Cannot insert with PENDING_COMMIT_TIMESTAMP if that column is part of an index

[bloveless]

This issue is about a regression that was introduces in 1.5.17 which fixed #25.

I've reproduced this issue in the following example.

go test -v shows the following (condensed) output

=== RUN   TestSpanner1516Passing
    main_test.go:85: Created database [testdb]
--- PASS: TestSpanner1516Passing (21.97s)
=== RUN   TestSpanner1517Failing
    main_test.go:85: Created database [testdb]
    main_test.go:99: failed to insert data: spanner: code = "InvalidArgument", desc = "Table _index_data_table_AK_SingersCreatedTime cannot be accessed because it, or its associated index, has a pending CommitTimestamp", requestID = "1.77b04ab44b49316e.2.3.6.1"
--- FAIL: TestSpanner1517Failing (21.48s)
=== RUN   TestSpanner1529Failing
    main_test.go:85: Created database [testdb]
    main_test.go:99: failed to insert data: spanner: code = "InvalidArgument", desc = "Table _index_data_table_AK_SingersCreatedTime cannot be accessed because it, or its associated index, has a pending CommitTimestamp", requestID = "1.77b04ab44b49316e.3.2.6.1"
--- FAIL: TestSpanner1529Failing (21.49s)
FAIL
exit status 1
FAIL    commitrepro     65.951s

commitrepro.tgz

In this example you can see that using spanner emulator 1.5.16 it works to insert a pending commit timestamp into a column which exists in an index. All versions after that do not allow this functionality. I specifically tested the next version 1.5.17 and the latest version 1.5.29 and they both fail.

The summary of the issue is that the following used to work in the emulator but doesn't anymore:

CREATE TABLE Singers (
    SingerId   INT64 NOT NULL,
    FirstName  STRING(1024),
    CreatedAt  TIMESTAMP OPTIONS (allow_commit_timestamp=true),
) PRIMARY KEY (SingerId);
CREATE UNIQUE INDEX AK_SingersCreatedTime ON Singers (CreatedAt);
INSERT INTO Singers (SingerId, FirstName, CreatedAt) VALUES (1, 'Marc', PENDING_COMMIT_TIMESTAMP());

I believe this is was mentioned as the final comment on issue #25.