Split buildGroupFromRequest() for readability

engine.go

@@ -179,11 +179,9 @@ func (e *Engine) CreateGroup(req *CreateGroupRequest) (*CreateGroupResponse, err
 	}
 	gctx := newGroupContext(ttl)
 
-	tx := newTX()
-
 	// Populate the groupContext with the final Group object, and
 	// register it while we're still holding the lock.
-	err := e.buildGroupFromRequest(gctx, tx, req)
+	tx, err := e.createAndBuildGroup(gctx, req)
 	if err == nil {
 		if _, ok := e.groups[gctx.group.ID]; ok {
 			err = errors.New("group already exists")
@@ -211,13 +209,24 @@ func (e *Engine) CreateGroup(req *CreateGroupRequest) (*CreateGroupResponse, err
 	}, nil
 }
 
-func (e *Engine) buildGroupFromRequest(gctx *groupContext, tx Transaction, req *CreateGroupRequest) error {
+// Create a new Group from the CreateGroupRequest, and prepare its
+// environment in a new Transaction.
+func (e *Engine) createAndBuildGroup(gctx *groupContext, req *CreateGroupRequest) (Transaction, error) {
+	if err := e.newGroupFromRequest(gctx, req); err != nil {
+		return nil, err
+	}
+
+	return e.buildTransaction(gctx)
+}
+
+// Build a Group from a CreateGroupRequest.
+func (e *Engine) newGroupFromRequest(gctx *groupContext, req *CreateGroupRequest) error {
 	req, err := fillCreateGroupRequest(gctx, req, e.subnets)
 	if err != nil {
 		return err
 	}
 
-	// Create the group.
+	// Create the group and attach it to the groupContext.
 	group, err := newGroup(req, e.registry)
 	if err != nil {
 		return err
@@ -229,8 +238,7 @@ func (e *Engine) buildGroupFromRequest(gctx *groupContext, tx Transaction, req *
 		return err
 	}
 
-	// Checking if the memory is available and agreeing to start
-	// the group should both happen while the lock is held.
+	// Allocate memory, if we're keeping track of it.
 	if e.memory != nil {
 		ram := group.ram()
 		if !e.memory.Acquire(ram) {
@@ -241,10 +249,20 @@ func (e *Engine) buildGroupFromRequest(gctx *groupContext, tx Transaction, req *
 		})
 	}
 
-	// Build the group resources and set up the transaction.
-	return group.BuildWithProviders(tx, e.net, e.vm)
+	return nil
+}
+
+// Setup the group resources by attaching them to a new Transaction.
+func (e *Engine) buildTransaction(gctx *groupContext) (Transaction, error) {
+	tx := newTX()
+
+	err := gctx.group.BuildWithProviders(tx, e.net, e.vm)
+
+	return tx, err
 }
 
+// Fill in missing values in the CreateGroupRequest, possibly by
+// allocating shared resources such as subnets, IP addresses, etc.
 func fillCreateGroupRequest(gctx *groupContext, req *CreateGroupRequest, netAlloc *allocator.SubnetAllocator) (*CreateGroupRequest, error) {
 	var ipnet *net.IPNet
 	var err error