Driver v2: migrate connect.go (mongo.Connect, Disconnect, BSONOptions, write concern)

[TopherGopher]

Part of the easymongo refactor (see PLAN.md on branch claude/mongotest-easymongo-refactor-wu2vhm). First step of the go.mongodb.org/mongo-driver v1 to go.mongodb.org/mongo-driver/v2 migration. The existing test suite is the regression net and must stay green after each issue.

Working context (read this first)

• Repository github.com/tophergopher/easymongo. Start from branch claude/mongotest-easymongo-refactor-wu2vhm; create a branch issue-<number>-<short-slug> from it and open a pull request back into it. PLAN.md on that branch is the source of truth; read it before writing code.
• Go project, single module, all packages at the repo root except the new easymongotest/ subpackage. The final toolchain target is Go 1.27 (set by Toolchain: go 1.27, latest dependencies, tidy, remove replace directives #10); any Go 1.24 or newer works until then. Before pushing: gofmt -l . prints nothing, go vet ./..., go test -race ./....
• The existing test suite (*_test.go at the root, package easymongo_test, every test function calling setup(t) from common_test.go) is the regression net for this refactor. It must pass after your change. It needs a Docker daemon reachable through the normal Docker environment and the mongo:8 image.
• Order matters. The driver package cannot compile half-migrated (the *mongo.Client type flows from Connection to Database to Collection), so the four driver issues Driver v2: migrate connect.go (mongo.Connect, Disconnect, BSONOptions, write concern) #2, Driver v2: options struct literals become builders; drop MaxTime; ArrayFilters and Delete option changes #3, Driver v2: primitive package merged into bson; replace x/bsonx in index.go #4 and Driver v2: Distinct returns a DistinctResult that must be decoded #5 land together in one pull request, worked in that order. Before them, New easymongotest subpackage wrapping mongotest/v2 #7 and Switch the easymongo test suite to easymongotest #8 must land so the test suite no longer depends on the old github.com/tophergopher/mongotest v0.1.0 module (which itself imports easymongo and breaks the moment easymongo's types change). Until mongotest's v2 module is tagged, use a local replace github.com/tophergopher/mongotest/v2 => ../mongotest/v2 in go.mod and say so in the pull request; Toolchain: go 1.27, latest dependencies, tidy, remove replace directives #10 removes it.
• Method is test-driven: write the tests listed under "Tests first", confirm they fail for the expected reason, then implement. Never delete, skip or weaken an existing test to get green; if a test must change because the public API changed (for example primitive.ObjectID to bson.ObjectID), change only the types.
• Driver v2 reference: https://pkg.go.dev/go.mongodb.org/mongo-driver/v2 and the migration guide https://github.com/mongodb/mongo-go-driver/blob/master/docs/migration-2.0.md. Signatures quoted in these issues were checked against v2.9.0.
• Do not add third-party dependencies unless this issue lists them.
• Tracking issue for the whole refactor: Tracking: easymongo refactor (mongo-driver v2, easymongotest helpers, toolchain) #13. The mongotest side is tracked in Tracking: mongotest refactor (stdlib Docker client, driver v1/v2 modules, lighter dependencies) mongotest#48.
• Commit messages: imperative summary line, blank line, body explaining why. Mention this issue number in the pull request description.

Scope (connect.go)

• Import paths to go.mongodb.org/mongo-driver/v2/....
• mongo.NewClient(opts) + client.Connect(ctx) (currently lines 281/290 and 315/326) become mongo.Connect(opts), which takes no context.
• Add Connection.Disconnect(ctx context.Context) error. Nothing in the module calls Disconnect today; easymongotest and tests will.
• EnableDebug currently does *conn.client = *client (copies a client struct). Replace with: build new options with the monitor, Disconnect the old client, mongo.Connect a new one, swap the pointer.
• Custom registry (bson.NewRegistryBuilder, bsoncodec.NewSliceCodec, bsonoptions.SliceCodec().SetEncodeNilAsEmpty(true), lines 91-107) becomes opts.SetBSONOptions(&options.BSONOptions{NilSliceAsEmpty: true}), still gated on nilSlicesAreNull.
• writeconcern.New(writeconcern.W(n)) (line 116) becomes &writeconcern.WriteConcern{W: n}. The []writeconcern.Option accumulation in mongoDriverDatabaseOptions and mongoDriverClientOptions (lines 471-484, 520-535) becomes direct struct construction: W as int or "majority", Journal: &true for WriteConcernJournal.
• readconcern.*() and readpref.*() constructors are unchanged in v2; keep them.
• event.CommandFailedEvent.Failure is an error in v2; adjust the debug monitor formatting.
• options.Database() builder and options.ListDatabases() still exist; verify SetReadConcern/SetReadPreference/SetWriteConcern signatures.

