THRIFT-5845: Return TException for union check in Write

Client: go

In compiler generated Write method for union types, return a TException
(TProtocolException) when the number of fields set is not exactly 1, to
help customer logic to decide whether to reuse a connection after an
error.

While I'm here, also do the same thing for the uniqueness check failure
for set fields in Write as well.
diff --git a/lib/go/test/UnionBinaryTest.thrift b/lib/go/test/UnionBinaryTest.thrift
index f77112b..8bce38d 100644
--- a/lib/go/test/UnionBinaryTest.thrift
+++ b/lib/go/test/UnionBinaryTest.thrift
@@ -21,5 +21,6 @@
 union Sample {
   1: map<string, string> u1,
   2: binary u2,
-  3: list<string> u3
+  3: list<string> u3,
+  4: set<string> u4,
 }
diff --git a/lib/go/test/tests/write_texception_test.go b/lib/go/test/tests/write_texception_test.go
new file mode 100644
index 0000000..3241364
--- /dev/null
+++ b/lib/go/test/tests/write_texception_test.go
@@ -0,0 +1,69 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package tests
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/apache/thrift/lib/go/test/gopath/src/unionbinarytest"
+	"github.com/apache/thrift/lib/go/thrift"
+)
+
+func TestWriteUnionTException(t *testing.T) {
+	// See https://issues.apache.org/jira/browse/THRIFT-5845
+	s := unionbinarytest.NewSample()
+	proto := thrift.NewTBinaryProtocolConf(thrift.NewTMemoryBuffer(), nil)
+	err := s.Write(context.Background(), proto)
+	t.Log(err)
+	if err == nil {
+		t.Fatal("Writing empty union did not produce error")
+	}
+	var te thrift.TException
+	if !errors.As(err, &te) {
+		t.Fatalf("Error from writing empty union is not TException: (%T) %v", err, err)
+	}
+	if typ := te.TExceptionType(); typ != thrift.TExceptionTypeProtocol && typ != thrift.TExceptionTypeTransport {
+		t.Errorf("Got TExceptionType %v, want one of TProtocolException or TTransportException", typ)
+	}
+}
+
+func TestWriteSetTException(t *testing.T) {
+	// See https://issues.apache.org/jira/browse/THRIFT-5845
+	s := unionbinarytest.NewSample()
+	s.U4 = []string{
+		"foo",
+		"foo", // duplicate
+	}
+	proto := thrift.NewTBinaryProtocolConf(thrift.NewTMemoryBuffer(), nil)
+	err := s.Write(context.Background(), proto)
+	t.Log(err)
+	if err == nil {
+		t.Fatal("Writing duplicate set did not produce error")
+	}
+	var te thrift.TException
+	if !errors.As(err, &te) {
+		t.Fatalf("Error from writing duplicate set is not TException: (%T) %v", err, err)
+	}
+	if typ := te.TExceptionType(); typ != thrift.TExceptionTypeProtocol && typ != thrift.TExceptionTypeTransport {
+		t.Errorf("Got TExceptionType %v, want one of TProtocolException or TTransportException", typ)
+	}
+}