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/agentapi/cached_workspace.go

@@ -1,9 +1,13 @@
 package agentapi
 
 import (
+	"context"
 	"sync"
 
+	"golang.org/x/xerrors"
+
 	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/database/dbauthz"
 )
 
 // CachedWorkspaceFields contains workspace data that is safe to cache for the
@@ -50,3 +54,19 @@ func (cws *CachedWorkspaceFields) AsWorkspaceIdentity() (database.WorkspaceIdent
 	}
 	return cws.identity, true
 }
+
+// ContextInject attempts to inject the rbac object for the cached workspace fields
+// into the given context, either returning the wrapped context or the original.
+func (cws *CachedWorkspaceFields) ContextInject(ctx context.Context) (context.Context, error) {
+	var err error
+	rbacCtx := ctx
+	if dbws, ok := cws.AsWorkspaceIdentity(); ok {
+		rbacCtx, err = dbauthz.WithWorkspaceRBAC(ctx, dbws.RBACObject())
+		if err != nil {
+			// Don't error level log here, will exit the function. We want to fall back to GetWorkspaceByAgentID.
+			//nolint:gocritic
+			return ctx, xerrors.Errorf("Cached workspace was present but RBAC object was invalid: %w", err)
+		}
+	}
+	return rbacCtx, nil
+}