Implementation details

Verified v2.9.0 signatures you will use:

func mongo.Connect(opts ...*options.ClientOptions) (*mongo.Client, error)   // no context
func (c *mongo.Client) Disconnect(ctx context.Context) error
func (c *options.ClientOptions) SetBSONOptions(b *options.BSONOptions) *options.ClientOptions
type options.BSONOptions struct { NilSliceAsEmpty bool; NilMapAsEmpty bool; /* ... */ }
type writeconcern.WriteConcern struct { W any /* int or "majority" */; Journal *bool }
func writeconcern.W1() *writeconcern.WriteConcern; func writeconcern.Majority() *writeconcern.WriteConcern; func writeconcern.Journaled() *writeconcern.WriteConcern
type readconcern.ReadConcern struct { Level string }   // readconcern.Local(), Majority(), Linearizable(), Available(), Snapshot() still exist
func options.Database() *options.DatabaseOptionsBuilder  // SetReadConcern, SetWriteConcern, SetReadPreference, SetBSONOptions
type event.CommandFailedEvent struct { /* ... */ Failure error }

Replacement for the write concern flag mapping (mongoDriverClientOptions and mongoDriverDatabaseOptions):

var wc *writeconcern.WriteConcern
switch {
case connectFlag&WriteConcernMajority != 0: wc = writeconcern.Majority()
case connectFlag&WriteConcernW3 != 0:       wc = &writeconcern.WriteConcern{W: 3}
case connectFlag&WriteConcernW2 != 0:       wc = &writeconcern.WriteConcern{W: 2}
case connectFlag&WriteConcernW1 != 0:       wc = writeconcern.W1()
}
if connectFlag&WriteConcernJournal != 0 {
    if wc == nil { wc = &writeconcern.WriteConcern{} }
    j := true; wc.Journal = &j
}
if wc != nil { opts.SetWriteConcern(wc) }

Note the old client-options code applied W1, W2 and W3 cumulatively (later ifs overwrote earlier ones); pick the highest as above and mention it in the pull request.

EnableDebug rewrite:

conn.mongoOptions.debugMode = true
if conn.log == nil { conn.SetLogger(NewDefaultLogger()) }
ctx, cancel := conn.operationCtx(); defer cancel()
_ = conn.client.Disconnect(ctx)
client, err := mongo.Connect(conn.clientOptions())
if err != nil { return err }
conn.client = client
setGlobalConnection(conn)
return conn.Ping()

Write Disconnect so that a second call is a no-op returning nil (the driver returns mongo.ErrClientDisconnected on double disconnect; swallow exactly that error).

Tests first (connect_test.go)

• Disconnect succeeds and a subsequent Ping returns an error.
• EnableDebug keeps the connection usable (ping succeeds afterwards) and the debug logger receives at least one Debugf call for a command.
• Nil slice encoding: insert a struct with a nil []string, read back raw BSON, assert it is an empty array by default and null when nilSlicesAreNull is set.
• Write concern flags produce the expected WriteConcern values (unit test on mongoDriverClientOptions without a server).

Acceptance

• go build ./... succeeds with only connect.go and connect_test.go on driver v2 (other files may temporarily import v1 through a replace-free dual requirement; remove the v1 requirement in the final migration issue).

Depends on: #8 (test suite on easymongotest). Lands in one pull request together with #3, #4 and #5, worked in that order.