[SPARK-25700][SQL] Creates ReadSupport in only Append Mode in Data Source V2 write path

sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala

@@ -246,8 +246,8 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
             df.sparkSession.sessionState.conf)
           val options = sessionOptions ++ extraOptions
 
-          val relation = DataSourceV2Relation.create(source, options)
           if (mode == SaveMode.Append) {
+            val relation = DataSourceV2Relation.create(source, options)
             runCommand(df.sparkSession, "save") {
               AppendData.byName(relation, df.logicalPlan)
             }

sql/core/src/test/scala/org/apache/spark/sql/sources/v2/DataSourceV2Suite.scala

@@ -351,6 +351,24 @@ class DataSourceV2Suite extends QueryTest with SharedSQLContext {
       }
     }
   }
+
+  test("SPARK-25700: do not read schema when writing in other modes except append mode") {
+    withTempPath { file =>
+      val cls = classOf[SimpleWriteOnlyDataSource]
+      val path = file.getCanonicalPath
+      val df = spark.range(5).select('id as 'i, -'id as 'j)
+      try {
+        df.write.format(cls.getName).option("path", path).mode("error").save()
+        df.write.format(cls.getName).option("path", path).mode("overwrite").save()
+        df.write.format(cls.getName).option("path", path).mode("ignore").save()
+      } catch {
+        case e: SchemaReadAttemptException => fail("Schema read was attempted.", e)
+      }
+      intercept[SchemaReadAttemptException] {
+        df.write.format(cls.getName).option("path", path).mode("append").save()
+      }
+    }
+  }
 }
 
 
@@ -640,3 +658,14 @@ object SpecificReaderFactory extends PartitionReaderFactory {
     }
   }
 }
+
+class SchemaReadAttemptException(m: String) extends RuntimeException(m)
+
+class SimpleWriteOnlyDataSource extends SimpleWritableDataSource {
+  override def fullSchema(): StructType = {
+    // This is a bit hacky since this source implements read support but throws
+    // during schema retrieval. Might have to rewrite but it's done
+    // such so for minimised changes.
+    throw new SchemaReadAttemptException("read is not supported")
+  }
+}

sql/core/src/test/scala/org/apache/spark/sql/sources/v2/SimpleWritableDataSource.scala

@@ -43,13 +43,13 @@ class SimpleWritableDataSource extends DataSourceV2
   with BatchWriteSupportProvider
   with SessionConfigSupport {
 
-  private val schema = new StructType().add("i", "long").add("j", "long")
+  protected def fullSchema(): StructType = new StructType().add("i", "long").add("j", "long")
 
   override def keyPrefix: String = "simpleWritableDataSource"
 
   class ReadSupport(path: String, conf: Configuration) extends SimpleReadSupport {
 
-    override def fullSchema(): StructType = schema
+    override def fullSchema(): StructType = SimpleWritableDataSource.this.fullSchema()
 
     override def planInputPartitions(config: ScanConfig): Array[InputPartition] = {
       val dataPath = new Path(path)
@@ -116,7 +116,6 @@ class SimpleWritableDataSource extends DataSourceV2
       schema: StructType,
       mode: SaveMode,
       options: DataSourceOptions): Optional[BatchWriteSupport] = {
-    assert(DataType.equalsStructurally(schema.asNullable, this.schema.asNullable))
     assert(!SparkContext.getActive.get.conf.getBoolean("spark.speculation", false))
 
     val path = new Path(options.get("path").get())