Uploading more than one package to Sandbox fails with Postgres backend

[da-tanabe]

Attempting to upload more than one package to the Sandbox (provided all packages have been built with the same version of the SDK) fails with this error in the Postgres logs:

2019-07-12 15:44:05.609 UTC [67] ERROR:  duplicate key value violates unique constraint "packages_pkey"
2019-07-12 15:44:05.609 UTC [67] DETAIL:  Key (package_id)=(a5264aaa710fe11a94ac80d71efc1083997ed09f39047888ae1c1d90cc4a549a) already exists.
2019-07-12 15:44:05.609 UTC [67] STATEMENT:  insert into packages(package_id, upload_id, source_description, size, known_since, ledger_offset, package)
	select $1, $2, $3, $4, $5, ledger_end, $6
	from parameters

Sandbox logs nothing.

The faulty code is here: https://github.com/digital-asset/daml/blob/master/ledger/sandbox/src/main/scala/com/digitalasset/platform/sandbox/stores/ledger/sql/dao/PostgresLedgerDao.scala#L1062

It rolls back the entire transaction on the event of any key conflict, which prevents the other DALFs in the same archive from being loaded. Because every DAR contains a copy of the stdlib, this effectively means that only one DAR can be uploaded.