[Spark] Fix char/varchar to string column conversions

Users are currently allowed to change the data type of their columns
from char/varchar to string. Even though the command succeeds, the
column metadata that indicates that the column is char/varchar is not
removed during the process. As a result, Delta enforces the old
char/varchar constraints on future inserts.

This PR fixes that by removing this metadata from any updated
char/varchar column when the column is updated.

## How was this patch tested?

Added a new test which was failing without the code fix.

## Does this PR introduce _any_ user-facing changes?
Yes, `ALTER COLUMN X TYPE STRING;` will now work correctly when X is
char/varchar.

spark/src/main/scala/org/apache/spark/sql/delta/commands/alterDeltaTableCommands.scala

@@ -41,7 +41,7 @@ import org.apache.spark.sql.catalyst.analysis.{Resolver, UnresolvedAttribute}
 import org.apache.spark.sql.catalyst.catalog.CatalogUtils
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.plans.logical.{IgnoreCachedData, QualifiedColType}
-import org.apache.spark.sql.catalyst.util.CharVarcharUtils
+import org.apache.spark.sql.catalyst.util.{CharVarcharUtils, SparkCharVarcharUtils}
 import org.apache.spark.sql.connector.catalog.TableCatalog
 import org.apache.spark.sql.connector.catalog.TableChange.{After, ColumnPosition, First}
 import org.apache.spark.sql.connector.expressions.FieldReference
@@ -578,12 +578,25 @@ case class AlterTableChangeColumnDeltaCommand(
                 case Some(newDefaultValue) => result.withCurrentDefaultValue(newDefaultValue)
                 case None => result.clearCurrentDefaultValue()
               }
+              val updatedColumnMetadata =
+                if (!SparkCharVarcharUtils.hasCharVarchar(newColumn.dataType)) {
+                  // Remove the char/varchar property from the metadata that
+                  // indicates that this column is a char/varchar column.
+                  // We construct this throwaway object because
+                  // CharVarcharUtils.cleanAttrMetadata takes an AttributeReference.
+                  val throwAwayAttrRef = AttributeReference(
+                      result.name, result.dataType, nullable = result.nullable, result.metadata)()
+                  CharVarcharUtils.cleanAttrMetadata(throwAwayAttrRef).metadata
+                } else {
+                  result.metadata
+                }
               result
                 .copy(
                   name = newColumn.name,
                   dataType =
                     SchemaUtils.changeDataType(oldColumn.dataType, newColumn.dataType, resolver),
-                  nullable = newColumn.nullable)
+                  nullable = newColumn.nullable,
+                  metadata = updatedColumnMetadata)
           }
 
           // Replace existing field with new field

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

@@ -1628,6 +1628,17 @@ trait DeltaAlterTableTests extends DeltaAlterTableTestBase {
       checkColType(spark.table("t").schema(1), VarcharType(5))
     }
   }
+
+  test("CHANGE COLUMN: allow change from char(x) to string type") {
+    withTable("t") {
+      sql("CREATE TABLE t(i VARCHAR(4)) USING delta")
+      sql("ALTER TABLE t CHANGE COLUMN i TYPE STRING")
+      val col = spark.table("t").schema.head
+      assert(col.dataType == StringType)
+      assert(CharVarcharUtils.getRawType(col.metadata).isEmpty)
+      sql("INSERT INTO t VALUES ('123456789')")
+    }
+  }
 }
 
 trait DeltaAlterTableByNameTests extends DeltaAlterTableTests {