Bug fixes, new schema diff interface

go/libraries/doltcore/diff/diff.go

@@ -57,7 +57,7 @@ type RowDiffer interface {
 	Close() error
 }
 
-// SqlRowDiffWriter knows how to write diff rows to an arbitrary format and destination.
+// SqlRowDiffWriter knows how to write diff rows for a table to an arbitrary format and destination.
 type SqlRowDiffWriter interface {
 	// WriteRow writes the diff row given, of the diff type provided. colDiffTypes is guaranteed to be the same length as
 	// the input row.
@@ -67,5 +67,16 @@ type SqlRowDiffWriter interface {
 	Close(ctx context.Context) error
 }
 
+// SchemaDiffWriter knows how to write SQL DDL statements for a schema diff for a table to an arbitrary format and
+// destination.
+type SchemaDiffWriter interface {
+	// WriteSchemaDiff writes the schema diff given (a SQL statement) and returns any error. A single table may have
+	// many SQL statements for a single diff.
+	WriteSchemaDiff(ctx context.Context, schemaDiffStatement string) error
+
+	// Close finalizes the work of this writer.
+	Close(ctx context.Context) error
+}
+
 // ColorFunc is a function that can color a format string
 type ColorFunc func(a ...interface{}) string

go/libraries/doltcore/table/typed/json/json_diff_writer.go

@@ -35,10 +35,15 @@ func NewJsonDiffWriter(wr io.WriteCloser, outSch schema.Schema) (*JsonDiffWriter
 	// leading diff type column with empty name
 	cols := outSch.GetAllCols()
 	newCols := schema.NewColCollection()
-	newCols.Append(schema.NewColumn("diff_type", 0, types.StringKind, false))
-	newCols.Append(cols.GetColumns()...)
+	newCols = newCols.Append(schema.NewColumn("diff_type", 0, types.StringKind, false))
+	newCols = newCols.AppendColl(cols)
 
-	writer, err := NewJSONWriter(wr, outSch)
+	newSchema, err := schema.SchemaFromCols(newCols)
+	if err != nil {
+		return nil, err
+	}
+
+	writer, err := NewJSONWriter(wr, newSchema)
 	if err != nil {
 		return nil, err
 	}
@@ -77,3 +82,16 @@ func (j *JsonDiffWriter) WriteRow(
 func (j *JsonDiffWriter) Close(ctx context.Context) error {
 	return j.wr.Close(ctx)
 }
+ type JsonSchemaDiffWriter struct {
+	 wr io.WriteCloser
+ }
+
+var _ diff.SchemaDiffWriter = (*JsonSchemaDiffWriter)(nil)
+
+func (j JsonSchemaDiffWriter) WriteSchemaDiff(ctx context.Context, schemaDiffStatement string) error {
+	return nil
+}
+
+func (j JsonSchemaDiffWriter) Close(ctx context.Context) error {
+	return j.wr.Close()
+}