[JUJU-1949] Dqlite DB worker connection caching

[manadart]

The proof-of-concept implementation of the dbaccessor worker had two methods, GetDB and GetExistingDB. GetDB was intended as a call through to Dqlite's Open method, with GetExistingDB used to acquire a connection to a previously opened DB.

As an exported API surface, this doesn't really make sense as it gives any consumer the ability to open (or more importantly create) any database.

Instead, we take the approach of assuming that the DB worker will know what databases it needs to open and cache connections for. GetExistingDB is removed in favour of GetDB only returning cached database connections, or an error if no such DB is known.

The first worker and manifold tests are introduced, with attendant indirections and mocks.

QA steps

Acceptance tests will accompany the patch following this one, where GetDB is used to supply a DB to the new lease store. This patch is verified by unit tests, which can be run as follows.

• Run make dqlite-deps-pull.
• Create a file in the project root, test.list, with the following contents:

github.com/juju/juju/database
github.com/juju/juju/database/migration
github.com/juju/juju/database/repository
github.com/juju/juju/worker/dbaccessor

• Run TEST_PACKAGE_LIST=test.list make test and ensure tests pass.

Documentation changes

None.

Bug reference

N/A

[jack-w-shaw]

QA is failing for me with

go test -mod=readonly -tags ""   -test.timeout=2700s $TEST_PACKAGES -check.v
# github.com/juju/juju/worker/dbaccessor.test
/snap/go/9994/pkg/tool/linux_amd64/link: running musl-gcc failed: exit status 1
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/12/libgcc_eh.a(unwind-dw2-fde-dip.o): in function `_Unwind_Find_FDE':
(.text+0x18f2): undefined reference to `_dl_find_object'
collect2: error: ld returned 1 exit status

# github.com/juju/juju/database.test
/snap/go/9994/pkg/tool/linux_amd64/link: running musl-gcc failed: exit status 1
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/12/libgcc_eh.a(unwind-dw2-fde-dip.o): in function `_Unwind_Find_FDE':
(.text+0x18f2): undefined reference to `_dl_find_object'
collect2: error: ld returned 1 exit status

I don't suppose you have any advice?

[manadart]

@jack-w-shaw If you delete the _deps directory in the project root and run again, it should work (verified locally and with Vitaly). I rebuilt the static deps with a 22.04 container and pushed them up to S3.

The 22.04 change is included here.

[anvial]

QA passed: https://pastebin.ubuntu.com/p/t3tDKhtCQK/

LG2M