  ////////////////////
+  // Helper Methods //
+  ////////////////////
+
+  private def checkSupported(
+      protocol: Protocol,
+      metadata: Metadata = null,
+      schema: StructType = createTestSchema()): Unit = {
     validateWriteSupportedTable(protocol, metadata, schema, "/test/table")
   }
 
-  def checkUnsupported(
-    protocol: Protocol,
-    metadata: Metadata = null,
-    schema: StructType = createTestSchema()): Unit = {
+  private def checkUnsupported(
+      protocol: Protocol,
+      metadata: Metadata = null,
+      schema: StructType = createTestSchema()): Unit = {
     intercept[KernelException] {
       validateWriteSupportedTable(protocol, metadata, schema, "/test/table")
     }
   }
 
-  def createTestProtocol(minWriterVersion: Int, writerFeatures: String*): Protocol = {
+  private def createTestProtocol(minWriterVersion: Int, writerFeatures: String*): Protocol = {
     new Protocol(
       // minReaderVersion - it doesn't matter as the read fails anyway before the writer check
       0,
@@ -111,7 +140,7 @@ class TableFeaturesSuite extends AnyFunSuite {
     )
   }
 
-  def createTestMetadata(withAppendOnly: Boolean = false): Metadata = {
+  private def createTestMetadata(withAppendOnly: Boolean = false): Metadata = {
     var config: Map[String, String] = Map()
     if (withAppendOnly) {
       config = Map("delta.appendOnly" -> "true");
@@ -140,8 +169,8 @@ class TableFeaturesSuite extends AnyFunSuite {
     )
   }
 
-  def createTestSchema(
-    includeInvariant: Boolean = false): StructType = {
+  private def createTestSchema(
+      includeInvariant: Boolean = false): StructType = {
     var structType = new StructType()
       .add("c1", IntegerType.INTEGER)
       .add("c2", StringType.STRING)