perf: support fastpath in dbauthz GetLatestWorkspaceBuildByWorkspaceID (#21047)

This PR piggy backs on the agent API cached workspace added in earlier PRs to provide a fast path for avoiding `GetWorkspaceByID` calls in `GetLatestWorkspaceBuildByWorkspaceID` via injection of the workspaces RBAC object into the context. We can do this from the `agentConnectionMonitor` easily since we already cache the workspace.

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/workspaceagentsrpc.go

@@ -227,10 +227,11 @@ func (api *API) startAgentYamuxMonitor(ctx context.Context,
 	mux *yamux.Session,
 ) *agentConnectionMonitor {
 	monitor := &agentConnectionMonitor{
-		apiCtx:            api.ctx,
-		workspace:         workspace,
-		workspaceAgent:    workspaceAgent,
-		workspaceBuild:    workspaceBuild,
+		apiCtx:         api.ctx,
+		workspace:      workspace,
+		workspaceAgent: workspaceAgent,
+		workspaceBuild: workspaceBuild,
+
 		conn:              &yamuxPingerCloser{mux: mux},
 		pingPeriod:        api.AgentConnectionUpdateFrequency,
 		db:                api.Database,
@@ -453,6 +454,13 @@ func (m *agentConnectionMonitor) monitor(ctx context.Context) {
 				AgentID:     &m.workspaceAgent.ID,
 			})
 		}
+
+		ctx, err := dbauthz.WithWorkspaceRBAC(ctx, m.workspace.RBACObject())
+		if err != nil {
+			// Don't error level log here, will exit the function. We want to fall back to GetWorkspaceByAgentID.
+			//nolint:gocritic
+			m.logger.Debug(ctx, "Cached workspace was present but RBAC object was invalid", slog.F("err", err))
+		}
 		err = checkBuildIsLatest(ctx, m.db, m.workspaceBuild)
 		if err != nil {
 			reason = err.Error()